Glyphosate applied to seedling Roundup Ready alfalfa outperformed a popular conventional herbicide in a 2012 Wisconsin study, killing more weeds and delivering a higher first-cutting alfalfa yield.

Mark Renz, the University of Wisconsin Extension weed scientist who headed the research, was surprised by the herbicide’s performance considering the drought and other problems that plagued the plots.

“Glyphosate was the most consistent treatment by far, especially when applied to small weeds,” says Renz.

The study, funded in part by the Midwest Forage Association, compared Roundup Ready and conventional alfalfa establishment methods. The treatments, all in Roundup Ready alfalfa, included glyphosate applied when weeds were 2-4” tall or two weeks later when they were 5-8” tall, Raptor (imazamox) used at those same weed heights, and untreated control plots.

The plots were seeded in early spring at seven locations throughout the state. Lambsquarter was the primary weed problem at most sites, and infestations ranged from light to very heavy.

Results varied by location, but, overall, the early glyphosate application controlled 93% of the lambsquarter; the later application, 82%. The early and late Raptor treatments were 75% and 66% effective against the weed, respectively.

As expected, total first-cutting yield favored the untreated control plots. On average, they yielded 1.2 tons of dry matter per acre, but more than half was weeds. Early applied glyphosate plots yielded just under 1 ton/acre of nearly pure alfalfa, and the other herbicide treatments produced slightly higher total yields but less alfalfa (see graph).

The benefits of herbicide treatment were most obvious in plots with heavy weed infestations. At one site with few weeds, none of the herbicide treatments improved total yield or percent alfalfa in the harvested forage.

Second-cutting yields were statistically the same among treatments, although the untreated plots still had the highest percentage of weeds. Fall stand counts weren’t significantly different, either. All were lower than the recommended 15-20 plants/sq ft, probably due to the drought and a severe potato leafhopper infestation, says Renz. Fall stand counts usually are driven by factors other than weeds, he adds.

The harvested forage was tested for quality, but results aren’t available yet. Lambsquarter is nutritious if harvested early, but the quality drops before alfalfa reaches bud stage most years. The crop matured early in 2012, but Renz thinks the first cutting was taken a little too late for quality weed forage.

If weeds are controlled early in the seeding year, only one herbicide application may be needed throughout the alfalfa stand’s life, says Renz.

“Alfalfa is a very competitive crop. If we can keep that crop healthy – no insect damage or disease and proper fertility – we really just need to control the weeds in that first month, when they’re small.”

For growers who plant Roundup Ready alfalfa, that means the higher seed cost – principally Monsanto’s technology fee – has to be recouped in the seeding year’s first cutting. Glyphosate is cheaper than Raptor or Pursuit, the other post herbicide often used on conventional alfalfa, and both stunt the crop, resulting in a 5-20% first-cutting yield loss. Those factors help offset the higher Roundup Ready cost, saysRenz.

Another possible cost-cutting measure: Lower the seeding rate. Wisconsin growers typically plant 16 lbs of alfalfa seed per acre, but 12 lbs/acre are recommended by Dan Undersander, University of Wisconsin Extension forage specialist.

“You clearly can reduce that seeding rate below 12/lbs/acre and still get a really good stand, but with that reduction you run added risk,” cautions Renz.

In the study, both herbicides were applied at recommended rates. To enhance their effectiveness, glyphosate was applied with ammonium sulfate and Raptor with methylated seed oil. The glyphosate rate was ¾ lb of acid equivalent per acre, or 1 qt/acre of a formulation containing 3 lbs of active ingredient per gallon. Glyphosate products come in different concentrations, so the rate should be calculated based on acid equivalent, he advises.

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