Issue No. 22, Article 5/September 1, 2006
Down the Home Stretch
Corn is starting to reach black layer in many fields in southern and central Illinois now, more or less on schedule, based on the growing degree-day (GDD) requirements, at least in fields where there has been adequate water available most of the season. Physiological maturity refers to the point in time when the kernels no longer accumulate dry matter and, thus, that grain yield for that ear has reached its maximum. The darkened layer of cells at the base of the kernel is an indication that these cells are no longer transporting sugars from the cob to the kernel. Kernels on the same ear reach black layer at different times, but it's not necessary to shell all of the kernels and scrape their base in order to know the exact day this happened. When the husks have dried and there is no liquid at the base of the kernel, sugar movement has, for all practical purposes, stopped.
Under good conditions, black layer is reached when the kernels are as large as they can get, after which some sugars might continue to accumulate in the cob or stalk before leaves dry up. In fields where the crop stopped filling early due to lack of water, kernels eventually show the black layer typical of mature kernels, though it can take a while for the dark color to develop. The end of grain filling in such fields is forced by the end of the sugar supply from the leaves, not from the "natural" cause of having kernels reach their maximum size. We would consider such a crop to be "source limited," since the source of sugars to fill kernels (photosynthesis) stopped before the kernels were filled.
While we normally consider corn to be source limited, meaning that kernels typically do not get quite as large as they could, there are some conditions under which kernels reach their maximum size. Many people have observed this year that ears do not have grain all the way to the end. Reasons for this are not entirely clear, especially where there was enough water available during July. High temperatures and silk-eating insects probably contributed in most fields. In any event, as we reach the end of the filling period under good conditions, it is likely that kernels on such ears will get quite large, and if leaves remain green after black layer is reached, it is likely that kernel number, rather than photosynthesis, will limit yields in such fields. The only practical consequence of this will be higher yields than kernel counts might suggest. If we end up with 80,000 kernels per bushel instead of the "default" 90,000 used in the corn yield calculator, that means 11% more yield.
An indication of how good filling conditions have been in parts of Illinois this year is provided by some data that Eric Adee at Monmouth provided. We have a project in which we are sampling ears at seven-day intervals, drying the ears and shelling them, and taking kernel weights. During the first two weeks in August, one hybrid added a total of 126 bushels per acre, or 9 bushels per day. Rates were the same for each of the two weeks and probably represent accumulation rates near the maximum, though even then there were probably some days better than others. But a crop would have to maintain such a rate for only 33 days to produce 300 bushels per acre and for only 22 days to reach 200 bushels per acre. This is during the "linear" phase of yield accumulation, starting perhaps 10 to 15 days after the end of pollination and ending a week or so before black layer; in most cases, this would last for about 40 to 45 days. That means that we normally get filling rates of maybe half the potential rates during this period. This puts some perspective on the importance of having healthy canopies and good kernel numbers, and also on the need for favorable temperatures and moisture.
When kernels get larger under good conditions at the end of the season, many people think of this extra yield as coming from an "increase in test weight." If kernels increase in weight by packing more starch into the same volume, then test weight and yield do increase together. But test weight is a measure of bulk density, while yield is based only on weight, so the two are not directly tied together. We can have very high yields without high test weight and low yields (think popcorn) with very high test weight. In fact, there is often very little correlation between test weight and yield among hybrids in our corn hybrid trials. How kernels fit together and how densely they are packed with starch determine test weight, while yield is the result of kernel number per acre and weight per kernel.
High test weights are nice because they mean we can put more "bushels" in a bin. Historically, a "bushel" was a fixed volume of grain (1.24 cubic feet), but today it refers to a fixed weight of grain (56 lb for corn). Thus the volume of a 10,000-bushel bin is about 12,400 cubic feet, but the bin will hold some 10,700 bushels of corn if the test weight is 60 lb/bushel. Because 60 lb/bushel is the standard test weight for soybeans, the same bin will hold only 10,000 bushels of soybean if they are at the standard test weight. We have not paid a lot of attention to soybean test weight, but it seems to vary less than corn test weight, probably because the spherical shape of the seeds means they pack together more uniformly and also because seed density may vary less.
Soybean fields are starting to lose their color, but the rainfall over the past two weeks has been helpful to the soybean crop in most areas, and there is considerably less premature end to seed filling in soybean than in corn. The rains partially revived the potential of double-cropped soybeans, but the late flush of leaves, flowers, and pods in these fields means that they will need good filling conditions during most or all of September, possibly into October, in order to fill these pods and to make good yields. Full-season soybeans may have lower than ideal pod numbers in some fields due to stress in late July, but the filling conditions have been good, and it appears that seeds will fill to their normal weight, and maybe a little above their normal weight, in fields where the canopy is still green and healthy. It would be better if daily high temperatures were higher than the upper 60s and low 70s we're getting in much of Illinois at the end of August, but as long as good sunlight returns and night temperatures do not drop into the lower 50s, temperatures a little lower than normal will mean only a delay in maturity, not a loss in yield.--Emerson Nafziger