Warm temperatures continue in Illinois, with growing degree day (GDD) accumulations since May 1 running from 150 above average in northern Illinois to about 250 GDD above average in the rest of the state. With GDD accumulations of 900 to 1,000 since May 1, the corn crop planted in early May is at V10 to V14, about 30 to 60 inches tall, and needing only about 350 to 450 more GDD to tassel and silking. With daily accumulations at about 25 GDD, much of the crop will be showing tassels and silks by the end of June and first days of July.
The corn crop condition ratings as of June 10 were 83% good + excellent, one of the highest early-season ratings we’ve ever had. In most fields, the crop looks outstanding, with probably the best stands we’ve ever had, and in most cases very good crop canopy development and color. On the other hand, plants are showing leaf curling in the afternoon in some areas, indicating that the water supply in the soil is not high enough to sustain maximum rates of photosynthesis now.
A lot of the rain over the past month has been from thunderstorms rather than broad movement of fronts; as a result, its distribution has been very uneven. During the first half of June, rainfall ranged from less than a half inch in parts of western and southwestern Illinois to more than six inches in southeastern Illinois. Even in those areas showing average or above-average rainfall, there are places that the storms missed, and where soil water is starting to run short. The US drought monitor shows “abnormally dry” conditions in several western Illinois counties, and in a small pocket in northeastern Illinois.
While the well-watered areas with deep soils have enough soil water now to get the crop through pollination, normal to below-normal temperatures will assure that the crop has high potential to set the kernel numbers needed for high yields. In areas where plants are showing stress in the afternoon now, we expect that this will set in a little earlier, and last a little longer, each day that temperatures remain high and there’s no rainfall. Canopy development is good so far, but a good canopy also means faster water usage, and a crop that’s head-high has a “crop coefficient” (the proportion of evaporation that the crop takes up) of about 0.7. This value reaches a maximum of about 0.8 at full crop canopy.
That means that if evaporation (also called “potential evapotranspiration” or PET) on a warm, sunny day is 0.25 inches – PET has been high as 0.3 inches on the warmest days in Illinois in recent weeks – the crop takes up 0.25 x 0.7 = 0.18 inches of water. Our best soils can store as much as 10 to 12 inches of plant-available water in the top three feet, so at field capacity the water supply can last six weeks or more without rainfall. That’s under ideal conditions, though – the crop will often show stress effects before the soil water is completely depleted. It’s been dry enough in parts of Illinois that the soil water supply is not enough to keep the crop well-supplied now.
As pollination approaches, the effect of water stress on the crop will increase. If it rained everywhere today, the crop could probably recover its full yield potential in most fields. But if leaf rolling starts by noon, the crop is producing less than half the normal amount of sugars through photosynthesis on that day, and the closer the crop gets to pollination the larger the effect of lost sugars will be. Today’s hybrids are bred to produce silks, pollen, and some fertilized kernels under stress conditions, but if it stays dry over the next weeks where the crop is already showing stress, kernel numbers will be lowered. Lower kernel numbers mean lower yield potential.
Corn plants that develop under high temperatures and with plenty of water tend to be taller than usual. In areas where the crop has been showing stress symptoms in the past week or two, though, we can expect plants to end up shorter than normal. Any water stress during rapid stem elongation – between V8 and tasseling – results in less elongation of cells in those internodes that are expanding during that time, and this results in shortened internodes and plants. As we saw in 2017, shorter plants can still yield very well, but that requires that they get adequate water by a week or so before tasseling to assure that the pollination process can proceed normally.
On a more positive note, the benefits of relatively dry weather include the development of good root systems, a good supply of soil nitrogen, and little development of diseases. The dark green leaf color shows that the N supply has been adequate. Where it has rained the soils have not stayed wet except in low-lying areas, and so there has been little potential for N loss. Some rains have been so intense that much of the water runs off, and dry period before the rain meant that the soils could take in several inches of water before they became saturated. Where water has stood or is standing now in fields, though, we can expect both some root damage and some denitrification, with the potential of considerable yield loss in those areas. Fortunately, this area is not as large in size as it’s been in some recent years.
By now the question of whether the corn crop needs more N to follow normal rates applied earlier should be answered: the deep green leaf color of the crop means that it’s not likely to run out of N so well-supplied with N as shown by its canopy color, there’s almost no chance that it will need more N than is on the soil now. Our N-tracking results from this spring confirm what the crop is telling us – that it has plenty of N. Those who put on a normal amount early with the idea that they’d come back to apply more if the yield potential looks good can skip the additional application this year.
The Illinois soybean crop in mid-June has the same high crop condition rating as the corn crop. Stands are good in most fields, and plants have begun to develop rapidly, after the usual lag that we often see, which in some cases might have been lengthened by application of certain herbicides. In a planting date study we have here at Urbana, early varieties planted on April 25 are at V6-V7 and 15-18 inches tall, with a lot of flowers now. As is the case in corn, canopy health is very good. Growth so far has been good even in dry areas, because soybean plants don’t use water very fast when they’re small so more water remains in the soil.
We see flowers appear before the longest day of the year when soybeans are planted relatively early, and when temperatures are warm in June. The appearance of flowers requires a certain night length, and if it takes 10 days after the longest day (June 21) to reach that night length, then that night length also occurs 10 days before June 21. But if plants aren’t past stage V3 or if nights are relatively cool, soybean plants won’t flower before the summer solstice. Even when they do flower in mid-June, limited numbers of flowers might turn into pods. We will need a lot of flowers appearing after June 21 along with good growing conditions in order to set the number of pods needed for high yields.
Our best hope now is for a return to normal temperatures and rainfall by July to get both corn and soybeans on the way to reaching their current potential for high yields. Little potential has been lost so far, but the next weeks will spell the difference between average and very good crops.