Would More Rain Have Made Good Corn Yields Even Better?

The Illinois State Water Survey weather recording site in Champaign near the South Farms provided the following rainfall totals (in inches) in 2013: May – 4.65; June – 5.33; July – 3.47; August – 0.49; September – 0.50. The dry weather along with high temperatures in late August into September – it reached 98 degrees on August 31 and again on September 10 – had many of us believing that yields would be lowered. I was also hoping that irrigation in a study we conducted here might bring us 300-bushel yields by preventing such a decline.

The irrigation study was conducted by grad student Josh Vonk, with help from others. It was planted following soybeans, with 180 lb of N applied as UAN before final tillage. On May 20, we planted about 40,000 seeds per acre of DeKalb hybrid DKC 62-08. Irrigation was done using dripline placed between row middles, with a row skipped between lines. We started to irrigate in mid-July, and from then to mid- September, irrigated plots received 9.66 inches of water, or about 1.2 inches per week. Rainfall totaled only about 2 inches over this period.

Besides comparing corn with and without irrigation, we applied as treatments 70 more lb of N on July 9 as Agrotain-treated urea, and foliar fungicide at silking. This made a total of eight treatments, ranging from nothing (the control) to all three added inputs – irrigated + extra N + fungicide. The idea was that extra N and fungicide would help yields more in irrigated plots, where water was not limiting.

We were much surprised when the control (nothing extra added) yielded 240 bushels per acre. Less surprising, adding extra N in non-irrigated plots failed to increase yield. Dry weather like we had resulted in little foliar disease, but many have come to expect that strobilurin fungicide will increase yield by helping to relieve stress under conditions like these. Fungicide did not increase yield in this case.

Adding water only – no extra N or fungicide – increased yield by 19 bushels (8 percent), to 259 bushels per acre. Adding 70 lb of N to irrigated plots added another 18 bushels, bringing yield to 277 bushels per acre. This N would easily have paid for itself under irrigation, but not without irrigation. As we saw in non-irrigated plots, adding fungicide failed to increase yields in irrigated corn, with or without extra N.

So in a season with good pollination conditions but a very dry second half, yields were outstanding even with normal input levels, and they were increased by less than 10 percent by water alone, and by about 15 percent by adding both water and extra N.

While 277 bushel per acre is nothing to complain about, it does leave us wondering what limiting factor prevented yields from being even higher. Temperature? Sunlight amount? We don’t expect we’ll ever know the answer for sure at these high yield levels. But we’ll keep looking.

It’s likely that soils have been “wrung dry” in the top 2 or 3 feet as they have had to provide the water that failed to fall from the sky. We’re also starting to see some evidence that soil nitrogen levels might also be low, presumably because N was taken up with water from deeper in the soil.

 


Thoughts at Harvest

Corn and soybean harvest in Illinois stood at 5 and 1 percent, respectively, on September 22, 2013. These are behind the 5-year averages, and far behind the 51 percent of the corn crop harvested by this date in 2012. Using 5-year averages may be reasonable, but corn harvest progress by October 1 ranged from 4 percent to 71 percent over the past five years, so “average” does not describe “typical” very well.

Harvest of both corn and soybean have ramped up in Illinois the last week of September, with corn leading the way but soybean harvest starting to pick up. Following are some observations as we start to get into fields to harvest both crops:

