Glyphosate-tolerant alfalfa (Roundup Ready alfalfa) is genetically engineered to tolerate over-the-top applications of glyphosate. Roundup Ready alfalfa allows growers a weed management option for many difficult-to-control weeds. Glyphosate controls most winter and summer annual weeds associated with alfalfa and suppresses or controls some problematic perennial weeds such as bermudagrass, nutsedge, quackgrass, and dandelion that are not currently well controlled with conventional herbicide systems.
While glyphosate controls a broader weed spectrum than the conventional herbicides used on alfalfa, it does not have residual control. Therefore, it is important to either tank-mix glyphosate with a soil residual herbicide or spray glyphosate after weeds have germinated and the alfalfa is large enough to suppress further weed germination. On the other hand, do not delay glyphosate applications so long that weeds become large and difficult to control.
Generally, the best time to spray seedling alfalfa is when the alfalfa is between the three- and six-trifoliolate leaf stage. With later applications, weeds may be too large for adequate control and alfalfa vigor may already be affected. Sometimes it may be necessary to make two applications of glyphosate during alfalfa stand establishment if a soil residual herbicide is not used.
Glyphosate can also be used for winter weed control and between cuttings to control summer annual grasses, such as yellow and green foxtail (Setaria spp.) and barnyardgrass (Echinochloa crus-galli). The number of applications depends on the geographic area and the infestation level. The overuse of glyphosate can lead to shifts of tolerant weeds with poor control and or glyphosate-resistant weeds developing.
For more information on glyphosate resistance, see Herbicide resistance: Glyphosate.