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Corn Comments: Uneven Plant Height
Dale R. Hicks
Department of Agronomy and Plant Genetics, University of Minnesota
July 27, 2004
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.
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