  • Corn yields: As anticipated, yields of early-harvested corn have ranged from good to excellent, with many reported to be 200 bushels per acre or higher. This is truly remarkable in areas (such as here in Champaign-Urbana) where total rainfall for August and September has still not reached 1 inch. It’s certainly possible that yields would be even higher if we had gotten normal rainfall in August, especially on lighter soils. It’s also possible that any effect of dry soils might be larger – that is, yields lower – in corn following corn compared to corn following soybean. As I’ve noted before, ability to pull water from the soil seems often to be a little less in corn that follows corn, for reasons we don’t very well understand. It’s also likely that in areas where some corn got planted in late May or into June, stress effects will be more severe. So there will be considerable yield variability among fields, and late-planted corn could have considerably lower yields, especially if it died early from stress.
  • Stalk quality: Stalk strength is good in most fields, and there have been few reports of stalk lodging. It’s possible, though, that some of the late-planted corn could have a problem in areas where stress started earlier in life cycle of the crop. What we have seen more commonly this year are “snapped” stalks, broken off at a node. Some of this was from wind events in the first half of the season; the typical greensnap period tapers off at about silking time as plant nodes strengthen, and involves breakage at nodes ranging from the ground level to about the ear node. But we are also seeing some fields where breakage occurred weeks after pollination, including some that appears to have happened past the middle of grainfill. This is an unusual phenomenon, and probably resulted from having strong internodes that don’t bend very much, so the stalks broke at the nodes instead. This might not decrease yield too much if it happened late in grainfill and if the stalk snapped above the ear. But in some fields stalks snapped below the ear, and the ears are on the ground attached to the upper stalk. These may be nearly impossible to pick up with the corn head. In some fields I’ve seen, this is a problem mostly on end rows that run north-south, but there are other fields where it’s more general.
  • Roots: Most fields are standing well with good roots anchoring the plant, and they’ve obviously been able to take up water from drying soils in order to get grain filled. One issue that Dr. Mike Gray has written about is that of rootworm damage resulting in considerable root lodging in some fields in some areas. Such damage has been found under heavy CRW pressure in hybrids with different CRW Bt events and sets of events, including in SmartStax fields. Insect resistance has been confirmed only to the protein Cry3Bb1, a protein produced by a CRW Bt event that is widely used, including in SmartStax hybrids. But SmartStax hybrids also include the event that produces the Cry34/35 Ab1 protein, and CRW resistance to this protein has not been found. As is the case with weeds, insects, or diseases, the presence of the pest or of damage is never proof that the pest has developed resistance to the control methods used; such proof requires careful lab tests that confirm a change in pest genetics resulting in resistance. There have also been some observations of roots that simply appear to be smaller than normal, in some cases in corn that follows corn. By maturity, root systems usually have deteriorated to a considerable extent, and it’s never safe to assume that we can know the cause this late in the season, especially when possible causes (such as insects or diseases) are not present.
  • Kernels: One indication that the crop retained enough green leaf area to fill kernels to close to their maximum size is having kernels that are deeper (longer from the point to the crown) than normal. Kernel depth is under genetic control to some extent, but high-yielding conditions often produce heavier kernels, and about the only way kernels on well-pollinated ears can increase in weight is by getting longer; their expansion is limited on all sides by other kernels. They can increase some in density of starch, but this is often high already, and there might be little ability to add weight without adding volume. In response to a question about how much an increase in kernel depth increases yield, I did the following calculation: Assuming that kernel density and how kernels “pack” on the ear stay the same, increasing kernel depth by 10 percent, from 0.4” to 0.44”, on a cob that’s 0.8” in diameter would mean a 21% increase in weight of kernels. Actual yield increases from deeper kernels could be less than this, but it illustrates the value of having a crop fill kernels right up to maturity. Another issue related to kernel development that has been reported this year is failure of the kernel to form a “black layer” even when grain moisture drops below 30%. This is related to the “extended” filling period, in that sugars may remain in the cob even after kernel fill would normally have stopped, so the layer of sugar-conducting cells at the tip of the kernel does not die and turn dark as it normally would. At the same time, starch in the endosperm is drying down, and such kernels have reached or are very close to reaching their maximum weight; even if some sugar is still present in the cob and tip of the kernel, there are no more cells with starch granules still forming, so the sugar has no place to go. Kernel tips with sugar like this will darken with artificial drying if not before, and I know of no negative consequence of failure to form a black layer at the exact time that kernel fill stops.
  • Test weight: Many people consider the main benefit of deeper, heavier kernels to be their contribution to higher test weight. While we have no reason to believe that test weights are not within normal ranges for corn that matured normally, it is not always the case that deeper, heavier kernels will mean higher test weight. In some cases, larger kernels may not fit together as well as smaller ones, and could produce lower test weights, even as they produce higher yields. High yields, though, almost always have test weights within the normal range, typically between 54 and 58 lb per bushel. Under such conditions, while test weight does determine how many bushels will fit into a bin, it is of little importance as long as the grain meets the minimum. The minimum is not fixed because elevators can blend corn to meet export requirements, but it’s rare for high-yielding corn to get docked in price due to low test weight. We did see some of this in 2009, but that was following a very late end to filling and low starch density.
  • Aflatoxin: We’ve continued to get questions about the potential for aflatoxin following the dry weather of the last two months. We have heard no reports of aflatoxin problems so far in 2013. As a reminder, the organism that infects kernels and produces the toxin infects in mid-season, and infection is much greater when the conditions are hot and dry at that time. It was hot and dry in July in 2012, but not in 2013. It is possible that the earlier-harvested corn from earlier-planted fields might have escaped infection a little better than late-planted corn. But it was never very hot through mid-August, and it wasn’t very dry until after that. So our hope is that, as is the case most years, aflatoxin will not be an issue this year. One hint: if elevators are not testing, it’s probably a non-issue.
  • Drydown: There have been many observations that even after this extended dry period and good drying weather, corn seems to be drying down slowly. This is connected to the fact that sugars have remained available to fully fill kernels out, and that yields are good. Under dry conditions, we often expect the crop to mature earlier than normal (and with lower yields) due to stress. That’s not happening. Instead, the crop is using its full complement of growing degree days. Corn planted on May 15 at Champaign reached 2,700 GDD (the amount needed to reach maturity for typical hybrids used here) only on September 17, and corn planted on May 30 will not reach that total until the first week of October. With deep kernels that might dry a little more slowly, and in some cases with sugar at the base of the kernel that tends to hold moisture, it is not unexpected that grain moisture is at current levels. While it can be worrisome to wait for grain to dry down and to have drying costs on top of lower prices this year, good yields and a crop that is standing well are pluses in many fields. It’s also a good time to remember that very dry grain in the field usually means much higher shelling loss, and corn harvested at 20% moisture without much shelling loss and dried may often net more than corn harvested at 15% moisture. As we move into October, we can expect drying rates to slow, so corn that is not yet below 30% moisture will probably not get to 20% very fast.
  • Soybeans: Early indications are that yields are fair to good, but there is some thought that the early-maturing soybeans (planted early or early maturity varieties) might have escaped some of the drought stress, and that later-maturing fields might not yield as well. A key indicator will be seed size; with pod numbers mostly better than last year and without the rain to prolong seedfilling this year, we expect to see smaller seeds in fields where stress ended the filling period. One benefit of having seedfilling end because of leaf loss is that greenstem, which is brought on when plants have more capacity to produce sugars than ability to move them into seeds, should not be much of an issue this year. As we expected might happen, many fields, once leaves drop, are not showing the number of pods that we would like to see. But seedfilling appears to have been at a good rate before leaf drop in areas that had a little more rainfall or where the crop was planted early, it’s possible that yields will be a little better than we might have expected a month ago.

Late-Season Dry Weather and Soybean Prospects

Soybean planting was very late in Illinois in 2013; it was early June before 50 percent of the crop was planted, and well into July by the time planting was completed. Even so, the crop condition ratings were good by mid-season, with some 70 percent of the crop rated good or excellent in mid-July.

With the late planting and some cool temperatures in July, soybean flowering and pod-setting started late, with half the crop flowering by July 21, and half the crop setting pods by the end of the first week of August. The 5-year averages against which these numbers are compared include 2008 and 2009, when crop development was very late, and 2012, when the crop developed very quickly. So while the 2013 crop was not that far behind the 5-year average, it was still well behind what we would consider to be normal.

