Mowing in Fall Before Adding Herbicide May Help Control Weeds

Farmers who apply herbicides to perennial weeds late in the fall should get better results if chemicals are applied just after mowing.

At the Agricultural Research Service's Range and Meadow Forage Management Research Unit in Burns, Ore., weed scientist Michael F. Carpinelli recently completed a study showing that when applying chemicals to attack Russian knapweed, farmers should mow first to remove the current year's growth. Russian knapweed--which owes its name to its origins in the Russia-Ukraine region--is a problem in range and pastures in the western United States.

Carpinelli is leading an effort to improve control of this weed while reducing herbicide rates. Producers have traditionally applied chemicals on rangeland in the spring or summer when the weeds start to flower. But more recently, researchers have looked at a fall application.

On two study sites in eastern Oregon, Carpinelli tested a new piece of equipment that mows and applies herbicide in a single pass. Carpinelli used the herbicides picloram, at a rate of one quart per acre, and clopyralid, at a rate of one pint per acre. These herbicides don't kill rangeland grasses, but can provide months of control of weed foliage and roots.

By mowing immediately prior to applying herbicide, more of the herbicide goes on the intended target, the soil surface. Rains carry the herbicide into the root zone, where it is taken up by plant roots the following spring to prevent future weeds.

Since this method is more effective at getting the herbicide to the target weed, in the long run the fall mow-and-spray combination should reduce weed control costs because less chemicals would be used. Farmers don't typically mow their weeds, but expenses associated with this additional activity should be offset by the reduction in the amount of chemicals needed, and the weed would be more effectively controlled.

Russian knapweed can grow up to four feet tall and can take over otherwise productive pastures and haylands. Previous efforts to control this weed have not been successful.

ARS is the U.S. Department of Agriculture's chief scientific research agency.