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Plant height varies drastically in many cornfields in Minnesota. This has been the case for the past six weeks and is especially noticeable in fields where corn follows corn even though corn following soybean is also extremely variable in height. In some fields the plants are taller in the tractor tracks and tassels are coming out from those plants but not yet visible from other plants. This makes an interesting picture in many cornfields.

Why the height differences? Soil temperatures have been low for an extended time period because of soil moisture and lower than normal air temperatures. When temperatures are low, small differences in temperature where roots are growing make a major difference in plant growth. There is less water in the upper soil surface where the tractor tracks are and soil warms quicker when spaces in the soil are filled with air rather than water because it takes more calories to warm water than it does to warm air. As a result, the soil warms more quickly in those places with less water that has to warm up as the soil warms. Uneven distribution of residue from last year’s crop also has an effect on soil temperature; the residue is a mulch, which slows soil warming. The combination of residue placement and differential water content in the surface soil pores has caused differences in soil temperature and resulted in differential crop growth rates and plant height. As a result, plants are taller and greener in those places in the field where soil temperature was higher, even though only slightly higher.

Will the field even out? The shorter plants will not reach the same height as the taller plants. But, as tassels get out completely, the height differences will not be as visible and the field will look more uniform.

Will this affect yield? Grain yield should not be appreciably affected by the plant height differences. When all plants are the same height, light penetration into the corn canopy is restricted to the upper six to seven leaves. With the uneven heights of corn plants, leaves will not be uniformly at the same height in the cornfield, which should allow greater light penetration to lower levels into the corn canopy and, in effect, having more leaves with the opportunity to intercept sunlight. The plant height differences create differential competition among corn plants. As a result, the taller plants should produce more grain than the shorter plants, but the field total should not be significantly affected.

Uniform plants in a field are more pleasing to the eye and some of the later planted corn has that uniform nice look to it, but the yield potential is lower due to later planting. The early planted, non uniform corn may have the yield potential reduced because of the cold weather delay in pollination and shifting the grain filling later, but it has the highest yield potential for this growing season.