The periods of low temperatures in late July and mid- August were not kind to the soybean crop. Night temperature fell to the upper 40s and low 50s during both of these periods, and as I mentioned in my August 8 article, such low night temperatures interfere with soybean plant physiology, and set the crop up for lower photosynthetic rates the following day. The effect of this over several days tends to be cumulative, and during the mid-August round of low temperatures, which coincided with the ongoing (and already-delayed) pod-setting process, low temperatures very likely lowered pod numbers.

Soybean plants form racemes – flower-bearing branches – on 15 to 20 or so stem nodes. On each raceme, many flowers form, and most initiate small pods. The number of pods that make it past the initial “tiny” stage to get to full-sized pods with beans depends on the supply of sugars from photosynthesis. Flowers appear on a raceme over several days, and it’s common for the first one, two, or three flowers to develop productive pods; subsequent flowers result in pods that often abort, depending on how much sugar is available at that raceme at the time they develop. Thus cool temperatures that reduce photosynthesis during the flowering process can reduce pod numbers.

Anything that reduces the amount of photosynthesis, including drought, leaf disease, or insect injury to leaves can lower pod numbers. While most pods abort when they’re very small, it is possible for even full-sized pods to abort, up to about the time that seeds start to grow in the pods. The most common reason for abortion of larger pods is shortage of water; without enough water available to keep stomata open, photosynthesis shuts down, cutting the sugar supply. Some have invoked things like ethylene production to explain such “stress effects.” While ethylene might be an intermediary of some sort, plants maintain pod and seed development only if they can maintain photosynthetic rates. We can sometimes help relieve the causes of low photosynthetic rates by irrigating or by helping keep leaves stay healthy and intact, but there’s often little we can do.

The soybean crop canopy generally looked good throughout most of August, and crop ratings were still at 65 percent good-excellent on August 18. To some extent the appearance of the crop disguised what might have been low numbers of pods developing, though the August 1 yield estimate for Illinois, which is based partly on pod counts, was 47 bushels per acre, which suggests that pods numbers were not too low. The return of warm weather (including warmer nights) the last week of August provided a boost to photosynthetic rates, and should have helped the crop to retain more pods. This effect was limited by dry soils in some areas.

As the crop headed into September still without rain in many areas, soil moisture became more of a concern. In most fields, leaves tended to stay reasonably well “hydrated” even in the afternoon heat. In dryer fields or parts of fields, though, leaves at some point during the day took on the grey-green appearance that signals inadequate water to maintain photosynthesis. If this condition holds for only an hour or two in a day, it may not have much effect. But if the leaves remain inactive for half of the day, that represents lost productivity that often cannot be recovered.

Why would a few hours of decreased photosynthetic activity in a day, which would usually lower corn yields, have less effect in soybean? Soybean plants tend to have a little more leaf area than they normally need for maximum photosynthesis, and because they can’t move sugars away from the leaves fast enough, they typically form starch during the day to tie up the extra sugar. While much of that starch is converted back to sugar and goes to good use, small reductions in photosynthesis over the course of a day may not decrease the available supply of sugars enough to hurt yields. This is especially true if pod numbers are less than normal, which means less demand for sugars.

The amount of seedfilling that the soybean crop is able to do when it’s dry during the second half of the season is directly related to two things: 1) how many seeds are present to fill; and 2) how long the crop is able to maintain enough active canopy to fill seeds. Most reports in late August showed a lot of concern about pod (seed) number under the stress conditions. That remained a concern into September, especially in the driest areas, and especially in the latest-planted fields, some of which were still setting (or trying to set) pods at the end of August.

Counting pods is a messy and tedious exercise, with large variations in pod counts between plants, and so the need to count a lot of pods to get a reasonable estimate. Pod counts are part of the USDA-NASS yield estimating procedure for soybeans. In the September report just released, the average pod count in Illinois was 93 pods per square foot. Assuming 2.5 seeds per pod, this would calculate to 10.13 million seeds per acre, and using the September 1 yield estimate of 46 bushels per acre, that would mean 220,000 seeds per bushel, or about 3,700 seeds per lb. That is a very small seed size, so reflects either an expectation that seeds will not fill very well or (more likely) an estimate of only 2 seeds per pod, which would mean about 2,900 seeds per lb.

Pod numbers reported for Illinois in September have ranged over the past 4 years from 81 per square foot in 2012, which was the lowest-yielding of these years, to 110 in 2011, which was the highest-yielding year. So while there is some correlation between pod counts and yields, the pod count number and the extent of fill are both “subject to change” after the September estimate, as are the estimates of seed number and seed weight. Pod counts tend to go up between the September and October estimates in good years, and down in poor years.

In our soybean planting date study at Urbana, the first date (in early May this year) lost its green color very quickly around September 5 or 6, and there was almost no green left on plants by September 9. We expect very little further seedfill once the green color is gone. On September 9, I took several pods from these plants, shelled out the seeds, and oven-dried them to see what weight they had reached. Most of these seeds were still green, and they might have been able to take in a little more sugar from the stems and petioles had they stayed in the field. After adjusting for moisture, they weighed in at about 2,900 seeds per lb. That’s smaller than average, but not tiny; “BB” seed can sometimes have 4,000 to 5,000 seeds per lb.

The second and third plantings in this trial had also lost most of their green leaf area by September 9, while the fourth (June)planting was still green. The fact that seeds had reached a reasonable weight by this date in September provides some indication that the plants might have done better at filling seeds than we had expected, at least in this field. But in fields that started to fill pods only in late August and are starting to lose leaf color in mid-September, there’s little reason to expect that we’ll see 30 or more pods per plants with well-filled seeds once the leaves drop. The normal time required for the seeds to fill in an individual pod is about 3 weeks, and for all the pods on a plant about 4 weeks. Plants that started filling seeds late and finished early simply can’t yield up to their potential.

The situation this year reminds us that we don’t really have a very good understanding of the physiology of soybeans, particularly with regard to when and how the plants stop the seedfilling process, drop their leaves, and mature. We know soybean behaves differently than corn, where maturity is typically reached at a (usually) predictable number of growing degree days from planting. In both crops, however, stress conditions at the end of the season can mean earlier than expected maturity, almost always accompanied by a yield penalty because seeds don’t grow as large as they might have.

