June 8, 2007

The following article by Kraig Roozeboom, Crop Production and Cropping Systems Specialist at Kansas State University, is reprinted with the author's permission from the June 1, 2007, issue of K-State Agronomy e-Update.

Figure 1. The relationship between estimated yield loss and percent leaf area loss at different growth stages of corn (based on data from Corn Loss Adjustment Standards Handbook, USDA-FCIC-25080, pp. 7-98.)

Hail damage always makes corn look bad, and can make for some sleepless nights. But while the physical damage is apparent, the actual effect on yield is not as obvious. Potential corn yield losses from hail gradually increase as the crop gets more mature, up to the silk stage, when peak yield loss occurs. After silking, yield losses from hail damage normally decline.

There are several reasons for this decline, all based on the growth and development of the corn plant.

Emergence until stem elongation (VE to V5). Up through the 5-leaf stage of growth, the growing point of corn is below the soil surface. At the worst, hail damage would remove all five leaves, but typically not damage the growing point. A corn plant has 24 to 26 leaves at tasseling. If the plant loses five of those leaves early on, it will still have the potential to have 19 to 21 leaves at tasseling. Yield will be reduced but much less than one might expect from the appearance of the plant.

Figure 2. Yield loss from three levels of stand loss at three growth stages.

Figure 3. Ear number at three levels of stand loss at three growth stages.

Figure 4. Ear size at three levels of stand loss at three growth stages.

Stem elongation to tassel (V6 to VT). The growing point begins extending aboveground by the 6-leaf stage, although it is still protected by several layers of leaves and sheaths. The number of rows that will be in the ear is established by the 12-leaf stage. Stress during V8 to V11 can reduce row number. The number of kernels per row is not determined until about V17, just before tasseling. Hail damage and loss of leaf area during these stages of growth can cause increasing potential for yield loss. Hail can also cause stalk bruising during these stages of growth, but it is hard to determine the amount of damage from stalk bruising until later in the season.

Tassel to maturity (VT to R6). At VT to R1 (tassel to silk), the corn plant is more vulnerable to hail damage than at any other stage. The tassel and all leaves are exposed at that time. No more leaves will be developed, and the corn cannot replace a damaged tassel. Furthermore, the stalk is exposed, with only one layer of leaf sheath protecting it. Unlike wheat, corn cannot fill from the stem if leaves are lost at this stage of growth. The six to eight leaves above the ear are the most important, and provide most of the grain fill.

The four-week period centered around silking is critical to corn, and not only in regard to hail damage. Drought stress, excessive moisture, extreme heat, diseases, and even high winds can all stress the plant at this critical stage of growth and reduce yields. Early in this period, stress can reduce kernel number by limiting potential ear size. Stress right at silking can reduce the number of kernels fertilized. And stress just after silking can cause fertilized kernels to abort.

Effect of Stand Loss on Corn Yields

Stand loss usually causes relatively little yield loss, at least at populations greater than 24,000. The amount of yield reduction from stand loss depends on the stage to growth.

Research by Barney Gordon at the North Central Experiment Field evaluated the effect of stand loss on yields, ear number, and ear size by removing plants at various stages of growth.

This study showed that:

• Yield loss was much less than stand loss at every growth stage.
• Yield loss increased with stand losses that occurred at later growth stages.
• Ear number per plant increased slightly at 50% stand loss, and more at 75% stand loss.
• Seed weight did not change as a result of stand loss (data not shown)
• Ear size increased with greater stand loss at early stages of growth.

When considering replanting due to poor stands, keep in mind that planting corn in early June in much of Kansas can result in yield losses of up to 50% compared to a typical planting date. Based on the data above, it would probably be better to keep an existing stand even with as much as 50% stand loss than to replant in early June. Of course, much depends on the uniformity of the remaining stand and the weather for the rest of the growing season.

Kraig Roozeboom
Crop Production and Cropping Systems Specialist
Kansas State University