Late-season dry weather raises concerns
I’ve heard a number of comments coming from drier areas that “test weight” was going to be hurt by dry conditions, with yield lowered as a result. When kernel fill stops before kernels have as much starch as they can hold, kernels tend to be “shrunken” on the end that attaches to the cob. When canopy photosynthesis decreases and the supply of sugars starts to run out by late dough, starch tends not to pack normally in the kernel, and this can lower the density of the starchy part of the kernel – the endosperm. Misshapen kernels that don’t fit together well and kernels that are less dense than normal both contribute to lowering of test weight.
Of course, an early end to starch accumulation means lowered kernel weight. And lowered kernel weight, not lower test weight, directly translates to lower yield. It is certainly the case that test weight and kernel weight are often related, as explained above. But yield is the product of kernel number per acre and weight per kernel, while test weights are often not well-correlated with yield level, unless of course stress lowers both at the same time.
It’s time to remind ourselves that “black layer” – the darkened layer of cells at the tip of the kernel that indicates that the tissue that transfers sugars into the kernel is no longer active – always forms in corn, whether or not the grainfilling process come to its natural end or ends early due to canopy loss. For practical purposes, the disappearance of the milkline at the base of the kernel means that little or no additional weight will be added to the kernel.
While we would have preferred normal rainfall and normal temperatures during August, the periods of cool weather did help stretch the supply of soil water, which helped plants fill grain even under the recent spell of high temperatures. Any green leaf area on plants means that they are capable of producing sugars, and green leaf area that persists even after some weeks of stress conditions indicates that the crop has had access to at least some water. I also think that we are seeing the advantage of cool night temperatures for some periods during grainfill, and that this has translated into slightly better than normal photosynthetic efficiency.
I’ve also heard comments to the effect that low solar radiation might have decreased photosynthesis and yield potential in 2013. It’s certainly true that sunlight amounts have been low in parts of the 2013 season: according to the Illinois State Water Survey (ICN data), solar radiation in July 2013 was 20% less than the average of the previous three years, and in August was 13% below the 2010-2012 average. These two months were warm and dry in all three previous years, with the exception of August 2012, but sunlight amounts in 2013 were considerably less than normal.