As a comparison, the 2012 season was one of great stress through early August, with rescue in the form of rainfall and moderating temperatures thereafter, and a large rainfall event from the remnants of a hurricane in early September. The soybean crop went from what looked like a certain disaster to one that stayed green late and was harvested late, with relatively low pod and seed numbers (due to heavy loss of pods before the rain came) and large seeds. In some fields, soybean plants stayed green much longer than usual, illustrating the phenomenon that soybean plants with few or no pods tend not to receive the “senescence signal”, and so often stay green until frost or herbicide kills the plants.

The 2013 growing season has been a near-complete opposite, with late planting and good growing conditions followed by dry weather and periods of high temperatures the second half of the season, continuing (as it now looks) through to maturity for the crop in many areas. Probably as a result of water stress brought on more quickly by high temperatures, the crop is losing its color earlier than expected in dry areas, and it’s likely that the crop will come to an end before seeds are filled as completely as they might have been.

There have been a lot of efforts in recent years to try to keep the soybean crop green (and filling seeds) longer at the end of the season, thereby increasing seed size and yield. Fungicide, in-season nitrogen, or growth regulators have all been reported to do this, at least some of the time. The problem is that this generally requires a certain set of crop and weather conditions to work as advertised. Prolonging the seedfilling period would not have helped much in 2012 because there weren’t enough seeds to fill and maturity was already late. In 2013, dry soils are the primary reason for the crop’s coming to an end early, and anything that would keep the crop greener would have meant faster water use and probably even earlier onset of maturity.

I said several weeks ago that I expected most of the “surprises” in soybean – what we find when leaves fall and the combines run – to be on the negative side. I hope that’s wrong, and that we find that the crop has done a remarkable job of filling seeds in a shortened time under stress conditions. I’d be happy to get reports from either side of this coin.

 


Late-Season Dry Weather and Corn

The 2013 corn growing season has had its share of ups and downs, with late planting due to early rainfall, more rain in June, and temperatures that were at or below normal most of the season until recent weeks. Pollination conditions were good in most places, with adequate soil moisture and generally good temperatures. By late July most fields were in good shape, with good kernel counts and good canopy color and leaf health.

Much of the crop reached the middle part of August in good shape, helped along by continued cool temperatures. But rainfall became infrequent or stopped at some point in July or August, depending on location. At Champaign, July rainfall was slightly above average, with 5.03 inches at the airport, more than half of which fell on July 21. Many areas received much less than this.

Temperatures continued to be part of the weather story, with growing degree day accumulations well below normal during the last week of July (with barely more than 100 GDD at Champaign) and again the third week of August. That changed to above-normal temperatures the last week of August, and since then, weekly GDD accumulations have been above normal, with the total for the first 10 days of September 234 GDD, nearly half the normal total for the month.

Despite the hot, dry weather of recent weeks, corn in many fields has retained some green color, at least in the upper canopy, and it appears that kernels continue to fill. That may not be the case in some of the drier parts of the state, where the crop either ran out of water earlier in August or where canopy color wasn’t very good even before that.

Many have commented on the poor leaf color in some fields, even in areas where there has been some rainfall. Some have reported that adjoining fields with the same hybrid, planted and treated alike, show differences in canopy color and crop condition. Reasons for this are often not clear, but might be related to soil condition at planting, differences in how well nitrogen stayed in the soil and available to roots, or root damage due to flooding or insects. I think that root-related problems are more likely to be issues than loss of N. As is sometimes the case when roots have issues, corn following corn may not perform as well as corn following soybean again this year.

The high temperatures of recent weeks along with drying soils have acted to move the crop toward maturity faster than we had expected. In most areas this is the result of high temperatures rather than of lack of water. At Champaign, GDD accumulations from May 1, May 15, and May 30 through September 10 totaled 2,760, 2,590, and 2,310, respectively. Mid-season hybrids used in central Illinois typically need 2,700 to 2,750 GDD from planting to maturity, so early May plantings should be at or near maturity by now. Those planted in mid-May should mature by September 20 or so, while corn planted in late May or early June is still be several weeks from maturity, unless dry soils bring on early maturity.

As we have mentioned before and as the information above indicates, late planting this year has not decreased the number of GDD required to reach maturity, probably because of the periods of low temperatures during the season. One of the main reasons that late-planted corn often uses fewer GDD than early-planted corn is that late-season stress causes loss of canopy photosynthesis and brings an early end to grainfilling. In such cases the crop almost always loses yield.

The good news this year is that in fields where the crop is taking its normal number of GDDs to reach maturity, yields are likely not to be diminished much by the recent heat and dryness. While there has been a considerable amount of “tip-back” (tip kernel abortion) in fields in drier areas, most reports in areas with some soil moisture have indicated that kernel numbers are good. Of course, it’s never sure until maturity that kernels will end up heavy enough to produce the yields that their numbers would suggest.

To see how kernel weight was progressing, I took some kernel samples early on August 26, at the early dent stage (beginning R5), and oven-dried them. This crop was planted at Champaign on May 15. Adjusted to 15% moisture, these kernels weighed 241 milligrams, which translates to about 105,000 kernels per bushel. By early dent, kernels are expected to have 50 to 60% of their final dry weight. Since these kernels were already more than three-fourths the weight (316 mg) of kernels at 80,000 per bushel, it appeared that the actual filling progress was running ahead of the visual indicators.

I sampled the same plots again late in the day on September 4, or about 10 filling days (about 250 GDD) after the first samples were taken. The milkline – the separation between hard and soft starch – was about halfway down the kernel. According to the Iowa State University publication “Corn Growth and Development” (PMR 1009), kernels at half milkline have accumulated about 90% of their maximum dry weight, are at about 40% moisture, and have about 200 GDD left to go before maturity. These kernels weighed 306 mg, or about 83,000 kernels per bushel, and they were at 31% moisture, which is considered typical for grain at physiological maturity. Thus it’s likely that these kernels were at or very close to their final weight.

