The best way to keep weeds out of pastures and forage crops is to keep the grasses and legumes healthy, says a University of Missouri weed specialist.

"Practices that encourage a vigorous, thick stand of pasture or forage legumes are important for good weed control," says Bill Johnson. "Weed seeds germinate and become established wherever pasture or forage stands are thin."

Maintaining optimum soil fertility and pH is important.

"The use of herbicides without good cultural practices will generally give poor weed control results," Johnson says.

Rotational grazing and periodic mowing of grass pastures enhance the ability of the pasture to compete with most annual weeds, he says. Well-adapted and long-lived varieties, weed-free seed and proper seedbed preparation are critical to the establishment of hardy stands.

Missouri forage specialist Craig Roberts agrees.

"If a stand is thin or too short, light will penetrate the weak canopy and stimulate the production of weedy species," says Roberts. "We can reduce this by having a vigorous stand of grass pasture."

Deeply rooted, broadleaf perennial weeds are a common problem in pastures and forage crops, says Johnson.

"Crop rotation with cultivated crops can reduce some perennial weed problems," he says.

Herbicide application timing varies with the weed species. A fall or early spring application is usually best for winter annuals or biennials in the rosette stage of growth. Spring and early summer treatments are best for summer annuals. Established perennials are most susceptible to herbicides in the bud-to-bloom stage in mid- to late summer, or in fall when food reserves are moving into the underground portions of plants.

Woody species should be sprayed when they’re fully leafed out and actively growing.

"Use herbicides only where the pasture or forage stand is thick and vigorous enough to fill in areas where weeds are killed," Johnson says. "Thin or irregular pasture or forage stands won't be able to fill in areas left empty by weed control before new weeds become established."

Forage legumes are frequently grown with a companion grass crop such as orchardgrass or fescue, he notes.

"Most herbicides registered for use in forage legumes will severely injure or kill a grass companion crop and vice versa. Be sure the herbicide is registered for both forage species before using it."

If the weed problem is severe, re-establishment may be necessary.

Missouri extension publication MP581, Weed and Brush Control Guide for Forages, Pastures and Non-Cropland, gives detailed information on appropriate herbicides. The updated version is available from: University of Missouri Extension Publications, 2800 McGuire St., Columbia, MO. 65211. Or, call 800-292-0969.