Help for growers managing multiple herbicide resistant waterhemp

University of Guelph weed scientist Dr. Peter Sikkema.

Waterhemp that’s resistant to multiple herbicide groups continues to march across Ontario, but growers still have options to control the weed.

There’s also emerging proof that integrated management could help growers take the fight to the spreading yield robber, says University of Guelph weed scientist Dr. Peter Sikkema.

At the Ontario Agricultural Conference earlier this month, Sikkema noted that multiple herbicide resistant waterhemp was first found in 2014 on Walpole Island in Lambton County. At that time, the weed was resistant to Group 5 and Group 9 herbicides.

“We now have multiple herbicide resistant waterhemp from Essex County, adjacent to the Michigan border, to Glengarry County, adjacent to the Quebec border,” says Sikkema. What’s really concerning for Ontario farmers, he says, is the presence of  biotypes that have evolved resistance to five different herbicide modes of action, including Group 2, 5, 9, 14 and 27 herbicides.

“If you have five-way resistance on your farm, that probably eliminates 80 per cent of your soybean and corn herbicides. And so that makes it a far greater challenge for Ontario farmers to manage,” he adds.

In this interview with RealAgriculture’s Bernard Tobin, Sikkema comments on why the weed is becoming an increasing problem. One contributing factor is the trend toward reduced tillage, including strip-till and no-till, which favours small seeded broadleaf weeds like waterhemp. There’s also wide genetic diversity of individual weed plants that may have evolved resistance to these herbicides.

The emergence pattern of the weed also promotes resistance. The first waterhemp comes up in May and the plants continue to emerge throughout the season, as late as October. (Story continues after the video.)

When it comes to controlling waterhemp in corn and soybeans, Sikkema says growers need to be prepared to use a two-pass weed control program. Start with an effective soil-applied herbicide and then be prepared to apply a post-emergent herbicide, if needed. In the video, he shares his top herbicide picks based on research trials.

What can farmers do to reduce the likelihood of multiple resistant waterhemp gaining a foothold in their fields? Sikkema says growers can proactively introduce more diversity into both their crop and weed management programs. His to-do list includes: implementing a diverse crop rotation; using tillage at strategic points in the rotation; planting in narrow rows where possible, especially soybeans; planting cover crops after winter wheat combining; purchasing a combine with harvest weed seed control; and utilizing multiple herbicide modes of action.

Sikkema is hopeful that integrated weed management will play a key role in helping farmers control the resistant waterhemp. He shared early results from a nine-year study designed to assess the agronomic control methods, including different crop rotations. The goal of the study is to deplete waterhemp seeds in the seed bank by 95 per cent using weed management practices that can be implemented on most commercial farms in Ontario.

After the first three years of the study, the results are encouraging. “Where we use the the soybean-wheat rotation, the corn-soybean rotation, as well as the corn-soybean-wheat and cover crop after winter wheat combining, the number of seeds in the seed bank was reduced by about 60 per cent,” says Sikkema. At the study location at Cottam, Ont., using weed management tactics that almost every farmer in Ontario could employ without purchasing any new equipment, “we reduced the number of seeds in the seed bank from 165 million seeds per acre to 30 million seeds per acre,” he notes.


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