Kernels at 83,000 per bushel are considered to be normal sized, so the yield potential for this plot was probably realized, even though there has been no appreciable rainfall here in Champaign-Urbana for more than 6 weeks. These plots had an average population of about 40,000 plants, and at about 500 kernels per ear (20 million kernels per acre), the yield estimate on September 4 (20,000,000÷83,000) came to about 240 bushels per acre. This is a little dangerous due to the small sample size, but if it’s accurate it means that in 10 days of fill, the crop added about 91 bushels per acre, or about 9 bushels per acre per day. This is in line with previous rates we’ve measured, and it shows a well-functioning crop.

I’ve heard a number of comments coming from drier areas that “test weight” was going to be hurt by dry conditions, with yield lowered as a result. When kernel fill stops before kernels have as much starch as they can hold, kernels tend to be “shrunken” on the end that attaches to the cob. When canopy photosynthesis decreases and the supply of sugars starts to run out by late dough, starch tends not to pack normally in the kernel, and this can lower the density of the starchy part of the kernel – the endosperm. Misshapen kernels that don’t fit together well and kernels that are less dense than normal both contribute to lowering of test weight.

Of course, an early end to starch accumulation means lowered kernel weight. And lowered kernel weight, not lower test weight, directly translates to lower yield. It is certainly the case that test weight and kernel weight are often related, as explained above. But yield is the product of kernel number per acre and weight per kernel, while test weights are often not well-correlated with yield level, unless of course stress lowers both at the same time.

It’s time to remind ourselves that “black layer” – the darkened layer of cells at the tip of the kernel that indicates that the tissue that transfers sugars into the kernel is no longer active – always forms in corn, whether or not the grainfilling process come to its natural end or ends early due to canopy loss. For practical purposes, the disappearance of the milkline at the base of the kernel means that little or no additional weight will be added to the kernel.

While we would have preferred normal rainfall and normal temperatures during August, the periods of cool weather did help stretch the supply of soil water, which helped plants fill grain even under the recent spell of high temperatures. Any green leaf area on plants means that they are capable of producing sugars, and green leaf area that persists even after some weeks of stress conditions indicates that the crop has had access to at least some water. I also think that we are seeing the advantage of cool night temperatures for some periods during grainfill, and that this has translated into slightly better than normal photosynthetic efficiency.

I’ve also heard comments to the effect that low solar radiation might have decreased photosynthesis and yield potential in 2013. It’s certainly true that sunlight amounts have been low in parts of the 2013 season: according to the Illinois State Water Survey (ICN data), solar radiation in July 2013 was 20% less than the average of the previous three years, and in August was 13% below the 2010-2012 average. These two months were warm and dry in all three previous years, with the exception of August 2012, but sunlight amounts in 2013 were considerably less than normal.

Any effect of the lack of sunlight might have had on photosynthesis was moderated by the fact that the 2013 crop was behind in its development and the weather was cool in parts of July and early August. Conditions during and after pollination generally resulted in good kernel numbers, and by the start of rapid grainfilling, when the amount of daily photosynthesis becomes the critical factor for yield, sunlight had returned to more normal levels. This, along with higher temperatures, meant high rates of photosynthesis in fields with enough soil water. As is always the case, more sunlight usually means less rain, and on balance water tends to be more limiting than sunlight, so depending on when it happens and whether it means more rain, low sunlight is not always a negative factor.

Another concern as we head towards harvest is how well stalks will stand in fields where stress at the end of the season means an early end to grainfilling. Keeping stalks healthy depends on having enough sugars in the stalks up to the end of the grainfilling period. In years like this, when stress lowers the production of sugars, stalks can lose sugars to the ear, and stalk health can suffer. One factor that may counter this to some extent in 2013 is that the favorable conditions in July seem to have allowed the plant to produce more than normal amounts of lignin, which is the woody material that strengthens stalks. If there is enough lignin in stalks, they will often stand well even there is not enough sugars to keep their cells alive. One piece of supporting evidence for this is the fact that wind storms have tended to push stalks over (root lodging) this year, but not broken the stalks. Still, it pays to check stalk strength at or before maturity to identify fields for early harvest.

Corn drydown rates should benefit considerably from earlier maturity brought on by high temperatures. It helps that husks seem to be drying well as maturity comes on; this usually means that they will loosen so air can reach the grain to help drying. We can expect drying rates of a half point or more per (warm, sunny) day in September, but this will slow as the weather cools and if rain returns. As we often see when it’s dry in September, grain moistures often drop more quickly than expected.

Finally, I’ve had a few questions about the potential for aflatoxin problems given the recent dry weather and high temperatures. While there are no guarantees, the Aspergillus fungus that produces aflatoxin infects kernels in mid-season, and tends to be favored by hot, dry conditions and plant stress. Conditions in July 2013 were not hot and dry like those in 2012. We hope this means that we won’t see the problem this year.


Soybean rust: status and risk

Observations of soybean rust in southern states indicate that the pathogen (Phakopsora pachyrhizi) is beginning to move northward towards Illinois. Based on current movement, soybean rust likely will arrive in Illinois again this year, but it may not be in the state early enough to cause any yield losses. Late-planted fields would be the most at risk to losses caused by soybean rust. In general, once soybean plants reach the R6 stage (full seed stage), yield loss is unlikely to occur with any infections by the soybean rust pathogen. Many soybean fields in the state are now at the R5 stage (beginning seed stage). If spores of the soybean rust pathogen arrive in Illinois, favorable conditions must occur for infection and disease development to occur. Conditions that are favorable for soybean rust include moderate temperatures, frequent rainfall, and cloudy skies.

Soybean rust observations in North America can be viewed at the IPM PIPE website (http://sbr.ipmpipe.org). Although soybean rust sentinel plots are no longer maintained in Illinois (due to lack of funding), monitoring for soybean rust in Illinois is still occurring in targeted risk areas in the state. Any soybean leaves that are suspicious for soybean rust should be sent to the University of Illinois Plant Clinic (http://web.extension.illinois.edu/plantclinic/).

