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Issue No. 13, Article 4/June 29, 2012

Short Corn, Short Yields?

The Illinois corn crop condition continues to deteriorate, with less than 40% now rated as good to excellent on June 24. This is almost entirely due to low rainfall--84% of the state’s topsoil is rated as having low or very low soil moisture. As the crop enters the critical yield-producing stage, many are wondering what effect the lack of soil water has had until now, and what effect it will have over the next weeks.

Rainfall amounts since May 1 have ranged from some 6 inches in parts of north-central Illinois to less than an inch in parts of southern and southeastern Illinois (Figure 1). Deficits from normal for this period range from an inch or two to more than 6 inches; they are in the range of 4 inches, or about half of normal, in much of the central part of the state (Figure 2). As of June 27, the U.S. Drought Monitor showed most of Illinois in "moderate" or "severe" drought, with the southernmost counties in "extreme" drought (Figure 3).


Figure 1. Accumulated precipitation (in.), May 1 to June 27, 2012. (Source: Midwestern Regional Climate Center.)


Figure 2. Departure from the mean for accumulated precipitation (in.), May 1 to June 27, 2012. (Source: Midwestern Regional Climate Center.)


Figure 3. Illinois drought conditions as of June 19. (Source: droughtmonitor.unl.edu)

As of June 24, 17% of the state’s corn crop was pollinating--the highest percentage for this date on record. This week we would expect most of the crop that was planted by mid-April in central and southern Illinois to begin to pollinate, bringing the number by July 1 to perhaps 40% or so.

While the pollination period is considered the most critical in terms of yield potential, breeding for aggressive emergence of ear shoots and silks has considerably lessened the likelihood of complete failure of pollination. Still, the number of kernels set may be lowered on plants that have been undergoing stress from dry soils, and the number of fertilized kernels that survive the weeks after pollination may continue to decline as the weather stays dry.

A common observation is that corn is entering or approaching pollination while plants are shorter than normal. This raises questions about the connection between plant height and yield. As I’ve observed before, plant height is the best visible indicator of how well the plant has been able to take up the water it needs to expand cells up to now. Cell expansion is sensitive to water supply, and so shortened internodes are one of the first things we notice on plants that have struggled to take up enough water to keep growing.

In some of the driest areas this year, plants have remained extremely short, and there are reports that some of these fields have tried to pollinate at heights of only a few feet. Most such fields will produce low yields, and some may produce none at all. More commonly, plants are 5 to 6 feet tall at the time tassels appear. Plants typically add some height after tasseling and reach full height by the end of pollination.

Having plants end up shorter than normal after pollination is not by itself a guarantee of lowered yields. But it is difficult for short plants to form the complete canopies that plants need for maximum yield. In part this is because plants that have had trouble getting enough water to elongate stalks may also have leaves that are smaller than normal. Even if leaf area is normal, short plants with leaves stacked closer together on the stalk means less interaction among neighboring plants and less flexibility of leaf movement, and so reduced ability of plants to form the complete canopy needed to intercept nearly all of the sunlight. Coupled with ongoing water stress that limits photosynthetic rates, such fields are likely to yield less than normal.

Positives with the current corn crop continue to be the good color, lack of diseases, and uniform stands, with few or no drowned-out areas. These factors will help increase kernel set in fields pollinating now, at least where there is enough soil water at present. Cooler weather this past week has helped to prolong the period of adequate water, and cooler nights reduce respiration some, thus helping the sugar supply. The only negative, which is potentially large enough to cancel out all of these positives, is the lack of rainfall--and the forecast that this may continue for more days and weeks to come. The 2012 corn crop is well rooted, healthy, and tough, but it’s unrealistic to expect that it keep thriving as the soil water supply continues to decline in dry areas.

Is there anything we can do to help the crop get through this dry period? Not much. Some have suggested that applying fungicides can help reduce respiration and increase the plant’s sugar supply, thus providing a return even if there are no diseases to control. Strobiluron fungicides do act by reducing respiration, a small percentage of which is considered "wasteful." But plants that are not photosynthesizing very well don’t have much sugar to respire away, so a reduction in respiration probably won’t do much good. By the same token, applying products said to reduce the "ethylene effects" in stressed plants is unlikely to have a positive effect when there’s not enough water to keep open the stomata, the holes in the leaves that open to admit the carbon dioxide converted to sugars during photosynthesis.

Protecting the crop from anything that reduces effective leaf area can help retain the potential to fill grain should we get rainfall later. This might include insecticides if enough insects are present to do damage. Fungal diseases that would respond to fungicides are not much of a threat in most fields today, and even if rainfall returns to normal, the rapidly developing crop may not develop fungal diseases fast enough to pose limitations late in the season. Foliar nutrients are unlikely to be of much benefit under dry conditions, especially when the good canopy color in most fields indicates adequate nutrient levels.

While the focus has been on corn, soybean plants are also showing stress effects in many areas. As with corn, the best indicators of how much stress soybeans have been under are plant height and leaf size. Our earliest-planted soybeans here were planted around April 20 and are about 24 inches tall and at stage R2, or full flower. With fair to good growth and warm temperatures, soybeans are moving quickly into flowering, with 11% blooming by June 24, only four days after the longest day of the year. Under conditions like those we have this year, day length is less important than growth stage and night temperatures in determining when soybean plants flower.

An early start to soybean flowering is generally positive, but concern remains about how water shortages might affect pod formation. The period over which new flowers appear will last for up to a month as the soybean plants continue to increase node numbers and stem height, and flowering can even recur if stress is relieved after that. This longer flowering period makes the soybean crop better able to set pods and start filling seeds even if there is some stress during July, but if conditions continue with little or no rainfall, abortion of flowers or of pods will likely continue. As is the case with corn, applying various materials promoted to "reduce stress" in soybean is not likely to do much good as long as water supply remains the critical limitation to continued growth and yield.--Emerson Nafziger

Author:
Emerson Nafziger

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