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Using
the Economic Threshold Concept as a Determinant for Velvetleaf
Control in Field Corn.
E.L.
Werner and W.S. Curran, 1995. Proc. NEWSS 49:23.
Soil applied herbicides have been
the emphasis of corn weed control programs throughout the region.
The foundation of these preemergence herbicide programs have been
products such as atrazine and alachlor, however, these herbicide
programs are closely related to resistant weed populations and
the water quality issues. An alternative to treating an entire
field with a soil applied herbicide would be the "wait and see
approach" treating only the infested areas postemergence. A planned
postemergence herbicide program based on the economic threshold
concept offers several advantages over more traditional programs.
Postemergence herbicide performance is not affected by soil texture
and organic matter, so the problem of matching herbicide rate
to soil type is eliminated. Foliar applied herbicides do not require
a timely rainfall to move them to the site of activity, and these
products are less affected by tillage and residue which is especially
important in reduced or no-till agricultural systems.
Field studies were conducted in central
and southeastern Pennsylvania for the 1993 and 1994 growing seasons
to examine the effect of increasing velvetleaf density on corn
and velvetleaf growth and yield. Velvetleaf densities ranging
from 0 to 96 plants per 20 ft row were established in field plots
measuring 10 feet wide by 15 feet long with each plot containing
24 subplots measuring 30 by 30 inches. Parameters measured included
corn and velvetleaf height, velvetleaf leaf area, velvetleaf aboveground
biomass, corn silage and grain yield, and velvetleaf seed production.
At the southeastern site corn silage
yield was reduced 20 to 36% as velvetleaf density increased from
8 to 96 plants in 1993, and a reduction of 12 to 29% in 1994.
Grain yields in 1993 indicate a 9 to 25% loss as velvetleaf density
increased from 8 to 96 plants and a 2 to 30% loss in 1994. There
was no significant corn yield response from velvetleaf density
at the central PA site.
An attempt was made at establishing
an economic threshold by incorporating local corn grain prices
and yield loss data from the two study seasons. Based on local
corn grain prices an economic threshold of 3.4 plants per 20 ft
of row was determined for 1993 and 6.9 plants for 1994. The difference
in the number of plants tolerated for each study season is probably
due to environmental factors. Corn was less competitive with the
velvetleaf in the dry 1993 season, therefore was not able to tolerate
as many velvetleaf plants compared to the 1994 season which had
adequate moisture.
This study and others like it have
shown variability from year to year of yield loss caused by velvetleaf
and other weed species. Unless factors such as rainfall can be
predicted for individual growing seasons on a local basis there
is no definite way of knowing how an infestation will impact corn
yield. However, information from this study and others like it
combined with experience can be a useful tool in planning a control
strategy.
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