Control of Summer Weeds in Alfalfa

If you currently have a good alfalfa stand (more than 30 stems/sq. ft.), then you should not have any summer weed problems this year. Your alfalfa will essential shade-out and out-compete the emerged annual weeds and none will result in the hay. However, if your stand is getting thin, or you are not keeping it fertilized properly or not controlling insects, then expect summer weeds to start showing up in the third harvest. Your current summer weed management options are:  Do no weed control: This may be the most profitable option. The first and second cuttings will not have any summer weeds in them because the weeds are still small. Then, if it turns dry in July, there will not be that much growth of the summer annual weeds the rest of the summer. If weeds do grow, they can be utilized. Summer weeds in thinning alfalfa stands increase total production, and quality of weedy alfalfa hay can be good if harvested before weeds get too large. "Flash grazing" weedy alfalfa stands with stockers in the summer should also be considered. Some producers do this and report good gains with minimum bloat problems (see Alfalfa Harvest Management Discussion with Cost-Benefit Analysis. OSU Extension Circular E-943. HTML Format (faster) or PDF Format (Requires Adobe Acrobat Reader).  

Use herbicides for control of annual summer weeds: Pursuit^® applied after 1st or 2nd harvest can be very effective on broadleaf weeds. At 4 oz/A, small pigweeds and some other broadleaf weeds are usually controlled. However, it may only suppress some of the grasses at this rate. In our trials, we have had excellent season-long control of pigweeds with 4 oz/A applied after first harvest. However, grass control has been erratic. Should apply Pursuit^® at 4 oz/A on small emerged weeds (less than 3 inches tall) after the first or second harvest when alfalfa regrowth is about 6 inches tall. Pursuit^® has both postemergence and residual activity, so you can expect summer-long weed control of susceptible weeds. Also, be aware of rotational crop guidelines after using Pursuit^®. It is 4 months for wheat and 18 months for sorghum (see label for other crops).   

• Sinbar^® applied after 1st harvest gives fair to good control of pigweeds, but only fair control of summer grasses. For best result, it needs to be applied immediately after first harvest and receive within 1 week an inch of rain to incorporate it.   

• Gramoxone^® Extra is a contact nonresidual herbicide that can be used to control emerged weeds between harvest. Most of the alfalfa will be browned-off with application, so it has to be applied within 5 days of cutting to minimize damage. Small emerged pigweeds are killed, but emerged grasses are often only suppressed. Emergence of new weeds after spraying can also be a problem since there is not canopy of alfalfa to shade-out new weed seedings. Best use for Gramoxone^® Extra has been for dodder control after second cutting.   

• Butyrac^® 200(2,4-DB) is a nonresidual herbicide that can give good control of small emerged pigweed (less than 4 inches tall). Control of some other summer broadleaf weeds is only fair, so read the label for listing of susceptible broadleaf weeds.   

• Poast Plus^® should be applied between cuttings when the alfalfa regrowth is about 6 inches tall and there is enough emerged weedy grasses to merit control (1 to 2/sq. ft.).   

The first application for both Butyrac^® 200 and Poast Plus^® is normally made after the second cutting in mid-June, but could be after first cutting if first cutting is delayed to late May or early June. Since both of these herbicides have no residual, it will be necessary to scout the field for new emerged weeds after each harvest and respray if season-long weed control is desired.