Other diseases that can be confused with soybean rust, such as bacterial pustule and Septoria brown spot, have been observed in Illinois this year. Of these two diseases, bacterial pustule may be the most easily confused with rust. One major difference between symptoms caused by bacterial pustule and soybean rust is that the pustules on the leaves will occur on both the upper- and underside of the leaf with bacterial pustule, but only on the underside of the leaf with soybean rust. Continue to monitor the IPM PIPE website and the Bulletin for additional updates on soybean rust.

Symptoms of soybean rust on soybean leaflets (photo courtesy D. Mueller, Iowa State University).

 

Soybean rust pustules filled with spores on the underside of a soybean leaflet (magnified) (photo courtesy D. Pedersen, Univ. IL).

 

 

Symptoms of bacterial pustule on a soybean leaflet (photo courtesy D. Pedersen, Univ. IL).

 

 

Bacterial pustules that can be found on either the upper- or undersides of soybean leaflets (magnified) (photo courtesy D. Pedersen, Univ. IL).

 

Symptoms of Septoria brown spot on a soybean leaflet (photo by C. Bradley).


Southern rust of corn observed in Illinois

Southern rust of corn has now been observed in different areas of Illinois.  Southern rust is one of two different rust diseases of corn that can be observed in the state (the other is known as common rust).  Because nearly every corn hybrid grown is susceptible to southern rust, yield reductions can occur if infection takes place early enough in the season.  Late-planted corn fields are at the highest risk for yield losses associated with southern rust and should be scouted for the presence of this disease.  Warm and humid conditions are most favorable for the southern rust pathogen infection and disease spread.

It is important to be able to differentiate southern rust from common rust, since the latter generally is not considered a threat to yellow dent corn hybrids because most are fairly resistant to common rust.  Southern rust pustules generally are smaller than common rust pustules and are orange in color compared to cinnamon-brown in color for common rust.  Southern rust pustules tend to be more densely scattered than common rust pustules and more chlorosis (yellowing) around the pustules generally will be observed with southern rust.

Because nearly every corn hybrid is susceptible to southern rust, foliar fungicides are the only management tool available.  If corn plants are at the R3 development stage (milk stage) or beyond, then it is less likely that southern rust will cause yield loss; however, on late-planted fields that are not yet at that stage, it is important to continue to scout those fields for southern rust.  A southern corn rust observation map for the United States is available through the the IPM PIPE system (http://www.ipmpipe.org/); however, southern rust may be present in counties that are not highlighted on the observation map.

 

Pustules of common rust (left) and southern rust (right) on corn leaves. Note that southern rust pustules are lighter in color.

Southern rust pustules beginning to form. Note the chlorotic (yellow) tissue around the pustules.


Will The Corn and Soybean Crops Get Finished?

Late planting and weather that continues to be cooler than normal into August has many wondering if the corn and soybean crops will reach maturity and harvest moisture within a reasonable time this fall. Crop conditions remain good for both crops, but crop development, including pod formation and filling in soybean and grain fill in corn, remains well behind normal. Corn is 10 days to 2 weeks behind normal, and soybeans are 2 to 3 weeks behind normal. The number of days behind will “stretch” as the weather cools, so late crops get even later. Ten days behind in mid-August will be become 15 or 20 days behind in mid-September, even if temperatures are normal.

We have often pointed out that late-planted corn tends to require fewer growing degree days to reach maturity than does early-planted corn. That’s often reflected in lower yields, as the crop experiences stress during the high temperatures and dry weather we often experience in mid-summer. This year, however, with temperatures generally below normal in recent weeks, we do not expect to see this accelerated development. Instead, we are seeing that corn development is following closely the normal number of GDDs required to reach each stage. This means less chance of premature death and so a better chance to fill grain completely. But for late-planted corn it also means late maturity.

How late can we expect the corn crop to mature? If we assume normal GDD accumulations in August and September, corn planted in the Champaign area on May 1, May 15, and May 31 will accumulate about 3,020, 2,850, and 2,530 GDD, respectively, by the end of September. If we further assume that a mid-season (111-day RM) hybrid needs 2,700 GDD from planting to maturity regardless of planting date, we can expect corn planted on these dates to reach maturity (black layer, about 32% grain moisture) about September 5-10, September 15-20, and mid-October, respectively. The crop seems to be on course to do this: corn planted in early May is at stage R3 (milk stage) now while corn planted in late May or early June is just finishing pollination. This reflects ongoing cool temperatures so far in August, and if these continue, maturity dates will be even later.

With a 50% frost date of about October 20 here, we would expect a mid-season corn hybrid planted in early June to mature before frost, if frost does not occur before its normal (50%) date. But drydown slows quickly as we move into October, and even early-planted corn will dry slowly after maturity unless September is unusually warm. Corn planted in mid-June (some fields in central Illinois are still not pollinated) is unlikely to mature before frost if temperatures are normal and frost comes at its normal time.

While the soybean crop has a dark green, healthy appearance in most fields now, podsetting is later than normal, and the crop planted the second half of May has not yet reached, or moved past, beginning podfill (stage R5). If temperatures continue to be cooler than normal, we can expect the crop to reach maturity only by late September or early October. We hope not to see a repeat of 2009, when more than half the Illinois soybean crop was harvested after November 1. That followed a very wet October.

Beyond their effect on maturity, cool temperatures are continuing to have a somewhat negative effect on soybean crop. The crop has good photosynthetic capacity due to its healthy, complete canopy. And soil water use rates are less than normal, extending the water supply. But below-normal daytime temperatures (and clouds) mean less photosynthesis, and cool nights can physiologically limit growth rates and the photosynthetic rates the next day. The first effect we might see is below-normal pod numbers, or pods initiated in late August that might not fill.

