Issue No. 10, Article 6/June 2, 2006
Crops Moving into June
Soybean planting progress geared up late but fast in Illinois, with close to a third of the crop planted during the week ending May 28 and planting nearly complete by now, except in some localized areas where fields are still wet. Concerns about dry soils persist in parts of western Illinois, but this tends to be in areas where planting was early and so where most soybeans are emerged. Soybeans planted after May 20 have benefited from the recent return to warm weather and have taken only a week or so to emerge. Stands in most soybean fields are adequate to good.
In some cornfields, typically those planted in early May, cold, wet soils after planting caused stand problems. Some agronomists suggest that this stems, at least in part, from "imbibitional chilling injury," which results when the water taken up by seeds and small seedlings is cold, usually below 45°F or so. Soil temperatures didn't get or stay that low for very long, but some of the rain that fell was no warmer than that, and there is a possibility that some of the seedlings that were misshapen, disoriented, missing parts, or otherwise unable to emerge might have experienced this type of injury. Taking up cold water affects membranes in young plants and often makes it easier for diseases, some of which thrive in cool, wet conditions, to attack young seedlings.
After the high temperatures of the past week, all corn planted before May 22 or so should be up, and any that is not is unlikely to emerge. Because of the yield penalty associated with late planting now, stands have to be less than half of normal to justify replanting if the original planting was done before May 1. If the first planting was in mid-May, however, then the relative yield loss from replanting is less; replanting might be justified if the stand is as high as 15,000 to 16,000, as long as replanting costs are minimal.
Some have wondered if "repair planting" of low corn stands is ever a good idea. We don't have much data on that for corn, but we do know that it can be tricky. Consider an example in which a stand of 15,000 is "patched" by planting another 15,000 plants in rows 4 to 6 inches away from the original row. The planting operation will probably do some damage to the existing stand, especially if the existing plants are larger than V2 or so. A greater problem is when the existing stand is unevenly distributed, so that certain areas end up with stands that are too low and others with stands that are too high. Some plant a high population near the old rows, with the idea that one or the other planting can be destroyed with herbicide or cultivation, leaving the one that seems to have better potential after several weeks of growth. Many who have tried this end up wishing they had simply destroyed the old stand first, as it's often difficult to make the decision on which one to keep, especially after the original stand enters its rapid growth phase and is much larger than the later plants.
Unlike corn, soybeans tend to respond well to "repair planting," whether the original planting was in rows or drilled. It is helpful to do as little damage to existing plants as possible, since they often will compensate for low numbers and less competition by growing larger, and the fact that their development is at a different stage than the replanted plants will provide a slight cushion against later-season weather problems. We would rather have uniform stands planted in early May, but lack of uniformity does not appear to be as critical in soybean as in corn. We think most of this is due to the indeterminate nature of the soybean plant, which provides it the ability to flower and set pods over a period of several weeks. Even with normal emergence, soybean plants at populations of about 125,000 per acre and above undergo a great deal of plant-to-plant competition, with some plants losing out completely, and with no apparent loss in yield.
Corn has responded well to the higher temperatures, with good color and rapid growth rates. GDD accumulation for May is a little less than normal at most locations in Illinois, and more than half the GDDs for the month accumulated in the last 10 days of May. Much of the corn crop is moving well into its rapid growth phase, which starts at about the six-collar stage (V6). We would prefer to have GDDs accumulate at normal rates, but as long as there is enough soil water to keep the plants growing well, the warm days and nights needed to accumulate above-average GDDs per day are not a problem at this point in the season. Warm nights increase the loss of sugars in the plant, but sugars are not normally in critically short supply during early vegetative growth, and the return of healthy leaf color associated with warmer, drier weather has been a positive.
Corn planted in the first half of April ranges from about V5 in the northern part of the state to about V8 to V9 in the south. As I discussed earlier, stem growth starts to accelerate at about V7, reaching a maximum rate at about V10 to V11, when corn is 3 to 4 feet tall. Daily height increases can reach 3 to 4 inches, and the increasing rate of leaf appearance will mean that canopies appear to "close" very quickly, usually when corn is about 40 inches tall or so, and a few days sooner if rows are narrower than 30 inches. It's best when such canopy closure takes place before the longest day of the year, which in 2006 is June 21. Most of the corn in Illinois should manage to do this, as long as temperatures are at least average and there is enough water to keep growth rates high.
After the dry weather of last year, concern about crop water supply is growing, especially in areas of western and central Illinois where May rainfall has been less than half of normal. June will be the time to watch such crops closely, as their growth rate signals their water status. The good planting conditions and start that the crop has enjoyed in the drier areas are a real plus, in that roots of this crop are likely deep enough to be into soil moisture and are proliferating deeper as soil moisture is used up. This could be a repeat of last year, when the very dry June weather had less than the expected negative effect on corn growth. While moisture recharge this spring may not have been quite complete in some areas, we can be confident that the crop has at least 6 inches of available water in most soils now. The crop that reaches V6 by the first of June will typically use about 4 to 5 inches of water during the month of June, so even if there is enough water in the soil to keep the crop going now, we hope that rainfall patterns return to at least normal by the time the crop pollinates.
While soybean has been emerging quickly, the crop will enter June without having made very much growth in most fields. This makes it unlikely that we'll see much of the early June flowering that we have seen some years. The soybean plant needs to have about 3 trifoliate leaves before it can flower, but it can be triggered to flower by nights long enough to signal flowering before, as well as after, the summer solstice. It is usually of little importance whether soybean plants flower in early June, but when they are not large enough to flower, it does indicate that the plants got off to a slow start and that a considerable amount of June sunshine is going to fall on the soil rather than on leaves. We normally like to have about six weeks of good growing conditions between emergence and flowering in order to have adequate canopy for top yields. We may not get that this year, but it's too early to say whether yields are in any danger from this.--Emerson Nafziger