Issue No. 17, Article 5/July 29, 2011
Assessing the Corn Crop as Heat Continues
Most of Illinois received some rainfall the past week, with amounts ranging from 0.5 to 1.5 inches in the southern two-thirds of Illinois and 2 to 4 inches in the northern third. The corn crop rating still slipped slightly, with only 59% of the crop now rated as good or excellent. Much of the rain south of I-80 fell on Sunday, July 24, and so was not factored into the ratings released on Monday. Even with this rain, most of Illinois between I-70 and I-80 has received below-normal rainfall in July. In eastern Illinois, we've received only about 1.5 inches so far this month, less than half of normal.
While the rain was exactly what the crop needed, the high temperatures barely paused; at Urbana, the high on July 24 was "only" 87 degrees, following seven days over which the average high was 98, and the average low was 75. Due mostly to the high night temperatures (even average high temperatures this time of year reach the 86-degree cutoff, above which GDD is not increased), 214 growing degree-days accumulated during those 7 days. Exceeding 30 GDD for a day is rare, but such accumulation rates over a week are truly unusual. At 30 GDD per day, a hybrid needing 2,700 GDD to maturity could be planted on June 1 and mature before the end of August.
The temperature on July 25 was again above 90 degrees, and this is expected to continue for some days to come, with little rain in the forecast.
Although I've been saying that high temperatures are not a problem if the corn plant has enough water, there are some reports that the crop in some fields may not have fared as well by now as we had hoped. The main symptoms have appeared as lack of nitrogen, with firing of leaves and some death of leaf edges and the center of leaves, starting from the tip, and affecting lower leaves first. Corn following corn is probably more affected.
Any loss of leaf area or function (color) at any time during the season is a concern, but during and after pollination, when full photosynthetic rates are needed to set the number of kernels needed for high yields, loss of light interception will usually result directly in yield loss. The canopy position where leaf area is lost affects this; lower leaves, especially with tall plants, are often shaded and so contribute less, and their loss is less damaging. By this point in the season, most loss of leaf area is permanent; once leaves or parts of leaves start to dry out, they are dead. Leaves with pale yellow color seldom become functional again once we reach late July.
The only way to get a handle on loss of light interception is to look down the rows at midday on a sunny day and see if the crop is intercepting the light, in which case it will be fairly dark on the soil surface, or if there are streaks and patches of sunlight or lots of diffuse light, such that it seems fairly bright underneath the canopy. The best canopy at this time of the season should be intercepting more than 95% of the sunlight, with very few patches of sunlight reaching the ground. Anything less than that means lower yields.
Of course, high yields also require having enough kernels to fill; yields are much more closely tied to kernel number than to kernel size. We have not yet received many reports of kernel numbers, but in areas that have not suffered from dryness, they seem so far to be normal.
With pollination nearing completion, this is a good time to count kernels, especially in fields that pollinated by mid-July. With an improved water supply compared to last week, we expect fewer aborted kernels in the earlier fields now, but if leaf area has been lost, kernel numbers can still go down, all the way through the roasting ear stage (R3). We know this because we can see (as we did in 2010) aborted kernels that had reached up to half the dry weight of normal kernels. Kernels do not reach half their dry weight until they reach the dough stage.
Under good conditions, we would like to have about 15 million kernels filling per acre; if kernels reach a final weight of 70,000 kernels per bushel (360 mg/kernel), that would be 214 bushels. In practice, we find the number of ears in 1/1,000 of an acre and count kernels on three of those ears, multiplying average kernel number times ear count to get number of kernels in 1/1,000 of an acre. As an example, 30 ears with 500 kernels each would be 15,000, or 15 million per acre. So where are we with the corn crop? Under these temperatures the crop is using as much as 2 inches of water per week, so even with the rainfall we face the possibility of running into water deficits, as early as later this week in some places. Each day that has photosynthetic rates lowered by water deficits will reduce yields some. In most fields this would be through loss of kernels to abortion over the next week or two, and in late-planted fields it could still come as failure to fertilize kernels. By mid-August, we'll shift our concern to how well the kernels can be filled.--Emerson Nafziger