Farm Horizons, August 2012

Scouting for Roundup-resistant weeds

By Dave Schwartz, Certified crop advisor, Gold Country Seed

When walking fields this summer, I noticed fields with patches of waterhemp or giant ragweed. There is no question that some of these broadleaf weeds are becoming tolerant to Roundup.

I have been encouraging growers to develop a weed-control plan that, in addition to Roundup, incorporates a residual herbicide that provides good control of broadleaf weeds.

August is a good time to scout fields for weeds that are tolerant, or possibly resistant to Roundup.

Roundup-resistant weeds are becoming more common in corn and soybeans, so this is a problem we need to pay attention to, if we plan to use Roundup in the future as the foundation for a weed control program.

Roundup is a silver bullet for weed control, considering the number of weeds controlled and the tolerance crops have to Roundup.

When scouting for Roundup-resistant weeds, here are a few items to watch for:

• Look for patches of single weed species not controlled. This is the key point, so if you find a variety of weeds that survived, resistance is most likely not the cause.

• Following spraying, live plants are adjacent to dead plants of the same species.

• You notice a pattern of live plants in the field that look like they could have been spread by the combine.

• You notice over time a certain weed is becoming more difficult to control.

Once Roundup-resistant weeds become established on a farm, weed control costs soar, so use sound weed management principles. Dead weeds will not develop resistance, so use Roundup at full rates, apply on weeds no more than 4 inches in height, and use an additional residual herbicide that adds a second mode of action on both corn and soybeans.

Growers who used a residual herbicide this past year were much happier with their weed control than those who depended strictly on Roundup. Two weeds to pay special attention to are giant ragweed and waterhemp. It would be wise to pull and remove individual weeds that appear to be Roundup-resistant before they go to seed. One healthy waterhemp plant will produce at least 250,000 seeds. For the 2013 growing season, think seriously about using a residual herbicide for corn and soybeans.

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