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Corn Comments

Dale R. Hicks, University of Minnesota

This growing season is rapidly moving along and it continues to be cold and wet. For most areas of the state the growing degree days (GDD's) are lagging by more than 100 and as high as 300 GDD's in some areas as of July 11. As a result, corn is shorter than it normally is at this calendar date and growth stage. Corn is ten inches shorter than the last 5-year average and 14 inches shorter than it was at this time last year. Will corn height catch up? And will short corn have good yields?

Why are plants shorter?

Plants grow in height as cells in the internodes enlarge. When the temperature is low, cells don't grow as big, so internodes are shorter and the plant is shorter compared with normal for the rest of the growing season. So plants will never catch up in height. Leaves will also be smaller than normal for the same reason. But the leaves on the corn plant now (the first 6 that develop) slough from the plant as the plant grows. And the next few leaves (7 to 10) are on the bottom of the plant after tasseling and do not contribute significantly to grain development, so leaf size for the leaves on the corn plant now should not affect grain yield. The upper leaves that are growing now in size are the ones that will have the greatest impact on yield.

New leaves are initiated from the growing point with about 3 ½ days between the initiations of each leaf. All of the leaves are initiated while the corn plants are very small. Cool temperatures extend the time interval between leaves. The growing point shifts from initiating leaves to tassel because of the night length which is not affected by the cool weather, so there may be 1 to 2 fewer leaves that develop on corn plants this year due to the cooler temperatures that we have already had. A plant with fewer leaves is another cause of shorter plants.

Can short plants yield as well as taller plants?

Earlier hybrids are shorter than full season hybrids and yield less, so we associate lower yields with smaller plants. Plant size is the "factory" for producing yield so one might expect higher yield potential from larger, taller plants. However, short plants of a given hybrid should yield comparable to taller plants of the same hybrid unless the height difference is caused by nutrient deficiency or excessive moisture. In this case, the height difference is due to shorter internodes rather than a major stress and should not affect yield. We need to remind ourselves that in planting date trials, the late-April planted corn is shorter than corn planted in May, but produces higher yields so the shorter corn plants we have should not reduce yield potential.

Will tasseling be delayed because of the cool weather and wet soils?

Night length initiates tassel development, which should not be affected, but tassel emergence from the whorl may be delayed by the cool weather due to slower cell growth. However, tasseling will be delayed a few days on those plants in the low areas where plant development has been slowed due to excessive moisture. Corn was planted early this so tasseling should be occurring between July 14-20 which is normal for most early planted corn of full season hybrids.

Has the cold hurt the corn and what happens if it remains cold?

Yield potential should be good if the rest of the growing season gives average temperatures. We have an early-planted crop with good populations and adequate soil moisture. If we have normal temperatures for the rest of July, August and September, we should produce a crop that is above average. But if cold weather continues, yields will be affected and maturity will be delayed. The impact on yield will depend on how much the temperature deviates from normal.
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