Issue No. 15, Article 9/July 2, 2004
The latest crop report indicates that 16% of the Illinois corn crop was silking and 14% of the soybean crop was blooming by June 27. This is true even after temperatures during the week of June 20 averaged 10 degrees below normal, dropping the growing degree day (GDD) accumulations since May 1 to below normal for many reporting stations.
At Urbana, a midseason hybrid we planted on April 15 reached silking (R1 stage) by June 30. GDD accumulations since planting have totaled about 1,270, which is just slightly less than half the requirement for this hybrid and is very close to what the GDD model would predict. Let's assume that this hybrid requires 1,350 more GDDs to reach black layer, or physiological maturity, at which point grain filling stops and the crop dries to harvest moisture. GDD accumulation averages about 25 per day during July, or about 775 for the month. Daily accumulation slows slightly in August, to about 23.5, on average. Thus, if we get average temperatures in July and August, the crop will accumulate the 1,350 GDDs needed to complete grain filling by about August 20. That would mean reaching harvest moisture by early September.
The fact that we have adequate soil moisture almost everywhere in Illinois means that the crop is likely to pollinate successfully, with relatively high potential kernel number. Is there an advantage to pollinating early and finishing grain filling in August? The answer to this question is probably yes to pollinating early and no to finishing grain filling. It is great to have pollination finish before heat and dryness set in, so early pollination is good, especially during the moderate stretch of weather we're in now, with sunshine and cooler nights. If we can keep insects at bay, we should be able to count 650 to 750 developing kernels per plant (18 to 20 million kernels per acre) by about 2 weeks from now.
We would, though, prefer to have grain filling end in September rather than in August. That uses the sunlight we get during the first weeks of September. But more important, the cooler nights and more moderate temperatures in September are often better grain-filling conditions than we get in August. We certainly saw this last year and have generally noted that cooler-than-average temperatures during August usually mean higher-than-average corn yields. The chances of getting August weather cool enough to extend grain filling much are not large; even 3 degrees lower than average would only extend grain filling by less than a week. If black-layer development is delayed by a week or two, though, the crop will still come off in time, but very likely with higher yields. On the opposite end, if it's warmer than average and grain fill stops by August 15, it will likely mean lower yields.
Are all the flowers on soybean a positive sign, as well? We usually start to see flowers only about now, but in many fields this year, there have been flowers for several weeks already. This is unusual enough that we don't know how to predict the consequences very well. If an early start to flowering and podding means an earlier end to flowering, it could be positive, but only if pod numbers are high enough when flowering stops. We saw last year how delayed development in July followed by a lack of water in August hurt soybean yields, so we'll be cautiously optimistic that an early start to pod and seed development will be a good thing. We do not really expect soybean seed filling to extend longer than usual, though, so an early start to seed filling will likely mean early maturity and early harvest, much like we're likely to see in corn. If moisture supplies remain good and temperatures are average or slightly below, good yields should be possible. --Emerson Nafziger