June 12, 2012
U.S. Corn Entering Critical Growth Period
While pollination is certainly the most critical factor in determining the corn yield, the period leading up to pollination is also very important because that is when the potential size of the ear is determined. The components of grain yield for corn is a combination of the number of ears per acre, the number of kernels per plant, and the weight of each kernel.
The number of ears per acre is determined largely by the plant population. If there is going to be a reduction in plant population, it usually occurs early in the growing season due to flooding, ponding, saturated soils, diseases, or pests. Additional plant losses could occur later in the growing season due to strong winds or hail damage. Under exceptionally severe moisture stress, the plant may actually die prematurely before the ear develops. Under less severe moisture stress, the plant may survive, but the ears may be aborted resulting in what is called a "blank."
The number of kernels per plant is a function of the number or rows per ear and the number of kernels per row. When the corn plant has five leaves (V-5), it starts to initiate ear development and when the plant has seven leaves (V-7), which generally occurs 6-8 days after V-5, the number of kernel rows is determined. The row number is largely determined by genetics, but severe stress during the V-5 to V-7 stage can restrict the number of rows. The number of kernels per row is more subject to the growing conditions. The number of kernels per row is not complete until the plant reaches V-12 to V-15, so it is exposed to potential problems for a much longer period of time. Moisture and heat stress can reduce the number of kernels per row until after pollination. And lastly, the weight of the kernel is the final component of yield and it is a function of the growing conditions during the grain filling period. If a plant is undergoing stress, then the weight of individual kernels will be reduced.
The concern now is the developing dryness and resulting moisture stress for the corn that is either pollinating or in the pre-pollination phase. The only corn that is currently pollinating is in the southern locations, but much of the corn crop is going to be in the critical pre-pollination phase during the second half of June.
The plant size is also an important factor in determining the eventual yield. The size of the corn plant is determined during the rapid growth phase prior to pollination. Stress during this period can result in shorter than normal corn plants. If the stress is severe enough, the plant can cannibalize the lower leaves to maintain the health of the upper canopy. Shorter plants have a smaller leaf area which results in reduced photosynthetic activity that results in less photosynthate bring produced. If the "factory" produces less photosynthate, then the kernel weights will be lighter and the yield will be lower.