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Issue No. 3, Article 10/April 20, 2012

Will Frosted Corn Recover?

Temperatures fell into the upper 20s on one or more days during the week of April 16 over most of central and northern Illinois. The official low temperatures at Urbana were 29 on April 11 and 31 on April 12, but temperatures lower than this were reported by other observers.

According to official records, 1 percent of the Illinois corn crop was planted by March 25 and 5 percent by April 1. The corn that we planted at Urbana on March 16 was approaching the V3 stage by April 11, and the corn planted on March 29 was up and approaching the V1 stage. Based on this, we can safely conclude that 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.

By Friday, April 13, severe damage was visible on the early-planted corn, with most or all above-ground leaf area dead. 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.

Frost injury on corn planted March 16, 2012. Photo was taken on April 13 following temperatures in the upper 20s on April 11.

When corn suffers frost injury at this stage, we normally assure ourselves that, with the growing point protected beneath the soil surface, the potential for regrowth back into normal plants is high. One diagnostic is to see if the growing point--the tip of the stem at the base of the plant--remains white and healthy-looking. This appeared to be the case in damaged plants on April 13.

Growing point on a plant injured by frost on April 11. Photo was taken on April 13.

Temperatures have remained relatively cool since the frost, and it rained on April 13 and 15, totaling about an inch. By the morning of April 16, some of the badly damaged plants were showing some regrowth of green leaf area. Others showed no regrowth and will probably not survive.

Regrowth of corn leaf tissue 5 days after a frost event.

How can cold air temperatures damage a growing point some 3/4-inch deep in the soil? Soils were dry when the low temperatures occurred, and dry soils have low heat-holding capacity, so they cool quickly. Dry soils also let cold air infiltrate the soil more easily. But the likely cause of plant death is that, with seed reserves gone and plants dependent on photosynthesis, plants without leaves simply ran out of sugars that they need to stay alive.

This situation is similar to what we saw in 2005, when temperatures reached the upper 20s during the first few days of May. Corn planted in early April in 2005 was in the V2-V3 stage, and just as we saw this year, plants were randomly damaged or killed down the row. In that event, about a third of the plants died, and the rest regrew slowly as it stayed cool and dry for some time after the frost.

Corn planted in 2005 on March 31 and damaged by frost on May 3 and 4. Photo was taken on or about May 13.

It's risky to draw parallels between the 2005 event and this one, but if this year's crop responds like that one did, the damaged plants may fail to recover fully, and yields may suffer even if plants stay alive. In 2005, the frost-damaged crop was part of a planting date study with different plant populations. In that dry year, 25,000 plants per acre produced more grain than did higher populations. Thinning to 25,000 plants removed some of the damaged plants, but even so, the yield from the March 31 planting was only 58 bushels per acre, compared with 130 bushels from the April 19 planting and 140 bushels from the May 9 planting. So plants that survived the frost yielded less than half as much as undamaged plants from later plantings.

While we hope for better recovery of the crop this year, there's no way to predict it. We do have the advantage that it's still early enough to allow watching the crop for another week or so before deciding whether to replant. The first task as you wait is to count the remaining plants. If by April 20 or so there is little green leaf area on a plant, it's probably best not to count it.

Once you know the count of healthy plants, you can use replant data (Illinois Agronomy Handbook, Table 2.3) to estimate how much yield loss to expect due to reduced stand. 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.

Another factor likely to reduce productivity of the frost-damaged crop is the unevenness in plant size. This will result from the uneven damage of plants down the row, and the slow regrowth of more-damaged plants will increase the size discrepancy over the next weeks. Roots will also grow slowly because damaged plants suffer from lack of sugars. In general, any unevenness in plant size causes some yield loss, but when the smaller plants are also damaged plants, they may well contribute little or nothing to final yield.

Besides assessing stands and the state of the remaining plants, this would be the time to see if replant seed can be lined up. As I noted earlier, the seed supply is not great this year, so finding replant seed of good hybrids may not be easy. It also may be expensive, depending on seed company policy on replant seed price when the first planting is as early as it was this year. Still, if we continue to see damaged plants struggle to regrow, and if the number of plants we count as healthy actually declines, then replanting may be the more profitable choice.--Emerson Nafziger

Emerson Nafziger

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