Time to walk fields
August weather is not often conducive for walking cornfields. There are worse places to spend an August morning or afternoon — but we won’t discuss them here! Nevertheless, one gains much by strolling through fields in August.
To prepare for this, let’s walk through corn’s six reproductive stages, examining the effects of stress on the crop during each stage.
Silking, designated as R1 by agronomists, occurs immediately before or within a few days after pollen drop. Silking in older corn hybrids always followed tasseling. Incidentally, “R” stands for reproductive, in contrast to earlier growth stages denoted by “V,” or vegetative, which occur at and prior to silking.
Improved stress tolerance of modern corn hybrids relative to older hybrids directly relates to fewer days between tasseling and silk emergence, on average.
Since silking is more sensitive to stress than pollen drop, the occurrence of drought or other environmental stresses will slow silk emergence but may not affect pollen drop much, if at all. Silks from tip kernels emerge last. Therefore, if silking is delayed too much by stress, fertilization of tip kernels may not occur because of pollen shortage; this results in reduced kernels per ear and reduced yield.
To gain an appreciation of how stress affects the crop during silking, 50% leaf loss from hail at this time would result in a 31% yield loss; 100% leaf loss would result in a 97% yield loss (see graph).
Are ear tips filled out?
The blister stage, R2, occurs about two weeks after silking. Kernels resemble a blister filled with clear fluid and contain 85% moisture. Stress during R2 results in aborted tip kernels. If the crop is in either the R1 or R2 stages in mid-August, a later-than-normal frost is necessary for complete crop maturity.
Milk stage, or R3, takes place about three weeks after silking. Yellow-colored kernels filled with milky-white fluid characterize this growth stage. Starch rapidly accumulates in the kernels, and kernel moisture content has decreased to around 80%. Stress during R3 will cause abortion of the tip kernels.
Half of kernel dry weight accumulates by the dough stage, or R4, a month after silking. Words like “doughy” or “paste-like” characterize kernel contents. Kernel moisture contents of about 70% are normal.
Stresses at R4 reduce kernel weight since less starch is accumulated into individual kernels. Fifty percent leaf loss from hail during R4 would result in a 12% yield loss; 100% leaf loss would result in a 41% yield loss.
About 40 days after silking, the ear is at dent stage, R5. Kernel tops collapse, forming the characteristic dent. The kernel milk line slowly works its way from the kernel top to its base, marking the separation between solid starch and the liquid portion of the kernel. Kernels contain about 55% moisture at dent stage. Stress during this growth stage reduces kernel weight.
Kernels form black layer
Physiological maturity, or R6, commonly called the black layer stage, occurs about two months after silking. Kernels now contain their maximum dry weight and have about 30% moisture content. Grain moisture continues to drop if fall conditions are favorable.
A black abscission layer forms at the base of each kernel and acts as a physiological barrier separating the kernel from the cob. Although the formation of the black layer is evidence that the grain-fill period is over, it isn’t always easy to see. Instead, the stabilization of kernel dry weight actually best indicates R6.
Stalk lodging and ear drop from weak shanks associated with weather-related events (such as wind and snowstorms) will reduce machine-harvested yield at this time.
Walk your fields, making observations and notes on ear fill. This information will help you make sense of yield monitor data at harvest, as well as identify changes to consider for your 2011 corn management program. Let’s hit the fields now. Grab your water bottles and sunscreen!
Elmore is the Iowa State University Extension corn agronomist.
For more corn management information, visit www.agronext.
This article published in the August, 2010 edition of WALLACES FARMER.