Good soybean yields are still possible if the weather remains good into September, but seed filling rates will remain slow as long as temperatures remain low. We have sometimes seen cool temperatures trigger maturity before seeds are fully filled. The best scenario for soybeans would be a return to temperatures – both day and night – return to normal or a little above normal, with enough rainfall to enable the crop to photosynthesize fully as seeds fill. Even with that, we’re in for a wait to see how the crop finishes this fall.


Check Corn Pollination

We are starting to get reports that corn pollination might not have been as successful as expected in parts of Illinois. While the weather was generally favorable during the peak period of pollination, it was warm during the third week of July, and soil water may have been limiting during this period in some fields. Thus we would expect to see this in the areas with low rainfall in July.

This is not the lack of silking that we saw in many areas under the drought of 2012. Silks generally emerged well in most fields this year, but tassel emergence was slowed by dry soil conditions in some cases. The warm third week of July was followed by unusually cool weather, with some lows in the upper 40s and lower 50s the last weekend in July.

It’s possible that these unusual conditions limited pollen production or that silks became non-receptive (unable to accept and germinate pollen) before the end of pollen-shed. Indications are that scattered kernels from poor pollination are being found at the base of the ear more than at the tip, which might point toward lack of early pollen production and possible loss of silk receptivity.

Regardless of the cause, it’s time to get into fields to see how successful pollination has been. Once silks start to dry, remove husks and shake or pull on silks. Those that detach easily are from fertilized kernels, while those that stay attached are on kernels that have not been fertilized. Silks that emerge more than a week after silks first appeared and seem to be fresh probably emerged after rainfall, and there will typically be little or no pollen available to pollinate these. The target is to have around 500 kernels per ear at populations in the lower to mid-30,000 plant population.

There is little to be done about lack of kernel set at this point, but it will help us know what’s coming, and it may help us identify causes. When different hybrids show different degrees of this type of problem, it is often more a matter of timing than genetics; hybrids that silked on a certain date are often affected more than those that silked a day or two later. But if the same hybrid planted on different dates shows the problem, it may be genetic in nature, or possibly related to soils or management. For example, a better root system that can pull water a little more effectively can make a large difference in pollination success.

I’d be interested to receive reports and observations on this so we can try to put the puzzle together.


Assessing the risk of white mold (Sclerotinia stem rot) of soybean in 2013

White mold of soybean (a.k.a. Sclerotinia stem rot), caused by the fungus Sclerotinia sclerotiorum, is a disease that can occur in the northern half of the state in cool, wet years.  The most recent white mold epidemic in Illinois occurred during the 2009 season, where several fields in the northern half of the state were affected.

The white mold fungus overwinters in the soil as, small, black, and dense structures known as sclerotia.  These sclerotia germinate and form mushroom-like structures known as apothecia when soil remains moist for several consecutive days and soil temperatures are at 60 degrees F or below.  These apothecia generally will not form until the soil is shaded from sunlight due to soybean canopy closure.  Spores of the white mold fungus are shot out of the apothecia and land on senescing flower petals, where infections first occur on the soybean plants.  The white mold fungus becomes inactive when temperatures within the soybean canopy are above approximately 82 degrees, so infection and disease development may cease or slow down during periods of hot and dry weather.  The 2013 season started out similar to the 2009 season, with frequent rainfall and cool temperatures, but warmer and drier conditions have been observed more recently.

 

Fig. 1. Apothecia of the white mold fungus germinating from a sclerotium. Image courtesy J. Venette, North Dakota State University.

 

Fig. 2. Soybean plant with symptoms and signs of white mold (a.k.a. Sclerotinia stem rot). Image by C. Bradley.

 

So, what does the risk of white mold look like for 2013?  This is not an easy question to answer since we have had fairly favorable weather for white mold initially with some stretches of non-favorable weather in-between.  In fields that were not planted late and have been in full-canopy for the last couple of weeks, it is likely that apothecia have developed and that some infections may have occurred already.  However, the progression of infection and disease development likely came to a stop with the hot and dry conditions observed last week.  For fields planted later that just recently closed their canopy (or have not yet closed their canopy), apothecia likely have not emerged because of the hot dry weather experienced last week.  The weather from this point forward will likely dictate how bad white mold will be in 2013, although, I believe that it is safe to say that it will be less severe and less widespread than it was in 2009.

In University of Illinois research trials, some fungicide products have shown efficacy against white mold.  Foliar fungicides will not provide complete control of the disease, but may reduce disease.  The results of University of Illinois trials conducted in 2009 and 2010 are shown in Tables 1 and 2.  Note that some of the more popular, frequently marketed fungicides are not listed in the tables since many do not have white mold on their label because of no to poor efficacy.  In these trials, the primary targeted growth stage to apply foliar fungicides was at R1 (beginning flower).  Because of the late-planted soybean fields this year, R1 may occur before canopy closure.  If this is the case, then an application at canopy closure (rather than R1) might be more effective in protecting against white mold.  Also note that some treatments in these research trials were applied twice during the season (R1 and again 7-9 days later).

 

Table 1. Results of soybean foliar fungicide research trials focused on white mold conducted in 2009 at the University of Illinois Northern Agronomy Research Center (DeKalb County).

Treatment Rate/A Incidence (%) 

8-11-09

Incidence (%) 

9-14-09

Yield (bu/A)
Untreated check 75 95 24
Topsin 4.5 L 20 fl oz 43 96 24
Proline 3 fl oz 38 95 24
Domark 5 fl oz 68 98 23
Cobra herbicide 12.5 fl oz 15 51 42
Endura (2x)* 8 oz 38 86 39
Aproach (2x)* 8 fl oz 35 80 40
LSD 0.05** 33 15 8

*All treatments were applied at the R1 growth stage (July 20, 2009).  Treatments followed by “(2x)” were applied again 9 days later.

**Least significant difference (alpha level = 0.05).  Treatment values that differ by this number can be considered significantly different from one another.

 

Table 2. Results of soybean foliar fungicide research trials focused on white mold conducted in 2010 at the University of Illinois Northern Agronomy Research Center (DeKalb County).  Funded in part by the Illinois Soybean Association.

