Issue No. 6, Article 7/April 30, 2004
Planting: Starts and Stops
With night temperatures dipping into the 30s this week, there is concern about the corn crop that is emerging in most fields. We planted corn in Urbana on April 15, and it spiked through about April 26. During those 11 days, we accumulated about 120 growing degree-days (GDDs), which is about the GDD it normally takes for emergence. The accumulation pattern has been unusual; we accumulated 23 GDDs on April 17 but only a total of 35 in the past week, including zero on April 27. With warmer soils early, the crop emerged more quickly than it would have had it been cool at the start. Stand counts and uniformity are good in the few fields I have seen.
Officially, 64% of the state's corn crop was planted by April 25. With progress since the weekend, that percentage is considerably higher by now, though there are still pockets that are too wet. Most of the 50% of the crop listed as planted a week ago has emerged by now, and most of it is within a fairly narrow size range, somewhere between spike and one-leaf (V1, with one leaf collar visible) in size. Dr. Bob Nielsen, from Purdue, has shown that it takes about 85 GDDs for corn to add each leaf up to V10 or so, when rapid stem growth reduces the GDD requirement to about 50 per leaf. That means that corn will grow fairly slowly until we pick up the pace of GDD accumulation.
One advantage to the small size of the corn plants is that cool night temperatures will have less effect on the health of the plant, even though continued slow growth from low temperatures is not ideal. Cold temperatures can kill small corn, but this usually requires air temperatures to be below 32 for some hours, long enough to drop soil temperature at the crown depth (3/4 inch or so) to near freezing. That seldom happens in late April, though plants in low places in fields, where colder air accumulates on still nights, can show such damage. Looser, drier soils store less heat and so are more prone to such damage. More often, night temperatures in the 30s cause leaf injury, ranging from loss of color to death of leaf tissue, without killing the plant. Leaves oriented vertically, as they are in small plants, are less likely to suffer such damage, because they are not exposed to radiative heat loss to the "cold sky" at night. As long as the crown of the plant stays healthy, corn plants usually resume growth once warmer temperatures return and seldom suffer yield loss from loss of the small amount of leaf tissue on young plants.
Unless the weather changes quickly for the worse, corn stands are likely to be good in most April-planted fields, and there may be little reason to consider replanting. Many people want to take stand counts for the records, though. In good, uniform stands, counting the plants on 1/1000 of an acre works well. I usually count plants in two rows once the length is marked, just for a second count.
If stands are more variable across the field or you want a bit more accuracy, you can use the measuring wheel method to count stands. To do this, set a measuring wheel to zero, then push it as you count plants, recording the distance traveled as you count to a fixed number. I use 150 plants and count by threes. To convert the distance occupied by 150 plants in 30-inch rows, divide the number 2,613,600 by the distance traveled in feet to give plants per acre. Thus, if 150 plants occupy 92 feet, the population is 2,613,600 divided by 92 = 28,409 plants per acre. (If you don't want to carry so many digits, divide the feet traveled into 2,613.6 to give thousands per acre.)
If soil conditions are good, there is little reason to delay planting soybean, though we would expect planting date to have little effect on potential yield over the next 3 weeks or so. Soybean planting in Illinois is listed as only 3% complete as of April 25, but that number is likely to grow quickly over the next couple of weeks. It is good to have higher soil temperatures for soybean at planting compared to corn, but the threat of heavy rainfall after planting is the real danger to soybean. It's not that soybean needs warmer soils to emerge than corn does, but rather that soybean seed doesn't stay viable as long as corn seed under cooler, wetter conditions. Nor do soybean seedlings have as much vigor or as much ability to emerge through crusted soils as do corn seedlings. Warmer soils simply mean faster emergence, which reduces the chance that soil conditions will turn unfavorable for soybean seed viability and emergence. We can do our part by not planting soybean into soils that are already cool and wet. If soils are cool (in the 50s) but dry enough, it is reasonable to begin planting, since we expect soils to warm up at this time in the season.
Wheat in most fields continues to respond well to the cool, relatively dry weather. Here at Urbana, our crop is in Feekes stage 9, meaning that the collar of the flag leaf has emerged. At this rate, it should enter heading a few days earlier than the May 12-14 we usually expect, though that will depend on the temperatures over the next 2 weeks. Wheat in far southern Illinois normally heads about two weeks earlier than in central Illinois, so it is at or near heading by now. Though there's not much we can do to make it happen, cool, relatively dry weather through heading will go a long way to assuring a good wheat crop. At least the current weather pattern is favorable.--Emerson Nafziger