Farm Facts for Fairfield County

for July 13 and 20, 2001; also seen on Fridays in the Lancaster Eagle Gazette

Late Postemergence Herbicide Applications in Soybeans

At this point in the growing season, emergence of new summer annual weeds should be nearing an end. It is still possible to observe emergence of foxtails, black nightshade, burcucumber, and a few other weeds, but this will be primarily in response to significant rainfall events or in fields that were planted late and the soybean canopy is not adequately shading the soil surface.

A well-developed soybean canopy allows soybeans to suppress and out compete most late-emerging weeds, making it difficult to justify a late postemergence herbicide application in most soybean fields. Mark Loux, OSU weed specialist, says one exception would be late-planted soybeans that are still relatively small, where weeds that emerged around the time of planting may have to be controlled. Loux suggests in soybeans that were planted by early June, the main purpose of a herbicide application at this time should be to prevent harvest problems, rather than reduce weed-crop competition.

Late-emerging foxtail, nightshade, and vines can interfere with harvest. The need for a herbicide application at this time to reduce problems at harvest is a judgement call based on the conditions in individual fields. If nightshade is a problem, keep in mind that preharvest application of Gramoxone or glyphosate will dessicate the foliage but will not cause berries to fall off plants. Fields with significant nightshade infestations may therefore be candidates for a herbicide application at this time.

And finally, remember that individuals with late planted soybeans should consider spraying very soon before the weeds get larger and more water-stressed at which time they will be harder to control. Herbicide injury to soybeans at this time of the season and later can cause significant yield reductions on small soybeans, so choose products and adjuvants wisely. When weeds are water-stressed, spray before the hottest part of the day and use the appropriate adjuvants to maximize weed control. If dry conditions persist only a single herbicide application should be necessary in late-planted soybeans.



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Farms Facts is a weekly newspaper column, written by the agriculture staff at the Ohio State University Extension office in Fairfield County.

Previous Issues of Farm Facts

updated on July 18, 2001 by Stan Smith
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