Treatment Rate/A Incidence (%) 

8-11-09

Incidence (%) 

9-14-09

Yield (bu/A)
Untreated check 18 95 62
Topsin 4.5 L 20 fl oz 9 83 61
Proline 3 fl oz 11 93 64
Domark 5 fl oz 7 76 63
Cobra herbicide 6 fl oz 6 86 56
Endura 8 oz 4 79 69
Aproach (2x)* 8 fl oz 11 79 66
LSD 0.05** 11 NS 8

*All treatments were applied at the R1 growth stage (July 10, 2010).  Treatments followed by “(2x)” were applied again 7 days later.

**Least significant difference (alpha level = 0.05).  Treatment values that differ by this number can be considered significantly different from one another.  “NS” indicates that no treatments were significantly different from each other.

 

Overall, the highest level of white mold control will be achieved when several management practices are integrated (i.e. choosing the most-resistant varieties, utilizing recommended seeding rates, applying a foliar fungicide, and applying a biocontrol product).  For more information about white mold and management of this disease, go to http://www.planthealth.info/whitemold_basics.htm, where a recently-developed 7-page publication on white mold can be downloaded and several podcasts on white mold can be accessed (all funded by the North Central Soybean Research Program).


Corn and Soybean Crops at Mid-Season

The third week of July brought above-normal temperatures to much of Illinois. This is expected to be temporary, with temperatures the fourth week expected to return to normal. Because soils in most areas had enough water to carry the crop through the week, we don’t think the high daytime temperatures were much cause for concern.

By July 14, 21 percent of the Illinois corn crop was pollinating. This included most fields planted before May 10 to 15. Because planting was so concentrated in the third week of May, we expect as much as half the crop to pollinate by July 21. That’s a week or so later than normal. Higher than normal night temperatures this past week might have hurt pollination success some in areas where soils are starting to dry out. By the heavy silking that preceded the full emergence of tassels that we have noted in recent years is very much in evidence again this year. This indicates that silks should be present when pollen is being shed.

In some fields that were planted in mid-May or later, especially those planted at high populations, stalk diameter is noticeably smaller than we often see in earlier-planted corn. Later-planted corn that has plenty of soil moisture often grows taller than early-planted corn because of higher temperatures during internode elongation. But photosynthetic rates (dry weight increases) are not higher in late-planted corn, and so taller plants usually mean less weight per foot of stalk length. Plants with smaller stalks often have less leaf area, and so less ability to set and fill a large ear. It’s too early to know if this will decrease yield potential, but it is one of the ways in which late planting leads to lower yields.

If we have a return to better soil moisture conditions along with lower night temperatures over the next two weeks, we can expect kernel set to be good. But after a wet June, soil moisture is becoming a concern in some places. July rainfall has been less than normal over much of Illinois, with less than an inch so far in parts of western and northern Illinois.

The soil water supply was good coming into July, but how well the crop is tapping into this supply will make a difference as we move into the second half of the season. Fields that were planted by mid-May have had a chance to produce a good crop canopy, which in turn has enabled deeper root growth. It’s doubtful that roots are as extensive as they were in July 2012, but as we saw last year, good root systems don’t help when there’s no available water in the soil.

The major issue with water supply now is in the crop that was planted in late May or early June, and that won’t pollinate until late July or early August. These plants are in mid-vegetative stages, and are often showing mid-afternoon drought stress symptoms (leaf curling) where recent rainfall has been limited. This crop has not grown enough to have used up the available soil water to 3 or 4 feet deep. Rather, the root system is simply not as deep or as well-connected as it would be had planting been earlier or if soils had not been so wet earlier.

Another concern with the wet weather earlier in the spring is nitrogen supply for the crop. Leaf color remains good in areas with adequate soil water where the crop has continued to grow well. But in areas where soils have dried and some drought effects are starting to appear, leaf color had lightened some. In most cases this is because nitrogen reaches the roots with water, and when water uptake slows, so does N uptake. Rainfall in such places should return leaf color to normal green. If leaves have been pale during the pollination period, however, success of kernel set can be lowered.

It is possible that enough nitrogen has been lost in some lower-lying parts of fields to mean shortages for the crop, even after root systems recover. Supplemental applications of N have been made in some such areas, usually as urea, sometimes with urease inhibitor. Such applications might produce enough added yield to provide a positive return if they are made before or even during pollination, but are likely to be effective only when rain falls to move them into the soil.

As we move through pollination and into the grainfilling stages, the major concern will be how well the leaf canopy holds up to provide maximum amounts of photosynthate (sugars) to fill kernels. Soil water supply will be the major factor, followed by leaf health and nutrient supply. We expect the crop to take about 8 weeks to reach maturity after pollination is complete, and maintaining green, healthy leaves through this period is the only way to maximize kernel fill and yield. If the crop takes less time from pollination to maturity, it will have stopped filling prematurely and will produce lower yields.

The soybean crop has had a slow start, but warmer temperatures have finally produced a surge of growth in fields planted by early June. According to NASS, 32 percent of the soybean crop was blooming by July 14. That’s a little behind average, and less than half of what it was in 2012, but for a crop planted as late as this year’s, it’s reasonable. As is typical with late-planted soybeans, plant height at the beginning of flowering is fairly short this year.

The soybean crop canopy is starting to develop and close, but some of the very late-planted crop will flower for some weeks before the canopy is effectively complete. As we saw last year and in some previous work, as long as flowering and early podsetting take place under good conditions, having the canopy close early might not be that critical. The long-held idea that failure to close the canopy by first flower means lowered yield potential does not seem to hold true, at least with indeterminate soybeans that continue vegetative growth for weeks after first flower.

While we want to have good canopy color and cover in mid-July, yield potential for the 2013 soybean crop is far from being set at this point. We might consider it as we would the corn crop a month before pollination. There’s a long ways to go, and we won’t have a good idea about yield potential for at least a month.