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Angie Peltier

Angie Peltier
Former Extension Educator, Commercial Agriculture

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Hill and Furrow

Current topics about crop production in Western Illinois, including field crops research at the NWIARDC in Monmouth.
Figure. Large volunteer corn in Warren County, IL, June 2012.
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Huge weeds in Western IL and why waiting to spray may rob yields

Posted by Angie Peltier - Weeds

In my daily commute between Galesburg and Monmouth, I have noticed many fields with high populations of very large weeds (Figure). The majority of these fields have either volunteer glyphosate-resistant corn or broadleaf weeds. Many of the weeds are knee high (~24 in.). Other fields are completely clean of weeds and others have obviously applied a post-emergence herbicide with some weeds escaping.

Dr. Aaron Hager, University of Illinois Weed Scientist, explained in a recent phone conversation why waiting to spray may be risking yield potential. Weeds compete with crop plants by robbing the soil of both water and nutrients and intercepting sunlight. This competition for moisture and nutrients is particularly fierce in dry years like 2012.

Some producers would like to take care of all of their weeds in one spray. Unfortunately weed biology makes this a risky policy. Not all weeds emerge at the same time because of their growth habits and all weeds are influenced by soil and air temperature and soil moisture. Due to unseasonably warm, dry soil conditions this year some producers began working the soil in March but did not rework the soil before planting. This interval allowed some weeds to become established earlier than in years past and it is those early established weeds that have the greatest effect on yield potential. Trying to kill all birds with one stone (or all weeds with one spray) may cost you more money in lost yields than would two or three sprays throughout the season.

Weed science research has shown that there is a point in the development of both weeds and crops at which measurable yield loss will occur. This yield loss is irreversible. The curve used to develop this yield loss threshold varies from year to year depending upon environmental variables and weed composition. In general, the recommendation is to spray for weed control 3 to 4 weeks after soybean emergence. In drier years such as 2012, this interval would be closer to 3 weeks after emergence. The longer one waits to spray for weeds, the longer the weeds have to develop a thick protective cuticle which serves to both protect leaves from water loss and protect leaves from herbicides trying to penetrate the leaf surface.

The program, developed and housed through the University of Nebraska, Department of Weed Science, is a great resource. This website has several tools including a Yield Loss Calculator, Tank Mix Calculator, BurnDown Analyzer, and Web Advisor. The Yield Loss can be used to estimate yield loss using information specific to your field. You input data including your crop, row spacing, growth stage, the yields that you would expect without weeds, and crop price. You also enter information on up to 7 weed species in a particular field including density and height. This is a useful tool to help estimate the yield cost in delaying spraying.

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