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Pre-harvest Weed Control in Small Grains Revisited

July 20, 2020

U of MN  Extension

Wet and windy weather has made for weed control challenges in small grains. Combine this with poor stands, poor tillering, and a shorter than normal crop, we will likely be faced with escaped weeds causing harvest challenges. You really only have two options if the weed pressure is so high that timely and efficient harvest becomes a challenge; swathing or herbicides.

Swathing is still a reliable option to quickly dry down weed biomass and get a crop harvest-ready.  Swathing is becoming somewhat of a lost art. A few pointers on how and when to swath can be found here.  Preharvest glyphosate has by far been the approach of choice in recent history to improve harvestability of weedy crops.

However, the increased frequency of glyphosate-resistant weeds makes this no longer a sure bet. If weed escapes include glyphosate-resistant biotypes of waterhemp, giant ragweed, common ragweed, or kochia, it is likely that pre-harvest glyphosate will not be sufficient to desiccate weed escapes. In these situations consider tankmix partners or alternatives that can partially offset the limitations of glyphosate, including 2,4-D, dicamba, flumioxazin (Valor), and saflufenacil (Sharpen).

The most common tank-mix partner for glyphosate is 2,4-D.  2,4-D can be used at 0.75 lb ai/A.  Extreme caution should be used when considering dicamba due to volatility issues during high temperatures.  Sharpen probably provides the quickest burndown of the large broadleaf weeds, other than Canada thistle and other members of the Asteraceae family. Sharpen should NOT be applied to seed production fields, however, and the maximum allowable residue levels in the export market are subject to change (as they are for glyphosate residues).  Also, caution should be used if applying Sharpen to fields that will be planted to sugarbeets next year.  Therefore, it is paramount to follow all label restrictions.

Remember, pre-harvest herbicide applications cannot work miracles.  Depending on the weather, burn-down applications may be slower than desirable.

Source: U of M – MN Crop News

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