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Glyphosate for Pre-Harvest Drydown

Late-season rains this year resulted in late germinating weeds. Herbicide options were not available to control these weeds because of the growth stage of the crops. To avoid problems with green material during harvest and to reduce the risk of increased weed issues in the future, many producers may consider applying a pre-harvest desiccant to wheat and pea crops. Paul Johnson, SDSU Extension Weed Science Coordinator, noted in a recent iGrow article that some of the dessicants, specifically, glyphosate are not recommended unless the grain is below 30% moisture. This timing often occurs about 7-14 days prior to harvest. This timing corresponds to the hard dough stage in small grains and 75-80% brown pods in field peas.

A primary concern with this sort of application is the effect on seed production. For example, researchers at Washington State University applied glyphosate at the milk, soft dough and hard dough stages of spring wheat (Fenish and Young, 2000). The results showed that wheat yield was affected only during the milk stage application; ranging from 20-77% yield reduction. Moreover, planting seeds from the milk stage treatment the following year, the authors reported seed yield reductions of 12-97% compared to unaffected seed, likely due to poor germination. Seeds from the hard dough stage treatment did not show any quality effects.

Likewise, researchers (Mirza et al., 2003) found that below 30% moisture, seed viability was not affected. However, at moisture contents above 40%, they found a significant reduction in seed viability. In practice, however, these results lead to a general recommendation of not applying a glyphosate as a pre-harvest aid for crops used for seed because of unequal dry down in the crop. Where weed pressure is high, it is likely that seed moisture is extremely variable and would decrease seed production.

There are various other products used as desiccants, with similar timing methods and restrictions on seed production and grazing. Moisture content is currently the best indicator to guide in the use of such methods. With any product, however, there is an associated risk that must be taken into consideration. Restrictions will apply and will be addressed by the product label. These risks must be balanced against the necessity for weed control, the cost of potential yield loss, residue carryover and the cost of application.

Herbicide products labelled for use as preharvest dissicants in wheat are listed in the 2015 Pest Management Guide for Wheat.

Source: Christopher Graham, South Dakota State University 

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