Source: University of Minnesota Extension

This year’s early and dry spring has facilitated some of the earliest corn planting dates of all time in Minnesota. This early start to the growing season should allow the corn crop to pollinate earlier than normal. Early corn planting also allows kernels to fill when days are longer, reducing the risk of injury due to an autumn freeze before crop maturity and allowing increased time for grain dry-down prior to harvest.

Assessing Frost Injury

Frost damage can occur when air temperatures are in the mid 30s on calm nights, as the lack of wind allows the transfer of heat from air near the ground to the air above, resulting in colder temperatures near the soil surface. In general, frost damage tends to be worse in low areas where cold dense air settles, near field edges where vegetation reduces the potential for heat transfer from the soil to the air above, and in fields where high levels of surface residue coverage limit heat transfer from soil.

Symptoms of frost injury to corn are initially discolored water-soaked leaves, which later dry and turn brown. Since the growing point of the corn plant remains below the soil until the fifth to sixth leaf-collar stage, frost prior to this stage typically does not kill the plant unless temperatures are low enough to freeze the upper part of the soil where the growing point is located. 

Frost-damaged corn plants generally show new leaf growth a few days after the frost if their growing point was not damaged, so assessment of damaged fields should be delayed until 3-5 days after a frost. Buggy-whipped plants generally break free and recover following new vegetative growth, and is influenced by air temperature, wind and the size of the plants when damaged (smaller plants tend to recover more quickly than larger plants, especially when warm and windy conditions occur after frost injury).

To determine whether frost-damaged corn will survive, dig up plants and split stems to examine the growing point and the tissue directly above the growing point. Healthy growing points will be firm and white to yellow in color. If the growing point or plant tissue within 1/2 inch above the growing point is damaged, it will be watery and orange to brown in color, and the plant will not likely recover. 

In general, crop recovery tends to be greatest when frost occurs before the third leaf-collar stage or when only a limited amount of leaf area is damaged after the third leaf-collar stage, since recovery is influenced in part by the amount of energy reserves in the seed and leaf area for growth. 

Yield Reductions Due to Frost

Yield loss due to early-season frost damage is influenced by the reduction in plant population and the severity of plant damage. In Minnesota, growers can expect yield losses of 5%, 12% and 24% when the final plant population is reduced to 28,000, 22,000 and 16,000 plants per acre, respectively. 

In addition to reductions in plant population, growers should consider the severity of frost damage on remaining plants. Research from Wisconsin found that yield was reduced by 8% when all corn plants were cut off at the soil surface at the second leaf-collar stage, but yield reductions were minimal when only half of the plants were cut off. In addition to yield losses, frost-damaged plants may reach maturity a few days later than normal.

Replanting Considerations

Before replanting, growers should consider the yield potential of the existing crop, replanting costs and the yield potential of a replanted crop. Replant costs include time, fuel, seed costs and penalties associated with hybrid selection when the best genetics are no longer available. 

Yield potential of a replanted crop will also be influenced by planting date. Corn planting date studies from 1988-2003 conducted at the University of Minnesota Southwest Research and Outreach Center in Lamberton show that on average, grain yield was reduced by 5% and 18% when corn planting was delayed until May 15 and May 30, respectively. If replanting, growers should also consider the length of the growing season that remains and select hybrids of appropriate maturity. 

The number of growing degree days available for corn production for various planting dates and locations in Minnesota, along with the relationship between growing degree days required for crop maturity and hybrid relative maturity is available in the article, Selecting corn hybrids for grain production.