Weed Control

Glyphosate (Roundup PowerMax, Durango DMA, Touchdown Total, etc.) provides a means of controlling quackgrass before forage establishment. Because spring seedings should be made before quackgrass reaches the four- to five-leaf stage, the recommended stage of development for treatment with glyphosate, fall applications are encouraged to control quackgrass and other perennial grasses before making spring seedings.

Fall applications of glyphosate are recommended between October 1 and November 15. Glyphosate should not be applied, however, if the average daily air temperature has dropped below 55°F for a seven-day period before application (glyphosate works best if soil temperature is 65° to 75°F). Spring applications can be made before late spring or summer seedings. Because glyphosate will not control weeds that germinate after application, this treatment should be used in combination with other weed control measures.

Scouting new legume seedings for weeds should be done shortly after the seeding emerges because herbicides for annual broadleaf weeds must be applied when the legume is 1 to 3 inches tall and weeds have two to four true leaves. In established hayfields scouting can be done at the time of each harvest. This should provide adequate information for decisions on dormant or between-cuttings herbicide applications.

Though it is relatively easy to show the value of herbicides during legume establishment, it is more difficult to deter-mine their value in established stands. To be economical, herbicide applications on established legumes must control the weeds, and the stand must have the potential for increased legume yields. If the legume stand is so poor that total forage yields decline when the weeds and/or grasses are controlled, the economics are questionable.

In a good stand, the removal of weed competition should increase the quantity and quality of the forage produced. Although the potential of a legume stand is difficult to evaluate, it is suggested that clear stands should have a minimum of five healthy crowns per square foot to justify herbicide application. Because grasses are sensitive to many of the herbicides available for use in established legumes, the guidelines given in Table 4.11.1 are for clear stands; in some cases, label restrictions limit their use to clear alfalfa.

Roughstalk bluegrass is a perennial, cool season grass that heads in May and early June and then goes dormant in summer. This weedy grass is a problem in established alfalfa because it matures prior to first cutting harvest and the mature, somewhat woody stems of the bluegrass reduce palatability and quality of first cutting dry hay. Its presence in alfalfa that is harvested and preserved as haylage is perhaps of less concern than in dry hay.  Research results show that seeding a perennial forage grass with alfalfa suppresses bluegrass and probably other weeds. The results show that the recommended seeding rate (4 – 6 lb. of seed per acre) for timothy or orchardgrass is adequate for this purpose.

Herbicide Resistance Management

Herbicide resistance management involves the use of crop rotation and cultivation along with herbicide rotation and/or use of herbicide combinations that include herbicides with different sites of action (how they affect weeds). These practices will help manage existing herbicide resistant weed populations and delay development of new resistant weed populations.

To effectively utilize herbicides with different sites of action, everyone involved in decisions about weed management must have site of action classification readily available. The Weed Science Society of America (WSSA) has approved a numbering system to classify herbicides by their site of action (Mallory-Smith, C.A. and Retzinger, E.J. 2003. Revised classification of herbicides by site of action for weed resistance management strategies. Weed Technol. 17:605-619). A group number is given to all herbicides with the same site of action. These “GROUP NUMBERS” are included in the “Chemical weed control tables” in each crop section. Since herbicide resistance management is most effective when practiced across all crops in rotation, a list of all herbicides in this guide with their “GROUP NUMBERS”  and mode of action/site of action and chemical families for site of action GROUPS can be found in the tables below.