How does drought affect corn?

Jeff Coulter, Extension corn agronomist
University of Minnesota Extension
A crop scout holds a corn plant.

Many people are concerned about how the continued dry conditions and recent hot weather are affecting the corn crop. This article discusses how corn growth and development are influenced by dry and hot weather before, during, and after pollination.

Currently, most of the corn in Minnesota has finished pollination and its kernels are well into the blister stage of development. The most critical period for the formation of corn grain yield begins about 10 days before the start of silking and lasts for about 24 days. Drought and other stresses during this period will reduce corn grain yield more than at any other time.

From about the eight leaf collar stage through the 15 leaf collar stage (which is about eight days before the start of silking), the number of potential kernels per row is determined. Therefore, drought stress during this time can reduce potential ear length.

Drought stress during the week before tasseling that lasts through the silking stage can accelerate tassel development and pollen shed while delaying silk emergence. This can lead to pollen shed being completed before all of the silks have emerged and in poor kernel set, particularly at the tip of the ear because silks emerge last from potential kernels at the tip of the ear.

Corn pollen is not viable when there is low relative humidity and air temperatures around 95 degrees or higher, but pollen viability is not typically a concern because most pollen shed occurs in the morning under lower air temperatures. Combined drought and heat stress shortly after tassel emergence can cause exposed silks to dry quickly and be less receptive to pollen. The progress of pollination can be evaluated during and soon after pollination by carefully unwrapping husks and gently shaking ears, as silks detach from kernels within a couple days after successful fertilization. Potential kernels with silks attached have either not been fertilized or have been fertilized very recently.

Corn kernels enter the blister stage at about 10 to 12 days after the start of silking. The blister stage lasts about eight days, then kernels enter the milk stage, which lasts about six days. Stress during the blister and milk stages can cause kernels to dry out and be lost, especially at the tip of the ear because these kernels are the last to establish. Stress during the blister and milk stages can also reduce kernel size, but the reduction in kernel number due to stress during this period has a greater impact on grain yield. Once kernels enter the dough stage, the number of kernels per ear is set. Therefore, stress on corn during the dough through dent stages of kernel development reduces yield solely through a reduction in kernel size.

In addition to drought, there are many other potential stressors that can occur after pollination and reduce corn yield potential. Some of these include factors that reduce the rate and duration of canopy photosynthesis, such as hazy or cloudy skies. Additionally, high night air temperatures after pollination can reduce corn grain yield because this causes corn plants to require more carbohydrates from photosynthesis to maintain themselves, leaving fewer carbohydrates available for the developing kernels. High air temperatures can also increase the speed of crop development, resulting in a shorter grain-filling period, leading to smaller kernels.

Drought stress study

In 2013, with funding from the Minnesota Corn Research and Promotion Council, others and I conducted a drought stress study on corn at the University of Minnesota Sand Plain Research Farm at Becker in central Minnesota. The soil was loamy sand in the 0 to 18 inch depth, and sand in the 18 to 42 inch depth. Drip irrigation was used to impose drought stress at different times. There were three experiments located within 0.3 miles of each other. 

Each experiment contained 18 treatments, representing all combinations of two hybrids (standard and drought-tolerant hybrids) times three levels of sustained moderate drought stress (a well-watered treatment representing no drought stress, drought stress from the 14 leaf collar stage until maturity, and drought stress from the blister stage of kernel development until maturity) times three nitrogen fertilizer rates (50, 100, and 150% of the expected economically optimum rate). In the two treatments with sustained moderate drought stress, irrigation was managed to achieve daily leaf rolling beginning mid-day, except on days when there was rainfall or irrigation. Averaged across experiments and nitrogen fertilizer rates, the results were as follows.

Results

Yield was not significantly different between the two hybrids in the well-watered treatment (average = 210 bushels/acre) or when there was sustained moderate drought stress from the blister stage of kernel development through maturity (149 bushels/acre = 29% less than the well-watered treatment).

Compared to the well-watered treatment, sustained moderate drought stress from the 14 leaf collar stage until maturity reduced grain yield by 30% for the drought-tolerant hybrid and 36% for the standard hybrid.

The findings from this study provide an indication of the corn yields that could occur under different scenarios. However, much of the 2021 growing season still remains and many growers may not experience yield reductions at this level, especially if rainfall improves.