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Late Planting and Crop Watching 2003

June 19, 2003

The latest report has Illinois corn planting almost complete but with some continuing delays in getting the soybean crop completely planted in southern Illinois. Now that the stress of getting the seed planted and fertilizer and herbicide applied is a memory for many, most people realize that whatever happens with the crop through the rest of the season is beyond human control; except for an occasional fungicide or herbicide application, most producers will visit fields only to see how things are going. For some, this will mean relaxing as the weather unfolds. Others will look for new things to worry about. They probably won't be disappointed.

The first concern is with those who still don't have all of their crops planted. The "prevented planting" date is upon us, so those with crop insurance might elect to take that option. Those without insurance who need to plant corn because of herbicides applied are in the most precarious position; we expect corn planted past the middle of June to yield about half what a timely planted corn crop would yield, though extra-good weather (read "wet almost all of the time") from planting through the season would reduce such loss, and stretches of dry weather lasting weeks would likely increase the loss. Grain sorghum might be a slightly better option for late planting in southern Illinois, but it is not a "miracle crop"it too will suffer from lack of water, and even early hybrids will be seriously limited by cool weather in September.

Late-planted soybean, including double-cropped soybean after wheat harvest, is one crop that still provides some hope in areas where it has rained 12 to 15 inches since May 1. In many areas, soil moisture is still excessive, with standing water in many fields. This will dry up slowly, especially where wheat is standing; as the wheat crop matures, it no longer takes up much water, and it reduces air movement over the soil, keeping soil wetter. This will bring a dilemma: either harvest when wet and deal with tracks, or wait until it's dry and face possible water shortage for emergence. We think the latter choice makes sense, especially if full-crop soybeans can be planted as we wait for wheat fields to dry. If it stops raining and dries out after planting, all of the late-planted crop will suffer, but a "mudded-in" crop will suffer more.

The conventional advice for planting soybean late is to use narrow rows and to increase seeding rates. Wide soybean rows are rare in many places, and no one should use wide rows for planting after early June. In light of lowered yield expectations, though, raising seeding rates should be done cautiously, especially if all of the seed needs to be purchased. We think that soybean planted after mid-June in rows less than 20 inches apart can usually maximize yield at populations of about four or five plants per square foot, or five to six plants per foot of row in 15-inch rows and about three plants per foot of row in drilled rows. This will take seeding rates of about 200,000 to 250,000 per acre if emergence is good. It might pay to consider seed treatment. And depending on the seed source, when it was tested, cold or accelerated aging germination when it was tested (low "stress" tests often mean declining vigor), and how the seed has been stored, it might also pay to run a warm germination test on the seed to make sure it still has germination as high as the tag indicates. Last year was a relatively good year for soybean seed production, but we only get one chance to establish late-planted soybean, and we should take all of the precautions that make sense.

For those ready to watch the crop grow, one concern has been the lower growing degree-day (GDD) accumulations that we have experienced this year. From May 1 to mid-June, accumulations were below average everywhere in the state, by an average of about 150 GDDs. Even our temperatures in the 80s this week are only average, so we are not catching up "lost" GDDs. To put this in perspective, it takes the first 2 weeks of May to accumulate 150 GDDs on average in central Illinois, but only about 1 week to accumulate that number in late June. For those able to plant early, April provided slightly more than average GDDs this year, so early-planted crops are right on schedule, if not a bit ahead.

It will be better for crops, in fact, if we did not get the high temperatures that it would take to gain back the GDD shortfall we have accumulated so far. Average GDD accumulation in July is about 25 per day, and because daytime temperatures in July average close to the 86° cutoff for GDDs, above-average accumulation comes mostly from high night temperatures. To gain back 5 GDDs per day in July (to total the 150 we're behind), minimum (night) temperatures would have to be almost 10° above average. Corn responds negatively to increases in night temperature, so catching up on GDDs in midsummer would cause much more harm than good. Of course, as the crop moves toward maturity in September, 150 GDDs again becomes 2 weeks' worth, so we can expect somewhat later maturity. If the crop is able to fill grain under those cooler conditions, though, later maturity will be more positive than negative. In fact, above-average July rainfall and below-average August temperatures are a formula for success for corn yields.

Soybean's response to summer weather is somewhat different from corn's, though on average good corn yields and good soybean yields tend to go together, with both tied closely to rainfall. Soybean plants are still small in most fields, reflecting the delay in planting and the cool weather after planting. The crop is finally starting to grow this week, but most fields are only at stage V1 or V2 (one or two trifoliolate leaves expanded). This is less than we would like as we reach the longest day of the year (this Saturday, June 21), but we expect growth to pick up rapidly now that it has warmed up. In our planting date experiment here at Urbana, soybean planted April 1 is about a foot tall, but leaves are small due to the low temperatures, so the canopy has not filled rapidly. Unlike last year, we are not likely to see any pre-June 21 flowering this year. Soybean plants flower when the length of the night reaches a certain minimum, so they can flower before longest day, when night length is decreasing, but only if they are large enough, at about stage V3. As it is, look for first flowers about the second week of July for fields planted in May.

We like to see soybean plants form a complete canopy by the time of the first flower in order to fully utilize the sunlight as more flowers and pods form. That probably won't happen in many fields this year, but if July rainfall is average, the plants should continue to grow rapidly and lack of canopy should not be a problem, except perhaps in fields planted very late or in wide rows. Compared to corn, soybean plants respond positively to warmer night temperatures, though this is probably more important during seed filling than in July. Starch accumulates in soybean leaves during the day, and we think that higher night temperatures help move this starch out, thus "clearing the deck" for higher photosynthetic rates the next day.

Soybean yield is largely made in August, during the time seeds are filling. If temperatures are moderate and rainfall and sunlight are adequate, soybeans can respond surprisingly well with higher yields, even if conditions during the first half of the season are not very favorable. Let's keep that in mind as we watch the crops grow during another Illinois season.

A final note about wheat: After the crop came out of the winter and early spring in good shape, wet weather in the main growing area has led to soggy fields and, in some fields, high levels of Fusarium head scab. Wheat in central and northern Illinois escaped the rain a little better at flowering time, so it has less scab and appears to have good yield potential. Where scab is a problem, especially if wheat has to stand past maturity as fields dry out, we expect problems in both yield and grain quality. Watch the grain carefully as you harvest, and set the combine to blow out most of the "tombstones"kernels that did not develop due to scab infection. If larger kernels have scab with pinkish discoloration, levels of vomitoxin/DON will be a concern. This is disheartening, but perhaps we can work up some optimism for the double-cropped soybean to follow in many of these fields.--Emerson D. Nafziger

Author: Emerson Nafziger

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