Warm-season Weeds in Established Alfalfa

Warm-season Weeds in Established Alfalfa

If you had summer weeds (pigweeds or weedy grasses) in your hay last summer, you will probably have them again this year. Most of these summer weeds will germinate after each cutting and will start having enough growth to show up in the hay by the 3^rd harvest if your alfalfa stand in not competitive enough to shade-out the weeds. This usually results in thinning stands (less than 30-alfalfa stems/sq. ft.), but can also result if alfalfa is not fertilized correctly and/or insect pest are not controlled.

Many alfalfa fields already have warm-season weedy grasses and pigweeds seedlings in them by May or early June. So, if you had a summer weed problem last year and have already taken your second cutting, you need to scout your fields now and make some quick decisions.

Summer weed control options include the following:

Do no weed control: It is very difficult to make a profit on controlling summer weeds for the following reasons:
A) Full stands of correctly managed alfalfa will out-compete the annual weeds, so no reason to spray these fields;
B) If it turns dry in July, there will not be that much growth of the annual weeds the rest of the summer;
C) Having weeds in thinning stands will increase the amount of hay, and a mixture of alfalfa-grass can be good hay for beef cattle; and
D) Herbicide treatments for control of summer weeds are expensive because it usually requires multiple treatments after each harvest, and/or some combination of herbicides.

Consider grazing: Producers should consider utilizing weedy alfalfa in the summer by grazing. Weeds in alfalfa minimize bloat problems, and good gains can be obtained by grazing (see Alfalfa Harvest Management Circular E-943).

Herbicide options: PURSUIT is active on broadleaf weeds. At 4 oz/A applied after 1^st or 2^nd harvest, it will control small pigweeds and some other broadleaf weeds, plus give summer-long residual control. However, grass control has been erratic. Be aware of rotational crop guidelines after PURSUIT. It is 4 months for wheat and 18 months for sorghum (see label for other crops).

SINBAR gives fair to good control of pigweeds when applied after 1^st harvest or after 2^nd harvest if pigweeds are still small (less 1 inch tall). However, grass control has only been fair. SINBAR should be applied at 0.7 to 0.9 pounds of product/A as soon as the hay is off of the field. A 1-inch rainfall is also required to move the herbicide into the soil so the roots of the weeds can take it up.

GRAMOXONE EXTRA is a contact nonresidual herbicide that can be used to control emerged weeds between harvest. Most 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 poor control of emerged grasses, and the emergence of new weeds after spraying can be a problem. 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 actively growing pigweeds (less 4 inches tall). Control of other summer broadleaf weeds is only fair, so read the label for listing of susceptible broadleaf weeds. BUTYRAC will not kill large or drought-affected pigweeds, nor will it kill mowed pigweeds that have resprouted. POAST PLUS is a nonresidual herbicide that can give good control of most emerged summer grasses. It is essential that 1 quart of crop oil concentrate/acre be added to the spray mix. It is important to use the rate listed on the label for the various grasses.

SELECT received label expansion for control of weedy grasses in alfalfa, tomatoes, peanuts & dry beans on April 8,1998. We have evaluated it for several years in alfalfa, and have found it to perform very similar to POAST PLUS. Both require an oil concentrate, both need to be applied to actively growing grass, and neither has any residual activity. Thus, getting a new flush of weedy grasses often results with both of them.

Use rate of SELECT is listed at 6 and 8 fl. Oz./acre. Valent recommends using the 6-oz. rate on smaller weeds in seedling alfalfa and the 8 oz. rate on grasses in established stands. We have found that timing of spraying (size of weeds, growing conditions, etc.) is more important than rate and believe that the 6-oz. rate is adequate if applied correctly. Our suggestion is to try both rates and decide for yourself.

If you are considering using postemergence herbicides such as BUTYRAC 200 for pigweeds and/or one of the grass herbicides (POAST PLUS or SELECT), then you should check fields about 10 days after cutting and apply herbicides just before the alfalfa canopy completely shades the soil. Since none of these three herbicides has residual activity, it will be necessary to scout the field for newly emerged weeds and respray in each cutting if season-long weed control is desired.

Jim Stritzke
Former Alfalfa Weed Control Specialist
Department of Plant and Soil Sciences
Oklahoma State University

	Oklahoma State University Stillwater, OK
Oklahoma Alfalfa Oklahoma Cooperative Extension Service Oklahoma Agricultural Experiment Station