Growers in warmer parts of Arizona may be able justify the higher seed cost of Roundup Ready alfalfa through higher-quality hay and extended stand life, say two University of Arizona experts. But weed scientist William McClosky and Lydia Brown, integrated pest management assistant, are less optimistic about economic returns for growers in cooler parts of the state.

They point out that growers in cooler areas frequently don’t need to use herbicides after establishment, because cutting and alfalfa competition suppress weeds. In warmer areas, though, summer temperatures are higher-than-optimum for alfalfa growth, and weeds like purple nutsedge and barnyardgrass compete with the crop.

McClosky and Brown say weed control during stand establishment is critical to assure a uniform, high-density distribution of alfalfa plants that will maximize yield and quality. Due to the polyploid genetics of alfalfa, 3-7% of seedlings will not contain the Roundup Ready gene. For weed control and to remove non-Roundup Ready plants, the best time to apply glyphosate is at the three- to four-trifoliate leaf stage. Apply 0.75-1.5 lbs active ingredient per acre. Their field research showed that this early application was sufficient to provide weed control up to the first cutting in the spring following fall planting.

A second application may be made up to five days before the first cutting. Sequential applications should be at least seven days apart. The combined total of all in-crop glyphosate applications per calendar year may not exceed 4.64 lbs active ingredient per acre. Potential per-acre rates and application numbers are: five applications at 0.93 lbs active ingredient, four applications at 1.16 lbs active ingredient and three applications at 1.54 lbs active ingredient, or 26, 33 and 44 fluid ounces an acre of a 4.5-lb active-ingredient-per-gallon glyphosate formulation.

Research in western Arizona on purple nutsedge found that multiple applications at 1.16 lbs active ingredient per acre provide better control than fewer applications at a 1.54-lb application rate. Often it is advantageous to use other post-emergence and pre-emergence herbicides to manage weeds in alfalfa, say McClosky and Brown.

As an alfalfa stand ages and density declines due to natural self-thinning, devote more attention to weed management to maintain quality, they advise.