(The following material has been adapted from Controlling Weeds in Alfalfa, Oregon State University Ext. Service Fact Sheet 267, S. Aldrich and L. Burrill)

Weed Control in Alfalfa Establishment

Proper weed control is important in establishing a new stand on alfalfa. When a field is cleared and otherwise prepared for planting, it usually presents an ideal environment for weed invasion. A wide variety of weeds may gain a foothold under these conditions, with the specific weed species depending on field history, location, and time of year. Weeds that are common to many alfalfa-growing areas of the U.S. include dandelion, pigweed, lambsquarter, Canada thistle, johnsongrass, and quackgrass.

Many weed problems in alfalfa can be avoided by using preplant herbicides. For example the herbicide glyphosate (Trade Name Roundup) can be applied up to 10 days before planting to control existing vegetation. Glyphosate can help control creeping perennials like field bindweed or Canada thistle. It is important to note that glyphosate does not have residual activity in the soil, so it will not control weeds that germinate after it is applied.

Another widely used preplant herbicide is EPTC (Trade name Eptam). EPTC is a volatile compound, so it needs to be incorporated into the soil immediately. The use of EPTC will prevent grass and broadleaf weeds from emerging and competing with alfalfa seedlings.

During the time it takes for the alfalfa seed to germinate and develop into small seedlings, some weeds may also germinate and produce seedlings within the developing stand. To control these weeds a post-emergence selective herbicide may be used. 2,4-DB (Trade names Butoxone, Butyrac) alone or mixed with bromoxynil (Trade Name Buctril) are two herbicides that have shown promise in controlling small broadleaf weeds. Sethoxydim (Trade name Poast) may be used to control grass weeds.

Weed Control in Established Alfalfa

Once a healthy, well managed alfalfa stand is established, weed problems should be minimized. However, this ideal is not always achieved and may not last for the entire lifetime of the stand. The overall key to keeping weed problems to a minimum is to maintain a healthy alfalfa stand that will outcompete weeds for space, light, water, and nutrients. This is accomplished by maintaining optimal growth conditions for the alfalfa, which means avoiding moisture stress and low soil fertility. It also means good harvest management. Despite reasonably good management practices, weeds may still gain a foothold. In the fall, when alfalfa begins to go into winter dormancy, winter-annual weeds may begin to grow. If the alfalfa is being grown for seed, mechanical control with a spring-tooth harrow may be used to uproot young weeds. However this may damage some of the alfalfa plants and so may not be ideal for alfalfa grown for hay. The alternative is to use post-emergence herbicides such as sethoxydim or pronamide. As an alfalfa stand ages and bare spots develop in the field, the only realistic alternative to controlling weeds may be to establish a new stand of alfalfa or rotate out of alfalfa for several years.

Finally, the specific type of weed present in alfalfa may influence the decision to utilize weed control. If the hay is not required to be weed free, a certain percent weed content in the baled hay may be acceptable. A number of weeds are not poisonous to livestock and may even be reasonably nutritious forage. Examples would include dandelion, lambsquarters, and quackgrass.

An Important Reminder!!!

Because of the differences in herbicide use regulations from state to state, the grower should consult with local weed control specialists.