Does Roundup need a partner on RR beans? Manufacturers of Roundup and other glyphosate products tell farmers that they don't need to add soil-applied residual partners to their Roundup Ready (RR) soybeans' dance cards. Yet manufacturers of residual herbicides say their products can be used to improve weed control and yields.

Both can be right. It depends largely on the tillage system used, along with the weed spectrum and weed pressure, say herbicide experts.

A two-year University of Illinois (U of I) trial looked at RR soybeans in 7" drilled rows and 30" rows using conventional tillage. Researchers compared treatments that included a full rate of pre-emergence (residual) herbicide, a reduced rate of pre-emergence herbicide plus early Roundup application, no pre-emergence herbicide but early Roundup, no pre but late Roundup, and no pre but early and late Roundup.

"The best net return - yield less herbicide cost - by $10 per acre was the treatment that had no pre-emergence residual herbicide and had Roundup applied at a 24-oz rate when weeds were 4" tall," reports U of I agronomist Lyle Paul. "That was true for both 7" and 30" rows, although the 7" rows showed a significantly greater profit than 30" rows due to higher yields."

University of Missouri agronomists did a two-year study with no-till soybeans. They compared half and full rates of Canopy XL plus 1 pint of Roundup as a burndown (with residual) followed by 1 or 2 pints of Roundup when broadleaf weeds were 2" or 4" tall. They also looked at Roundup alone as a burndown followed by 2 pints of Roundup when broadleaf weeds were 4" tall. And at no burndown but 1.5 pints of Roundup when broadleaf weeds were 2" tall followed by 1 pint of Roundup on 2" broadleaf weed regrowth.

The highest weed control rating (99%), as well as the highest yield (98% of hand-weeded checks) came with the half rate of Canopy XL-plus-Roundup burndown followed by 2 pints of Roundup when broadleaf weeds were 4" tall. The treatment with no burndown showed 93% weed control and the lowest yield (92%).

Bill Norden, certified crop adviser with TWAg, Walnut, IL, deals with both tilled and no-till RR soybeans when making herbicide recommendations.

"We generally suggest a burndown with residual on nearly all Roundup Ready beans, regardless of the tillage system," Norden reports. "We use Domain, Canopy XL or Roundup Ultra plus 2,4-D and either Prowl or FirstRate, depending on the weed spectrum. All those products have different modes of action from Roundup and can help control those weeds where Roundup has limitations.

"The residual also provides a wider window of application for the Roundup, and that can be a big help if there are weather delays."

Norden says, however, that some customers who till get by fine with a single application of Roundup when weeds are 4-6" tall. "It depends on the weeds and the amount of pressure," he points out.