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The growing season continues to be abnormally cold and now the cool temperatures of August 21 have caused frost injury to crops in Minnesota. This report gives an assessment of what I think is the situation.

Geographic area affected. From the MN state line in southwest MN in Pipestone County, follow State Highway 30 east to Highway 15 and continue north to Baudette and back west to the MN border. There is some damage to crops east of Highway 15, particularly north of Mpls/St. Paul, but this boundary encompasses the majority of the injury in MN. The affected area also includes two tiers of counties in South Dakota west of Pipestone and north of Highway 30. Parts of North Dakota are probably also affected.

Damage. Damage ranges from all leaves killed on plants completely to the ground in some low areas to very little, if any injury, in other areas or parts of fields. The typical injury for soybeans is only the top leaves killed. For fields that were canopied over, only the top leaves are affected. For fields that were not canopied over, the leaf injury is on the sides and tops of the rows. Typical injury for corn is the top 2 to 4 leaves killed or partially killed.

Impact on yield. Soybean yields have probably not been affected by the leaf injury; insect feeding studies have shown that 20 to 30% leaf removal does not affect soybean yield. The dead leaves will fall from the plant in a few days and will not affect sunlight penetration into the soybean canopy. Of course there will be a major effect on yield in those areas of soybean fields where most or all of the leaves have been killed.

The impact on corn yields will be different and will be dependent on the percent of the leaf area killed and the stage of kernel development. One can use the Corn Hail Yield Loss charts to estimate the effect on grain yield by determining the stage of development and percent of leaf area killed. These values may underestimate the effect on yield because in the case of corn the dead leaves will hang on the plant and interfere with sunlight penetration and capture by lower green leaves that can continue with photosynthesis. A portion of the loss chart is given in Table 1 for those that want to estimate yield loss on specific fields. For example, a 30% leaf loss at the Late Milk kernel stage results in a 6% grain yield loss. And one could add slightly to this because the dead leaves interrupt sunlight interception and efficiency, the yield loss could be higher. The upper leaves are the most efficient for photosynthetic efficiency, but the lower leaves will drive growth if they receive solar energy and temperature!

Kernel test weight should not be affected by 40 to 50% leaf killing at the milk to soft dough growth stages because kernels will abort from the ear tip due to the leaf injury. However, test weight will be lowered on those plants with 80 to 100 percent leaf killing. If test weight is not affected, kernel quality should also be good.

Time or temperature. The continued cold season (lagging 190 to 450 Growing Degree Days from south to north) has raised the question about the corn grain filling period. Some say that the time between pollination and maturity is day dependent and requires 55-60 calendar days and that temperature doesn’t affect the number of days. The 55-60 calendar time period is based on corn growth in Iowa with average temperatures. We have known for some time that cool temperatures will extend the grain filling period and warmer than average temperatures (even without moisture stress) will shorten the grain filling time in days. This season lets us learn that lesson over again. Corn is almost “treading water” with the cool temperatures; maturity is progressing at a snails pace. With an average pollination date of July 25, we should have half of the corn at the soft dough stage or beyond.

What we need. We need continued and constant heat accumulation without the extreme low night temperatures. Both crops continue to lag in development because of the low Growing Degree Day accumulation. Both crops need four weeks of good growing weather to reach maturity.

Other concerns. Some are asking about corn silage harvesting and baling soybeans. It’s too early to consider those options because if September brings average to above average (optimist!) temperatures the crops would mature and produce reasonably good yields as grain.

Table 1. Grain yield loss for combinations of kernel development and percent of leaf area removed by hail (Source: National Crop Insurance Services).

¹Milk stage is prime roasting ear; Late Milk is solids forming in the base of the kernel; Soft Dough is kernel contents pasty or semi-solid; Early Dent is when there is denting along the ear and contents are thick and gummy.