Efficacy of postemergence grass herbicides and their effects on yield and quality of alfalfa during the establishment yea

Postemergence grass herbicides may decrease the dependence upon the currently available preplant incorporated herbicides for weed control during alfalfa establishment. Sethoxydim, fluazifop, haloxyfop, quizalofop, fenoxaprop, SC 1084 and BAS 517 02H evaluated in eight separate studies effectively controlled both annual grass weeds such as large crabgrass, fall panicum and yellow foxtail and perennial grass weeds such as quackgrass in spring seedings of alfalfa at several stages of maturity.;The maximum first harvest alfalfa yields generally were obtained when the annual grass weeds were controlled prior to seven weeks after planting. In quackgrass-infested alfalfa seedings, herbicides were more effective when applied prior to the seven-leaf stage of quackgrass growth. Split and single applications of grass herbicides resulted in similar quackgrass control. Tank mixtures of grass herbicides with 2,4-DB controlled both the annual broadleaf and grass weeds. Preemergence treatments of several herbicides controlled annual grass weeds when applied at three to four times the rate required for postemergence control.;Herbicides decreased total forage yields, however removal of the weed component increased alfalfa yields. Greater alfalfa yield increases were obtained at the first harvest when the weed component of the forage ranged from 50 to 90%. The weed component of the forage in addition to the time of the first harvest affected the forage quality. The CP content of the untreated annual weeds and quackgrass (8 to 15%) was lower than alfalfa (18 to 20%). The CP content of the forage increased as the weed component decreased. The ADF content of the first harvest forage varied between the annual grass weeds and alfalfa, however the higher ADF content of quackgrass (40%) compared to alfalfa (33 to 35%) will reduce the digestibility of the forage. The higher NDF content of the untreated annual grass weeds (65 to 75%) compared to alfalfa (48 to 55%) will reduce the voluntary intake of the forage. Reduced heribicide rates resulted in lower weed control but increased the CP content and decreased the ADF and NDF content of the remaining weed component. The lower weed component of the second harvest forage and the higher quality of the weed regrowth after the first harvest generally had little affect on the forage quality of the second harvest.