Weigh pros, cons of spraying weedy fields

Due to a combination of conditions, some soybean fields aren’t as clean as usual, even with Roundup Ready varieties sprayed with glyphosate. Is this a year when it might pay to spray fields with green weeds early so harvest goes smoother?

The answer isn’t as simple as it sounds. Here are responses from this month’s panel of Indiana Certified Crop Advisers.

My soybean fields are weedier than usual. Will applying a dessicant to dry them up pay for itself?

Key Points

A dessicant could pay if weeds are so thick they will increase harvest losses.

Glyphosate and paraquat products have preharvest labels — read them carefully.

Applying a dessicant too early can affect soybeans and lower yield.


Andy Awald, Farm Fertilizers & Seeds Inc., Hamlet: It may pay for itself by increasing harvest efficiency or reducing dockage at the elevator. Usually, frost is a good enough dessicant, but some years the amount of green matter left when soybeans mature is too much to make harvest quick and easy.

Glyphosate and paraquat are among the most popular options. Glyphosate is broadest spectrum, but because it must translocate, timing could be an issue. You could expect up to 14 days before the weeds would be completely dried down enough to be beneficial. Glyphosate has a seven-day preharvest interval. Paraquat provides rather quick burndown and drydown, but with some tough annuals and perennials you may get an erratic control pattern.

Paraquat requires excellent coverage for the best control, so depending upon how thick the weeds are, it may not be the best choice. And despite the quick activity, the label requires a 15-day preharvest interval, so you will essentially be in the same time frame for harvest as a glyphosate application. Make sure your soybeans are physiologically mature before applying any dessicant. Spraying them too early could alter the plant’s maturity, resulting in decreasing yields.

Steve Gauck, Beck’s Hybrids, Westport: In most situations, applying a dessicant just helps with ease of harvest and getting a combine through the field. You would have to look hard to find products that are labeled for late application to clean up weedy fields. Do not spray until soybeans have lost all their green color.If you spray too early, you will reduce bean weight and lower yield.

David Taylor, Harvest Land Co-op, Portland: According to the University of Kentucky, weeds present at harvest can increase harvest losses by 5% to 7%, causing threshing problems and increasing losses at the header. Reducing the 5% to 7% harvest loss should allow the application to pay for itself. A 2- to 3-bushel-per-acre yield enhancement at $12 per bushel for soybeans adds up quickly.

Preharvest herbicides with paraquat should be applied only when 65% of the pods are brown, or when seed moisture is 30% or less. Other products such as glyphosate also have preharvest labels. Remember that these herbicides vary greatly in efficacy across different weed species, meaning results may be variable. Proper application techniques should be followed. Read the label, and use correct nozzles, the right carrier rates and recommended surfactants for best results.

This article published in the September, 2011 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.