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Issue No. 14, Article 4/June 25, 2004

Does Cooler Weather Now Help or Hurt?

Most areas in Illinois accumulated 150 to 200 growing degree-days (GDDs) last week (through June 20) and so remain 100 to 200 GDDs ahead of average since May 1. With early corn planting this spring, much of the crop received 200 or so GDDs before May 1, so much of the crop is 10 to 15 days (250 to 400 GDDs) ahead of normal. This is reflected in the fact that tassels are appearing widely on early-planted corn, and some corn is well into pollination now.

The return of cool temperatures this week will mean slower GDD accumulations and will thus slow the rush of growth that much of the crop has been maintaining for the past month. A high/low temperature combination of 75/55 means 15 GDDs, while one of 90/70 accumulates 28 GDDs. That's a relatively large change on a daily basis. Losing 80 to 90 GDDs in a week means that the "excess" we have accumulated would dissipate within 3 or 4 weeks. Of course, we have little reason to expect that temperatures will remain lower than normal for weeks at a time.

Would it be a problem if lower than normal temperatures persisted for the next several weeks? Probably not. Lower daytime temperatures mean slower rates of photosynthesis; with full sunlight, photosynthesis at 75° is about 30% slower than at 86°, and with cooler nights, daily rates might be reduced even more than that. Cooler nights mean less loss of sugars accumulated during the day and so are favorable, but on balance the plant produces more at 85° than at 75°. In soybean, cooler nights are more problematic, because they mean less movement of starch out of the leaves and so can inhibit photosynthesis the next day.

On the positive side, lower temperatures also cut water-use rates, in about the same proportion as photosynthesis is decreased. We also expect that the cooler, drier weather will tend to favor root growth on both corn and soybean, which might turn out to be very helpful later in the season. Weather like this also means slower rates of development of most diseases and insects, and the higher amounts of sunshine help to lessen the loss in photosynthesis from lower temperatures. Finally, slower growth allows plant stem tissue to mature some and so should lessen the likelihood of green snap. Given all this, and the advanced growth stage of corn already, we think the lower temperatures this week will have a modest positive effect on corn.

The soybean crop has improved some in appearance, and continued drier weather will help it come along, though cooler temperatures will not be very helpful. Its color should improve as nodules develop and become active, and canopies should continue to fill in, though ata slower rate than if it were warmer. Many people have noted some flowers on earlier-planted soybean plants this year. That's because the warm weather stimulated growth--soybean plants need to reach the 3-trifoliolate stage before they can flower--and warm nights also speed up the activity of a light-sensitive system in the plant that allows flowering to occur. Flowering will likely stop during these long days and cooler nights and will resume in early to mid-July.

Some of the flowers present now will form pods, and these will remain ahead in development of the main flush of pods that will form later in July. There are usually not many of these early flowers or pods, though, so their contribution to yield will not be great. You can use them as a general indicator of the state of the plant over the next few weeks, though: If they stay attached and develop seeds, it means that the plant is doing well. But if they fall off the plant, it's probably because the plant is struggling to maintain high rates of photosynthesis and growth.

There has been a bit of confusion about vegetative growth staging in soybean. Dr. Palle Pedersen at Iowa State University has revised the publication How a Soybean Plant Develops, and this should help clear up the confusion. We stage by counting the number of "unrolled" trifoliolate leaves (those with three leaflets, as opposed to the lowermost leaves on the stem, which have only single leaflets and are called "cotyledonary" leaves). So a V1 plant has a single trifoliolate leaf, with the trifoliolate leaf above it starting to unroll. Plants from an early May planting at Urbana are now in stage V6 or so. They will add about two new leaves per week, up to V16 or so. Once flowers appear, which usually happens at about V8, we usually stop paying much attention to V stage and start staging by R stage, which tracks flower, pod, and seed development.

We harvested the wheat variety trial at Urbana on June 22, and preliminary indications are that the average yield of the varieties exceeded 90 bushels per acre. That is almost 20 bushels per acre lower than the yields last year, but harvest this year was almost 2 weeks earlier than in 2003. Flowering was about 5 days earlier this year, but we expected the warm, often wet weather during grain fill to reduce yields much more than actually happened. We'll chalk it up to the outstanding weather in April that set the crop up for high yields and also to the ability of the crop to fill grain very quickly, even though it was warm, wet, and cloudy. We can still play "what might have been" had we had cooler, drier weather in May and June, but we'll double-crop earlier (and farther north) and be glad for the wheat yield we got.

I have not yet heard reports on field pea yields, but some fields in southern Illinois have likely been harvested by now. The weather over the past 6 weeks has probably not been very kind to the pea crop, but it could still surprise us like the wheat crop has done. RAPCO, the company that brought in seed and encouraged production of pea in Illinois this year, has reportedly suggested the possibility of planting pea in July for an October harvest. We know even less about the potential for such a crop than we knew about field pea planted in March, but we would have some concern about hot, dry (or wet) weather during vegetative growth, the potential for diseases, weed control, and the possibility of unexpected photoperiod sensitivity that could result in early or late flowering. Weather to fill seed may well be better in September and October than it was in May and June this year, but beyond that, we have little basis to even guess about crop prospects for summer-planted pea. --Emerson Nafziger

Emerson Nafziger

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