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The most critical stage of corn development is pollination.1 Stress during pollen shed and silking can cause more yield loss than almost any other period in the crop's development.
Corn yield potential is vulnerable to stress during several growth stages. Potential ear size (number of kernels) is already determined before silks emerge, from corn growth stages V6 to V15. Potential kernel row number is set by the 12-leaf collar growth stage. Potential kernel number per row is determined over a longer time period, from about the 12-leaf collar stage to about 1 week prior to silk emergence. Kernel number per row is more sensitive to stress, whereas row number is mainly determined by genetics.
Water stress around pollination can delay silking, reduce silk elongation, and inhibit embryo development after pollination.2 About 2 weeks before silk emergence, corn is very sensitive to drought stress.1 Moisture stress (leaf rolling by mid-morning) at this stage can decrease yield 3 to 4% per day. During the silking and pollen shed period, severe stress may reduce yield up to 8% per day. Silks remain receptive to pollen grain germination up to 10 days after silk emergence.1 Uptake of moisture and nutrients is critical at this time because cell division occurs.
Moisture or heat stress can interfere with the synchronization of pollen shed and silk emergence, potentially delaying silk emergence until pollen shed is finished. High temperatures, low relative humidity, and inadequate soil moisture can cause silk desiccation and nonresponsiveness to pollen germination. During the 2 weeks following silking, severe stress may reduce yield up to 6% per day.2
Severe drought stress that continues into the blister (R2, 10 to 14 days after silking) and milk (R3, 18 to 22 days after silking) growth stages can abort developing kernels.2 Aborted kernels will be shrunken and mostly white, compared to fertilized kernels. Severe stress during dough and dent stages of grain fill can decrease kernel weight and premature black layer formation.
Figure 1. The ear shake test. Fertilization has not yet occurred in ovules that still have silks attached.
There are a couple of tests to determine the success of kernel fertilization. Randomly sample several ears in a field and carefully unwrap the ear husk leaves and then gently shake the ear. The silks from fertilized ovules will drop off ( Figure 1). The proportion (%) of silks dropping off the ear indicates the proportion of future kernels on an ear. The second technique is to wait until 10 days after fertilization of the ovules (kernels). The developing ovules will appear as watery blisters.
Moisture-stressed corn may be more susceptible to stalk rots.
1 Nielsen, R. 2007. Drought and heat stress effects on corn pollination. Purdue University. 2 Lauer, J. 2006. Concerns about drought as corn pollination begins. Agronomy Advice. University of Wisconsin. 160629200634