Issue No. 8, Article 9/May 19, 2006
Stand Issues Following a Wet, Cool Week
The good news is that Illinois producers are nearly finished getting the 2006 corn crop planted, with 95% completed by mid-May. The not-so-good news is that soybean planting is starting to lag, with only 31% planted by May 14, and little planting progress by now in some areas. More negative news is the wet and cool conditions that we've been plagued with in most areas for the past week. Parts of western Illinois haven't gotten much rainfall so far in May, and much of the state has had close to normal amounts, while some places in eastern and southeastern Illinois have had twice as much rainfall as normal for the first half of May.
Most of the corn crop in Illinois has emerged, so unless there is water standing in fields, stands should end up being good. Cool temperatures might even help survival in areas with standing water. So even though growing degree-day (GDD) accumulations have been slow for the past two weeks, we don't think that corn stands will be compromised to any great extent in most Illinois fields. GDD accumulations have been lower for the first half of May than they were for the first half of April, though, so corn planted in May is taking longer to emerge than corn planted in April.
Corn that is up is also showing effects typical of those following cool, cloudy weather, with slow growth and color that has faded to pale, sickly yellow-green in many fields. This is not surprising when you consider that the GDD total for the past week was only 50 or so in much of the state, including a few days when 0 GDD accumulated. Sunlight for this past week was also much less than normal. We did avoid frost injury, and most corn leaf tissue in most fields is intact, so we expect a rapid return to healthy color and resumption of growth with warmer temperatures and better sunlight. Still, May weather this year has not helped maintain the rapid pace of crop development that we saw in 2004, and so we can expect early planting to provide slightly less advantage over later planting than it has in some years.
The stand situation in soybean is not nearly as settled or as favorable as in corn. With only about a third of the soybean crop planted by mid-May, planting is now behind normal, and it continues to slide in wet areas. Some people in drier parts of the state waited to plant until soils were warmer, if not moister. Now that soil temperatures (at 4 inches under bare soil) have started to move up toward 60°F, there is no longer much reason to wait, though the fact that nights are expected to be cool over the next week means that soil temperatures will not move up rapidly. In fields where dry soils continue to be a concern, planting without further tillage might help preserve enough moisture to get the crop to emerge.
Where soils have been both cool and wet over the past week, soybeans planted in early May that are not yet emerging might be in trouble. I had one report from western Illinois the morning of May 17 that close to half of the seeds of a planting done in early May had rotted without showing a sprout, even though there was no excessive wetness. I hope that's an isolated incident, but especially where it has rained more than an inch or so since early May, seeds need to be checked now for viability. If some seeds are sprouted in a field but others aren't, those that aren't sprouted within a few days under these cool soil conditions are likely not to produce plants. Even some sprouted seeds might not emerge, and so decisions on keeping stands in these fields will have to wait until emergence is complete.
Replant guidelines for soybean are less well defined than those for corn, but generally fields with less than about 80,000 healthy, uniformly distributed plants per acre should probably be replanted if replanting can be done during May. In 15-inch rows, 80,000 plants per acre is about 23 plants in 10 feet of row (counted as 5-foot sections of adjoining rows), or about 5.2 inches between plants. Such low densities require plants to grow unusually large to compensate for missing plants, so if there is anything that threatens the ability of plants to grow well and compensate (gaps, diseases, damaged plants), then the replant threshold (population to keep) increases. Existing plants become more valuable the later it gets, but stands below 70,000 or so on June 1 are likely to need replacement, and keeping fewer than 60,000 plants is unlikely to be beneficial unless it is past mid-June in the northern half of Illinois and replanting is not an option. Later planting will benefit more from narrow rows, and drilling in rows less than 8 inches apart may be appropriate after June 1.
For the large acreage of soybean yet to be planted the first time, we are starting to move into the "penalty phase," with yield declines expected as planting is delayed. Expected dates for this to begin are about May 20 in the northern third of the state, May 25 in central Illinois, and May 30 in the southern third. Yield declines will start slowly and then accelerate, with expected loss of about 1 to 2 bushels total for the first five days and 3 to 4 bushels total for the second five days, thus a total loss of 4 to 6 bushels if planting is 10 days after the dates mentioned. Yield loss rates accelerate to more than a bushel per day of delay by the time planting is two weeks late. In soybean more than in corn, weather patterns later in the season can reduce or increase losses from delayed planting. Since we have no way to know now how this will play out in 2006, we can use these averages. Note that losses are fairly modest for the first week or so after these dates, suggesting that we not "mud in" the crop just to gain two or three days, especially if the calendar still says May.--Emerson Nafziger