Missouri farmer Ryan Britt is working to diversify his weed program, but “it’s very difficult,” especially in soybeans, he says.

Britt raises corn, soybeans, wheat, alfalfa and cattle with his father and brother-in-law near Salisbury, Mo. Like many Corn Belt farmers, the Britts are battling ALS- and glyphosate-resistant waterhemp and marestail. Adding to their challenges, the Britts’ no-till cropping system includes some soybeans after soybeans fields, which limits their ability to rotate herbicide sites of action on those acres.

In soybeans, they had been applying a one-third to one-half “set-up” rate of Authority XL with their burndown of glyphosate and 2,4-D. Recently, they’ve had to step up to the full residual rate before planting. The past two seasons, they’ve followed with a postemergent application of glyphosate plus a tank mix of Ultra Blazer or Marvel.

This program deploys four sites of action (SOA): groups 9, 4, 2 and 14. But only two groups — 4 (2,4-D) and 14 (Authority XL, UltraBlazer or Marvel) — are considered effective on herbicide-resistant waterhemp, the Britts’ primary target weed.

To beef up control in 2013, Britt had planned to apply a full rate of Authority XL with the burndown, followed by an early postemergent application of glyphosate plus Prefix (SOA 14, 15). That would have provided overlapping residual control of waterhemp, and an additional site of action, group 15.

But late planting scotched that plan. The Britts’ agronomist, Jason Young, Agrivision, Salisbury, Mo., suggests a fall application of Anthem (SOA 14, 15) to control fall-emerged glyphosate-resistant marestail and winter annuals.

This past season, Britt got decent control of resistant waterhemp. But he worries about the selection pressure being placed on the PPO inhibitors as a result of repeated use, both preemergence and postemergence, in consecutive years. “We’ll have to be very careful that we don’t get resistance.”

Some of this selection pressure could be relieved by rotating to a Liberty Link system and substituting glufosinate (SOA 10), Young says. But the selection of Liberty Link soybean genetics for Missouri is limited, he adds, and the herbicide is also in short supply. “We’ve not used Liberty Link technology yet,” Britt says, “but it’s an option.”

herbicide diversity needed