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 degrees F for a seven-day period before application (glyphosate works best if soil temperature is 65 degrees to 75 degrees F). Spring applications can be made before late spring or summer seedings. Since 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 must be done shortly after the seeding emerges because weed control measures 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 determine 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 the 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 9 are for clear stands; in some cases, label restrictions limit their use to clear alfalfa.