If your corn was frost bitten on April 11 or 12, the impact will certainly be noticeable. It may have leaf damage. Some stalks will be brown and completely wilted, and dead with no hope of recovery. Some stalks will be brown with some green leaf tissue beginning to emerge. How do you assess the damage, and determine whether there is a need to tear up the field and replant it? Big decisions in a year that has seed priced at more than $100 per acre, and no insurance coverage on it.

Many farmers knew their decision to plant in mid to late March carried risks, but with the opportunity to take advantage of early delivery premiums, it made sense. However, cold temperatures moved in and damaged many fields to varying degrees. 

University of Illinois agronomist and crop production specialist Emerson Nafziger says, “Nearly all of the corn planted before April 1 was emerged when frost occurred, and the earliest-planted corn was 5 to 6 inches tall with 3 leaves fully emerged.” But Nafziger said not every corn plant displayed expected indications of being frost-bitten, “As we have seen before following frost on corn plants this size, plants with severe leaf loss often were randomly spread down the row, singly or in groups of two or three, alternating with plants that appeared to have little injury. Plants near the grass border were all damaged, and out into the field, 20 to 30 percent of plants were damaged.”

The survival of the plant depends upon the location of the growing point within the plant. Generally, if it was below the ground the plant will survive. If it were above the ground, recovery chances are minimal. However, there are exceptions to about everything, and some corn did not survive because the growing point was below ground and the soil was dry and was a non-insulator. Nafziger says look for the white healthy tissue. In some cases, corn has been reluctant to grow because of cold temperature.

Looking back, Nafziger says a similar frost in 2005 indicates how plants that were not killed in the frost did not fully recover and become productive. That can be helpful in making replant decisions. He says, “Set April 1 as the original planting date if planting was earlier than that. According to the data, 30,000 plants from an April 1 planting will yield about 95 percent as much as 35,000 plants from an April 20 planting. If 5 percent more yield is needed to cover replanting costs, then having fewer than 30,000 plants left may justify replanting, if replanting can be done by the end of April. If it takes 10 percent higher yield to cover replanting costs, then keeping 25,000 (healthy) plants may produce as much profit as would replanting.”

“One major concern is that, as we saw in 2005, plants that suffered freeze damage at about the V3 stage may not produce as much yield as undamaged plants even if they recover green leaf area. Physiological damage, coupled with the current slow growth of roots and tops due to loss of leaf area, may irreparably decrease plants’ productive potential. Plants from the March 29 planting are already approaching the size of the less-damaged plants from the March 16 planting, and I expect the later planting to yield more per plant than the earlier one.”

One of the keys to good yield is an even stand which does not have some plants that will rob others of moisture and nutrients. But after the frost, uneven stands may be prevalent, because root growth will be at different rates.

Nafziger directs your attention to a table he has published in the corn chapter of the Illinois Agronomy Handbook. In table 2.3 on page 24 he evaluates planting date with final plant population to determine the percent of maximum yield. Yield reaches its maximum yield if planted on April 10 with a 35 to 40 thousand population. It can also be achieved on April 20 with a 35 thousand population. 

He says, “For example, corn planted at 35,000 per acre on April 25 with its plant stand reduced to 15,000 by cutworm injury would be expected to yield 71% of a normal stand. If such a field were replanted on May 19 to establish 35,000 plants per acre, the expected yield would be 86% of normal. Whether it would pay to replant such a field depends on whether the yield increase of 15 percentage points would repay the costs to replant. In this example, if replanting is delayed until early June, the yield increase to be gained from replanting disappears.” 

Frost damaged corn will have a variable survival rate, but some survivors will be unable to make much progress. Determining the population of viable plants and the date of any replanting will help make a replanting decision, since cost of new seed will be a significant issue.

Source: FarmGate blog