Alfalfa Weed Management

Weed management in alfalfa production is very important to maintain and produce high quality feed (Figure 1). However, herbicide options for weed control are limited since alfalfa products with herbicide- resistance traits are limited. Also, alfalfa is a broadleaf crop and a good number of troublesome weeds in alfalfa are broadleaf species. For conventional alfalfa, control of winter annual weeds needs to be done during winter dormancy of alfalfa. There are options for in-season herbicide applications, but not targeted towards winter annual weeds that germinate in the fall, winter, or early spring.

Proper Alfalfa Management for Weed Management

The best management tactic to help keeping weed-free, high-quality alfalfa throughout the life of the stand is to keep the stands healthy through proper fertilization and management practices. Another piece of maintaining healthy alfalfa is proper field placement. The best suited fields are highly productive fields with good fertility and pH levels near 6.5 to 6.9. However, even on the best suited fields, as an alfalfa stand nears the end of its life there may be increased pressure from winter annual weeds as the alfalfa stand thins. Proper timing of cuttings can also promote healthy plants and quick regrowth to provide proper canopy closure to help prevent weed infestation.

Conventional Alfalfa Herbicides

There are a few herbicide options for managing troublesome winter annual weeds. Most of these herbicide options may only be applied while the alfalfa is in winter dormancy prior to spring regrowth. When dormancy breaks and regrowth starts, options for herbicide applications become even more limited. Another limiting factor for an alfalfa herbicide is that some herbicides can only be used on established alfalfa stands and not on seedling alfalfa. One of the more commonly used herbicides that can be applied during alfalfa dormancy is metribuzin which can help control winter annuals such as henbit, wild mustard, chickweed, and pennycress with the added benefit that it can be used in mixed grass and alfalfa stands. When metribuzin is applied, pH and soil texture restrictions on the herbicide label MUST be strictly followed. Crop response and injury is very likely if any parts of the herbicide label and/or soil restrictions are not followed. Timing of this application is very crucial to prevent or lessen potential alfalfa damage.

There are some residual herbicides that can be used preplant or early in the season to help prevent certain grasses and some broadleaves; however, these do not provide little or no control to emerged weeds. There are other herbicide options, but these are limited in the spectrum of weeds they control, on timing of applications, or in the environments which they can be used. It is best to do some checking to see which of these herbicides are local options.

Although conventional alfalfa varieties are not tolerant to glyphosate, products such as Roundup PowerMAX^® 3 Herbicide may be used in established alfalfa while the crop is still dormant to help provide control of existing broadleaf and grass weed species. Use 5 to 8 fl oz/acre of Roundup PowerMAX^® 3 Herbicide once spring temperatures start to rise enough for active weed growth but prior to alfalfa trifoliate leaf expansion. Glyphosate products can also be used prior to planting or pre-emergence in newly planted alfalfa fields with use rate depending on weed species present. Consult the specific product label for other use rates and restrictions before making any applications.

Roundup Ready^® Alfalfa

Another herbicide option may be the use of a Roundup^® brand agricultural herbicide or other labeled glyphosate herbicides in Roundup Ready^® alfalfa varieties to help control non-glyphosate resistant weed species. Roundup^® brand agricultural herbicides provide excellent control of most grass species that can be problematic in alfalfa production and control of many non-glyphosate resistant broadleaf weeds. It is important to remember that warm temperatures are needed for the proper control of weeds with Roundup^® brand agricultural herbicides. Roundup^® brand products can also be tank-mixed with several approved herbicides to provide multiple effective modes of action for weed control.

Key Highlights

Proper management practices that promote viable and healthy alfalfa plants are a key to providing high quality weed free alfalfa. There are limited alfalfa herbicide options for winter and spring control of weeds. Available herbicides have strict label requirements that vary by environment and location. Always read and follow all label restrictions and requirements.

Channel Agronomist
Austin Merz

1923_444599