Companion crops may compete with alfalfa (Medicago saliva L.) seedlings, but establishment with preemergence herbicides and without a companion crop may enhance the potential for soil erosion. Objectives of this study were to (i) compare alfalfa establishment techniques involving clipping or chemical control of weeds and (ii) determine differences in morphological development of alfalfa grown under several establishment techniques. Experiments were conducted at two locations in eastern South Dakota in 1989. Seven treatments included clipping the oat (Avena sativa L.) companion crop at boot stage, not clipping the oat, two treatments with oat sprayed with sethoxydim (2‐[I‐(ethoxyimino) butyl]‐5‐[2‐(ethylthio) propyl]‐3‐hydroxy‐2‐cyclohexen‐1‐one) at either S‐(SES) or 15‐cm (SE15) of oat growth, clear seeding (CS), hand weeding (HW), and a control treatment with no weed control. At Aurora, SD, alfalfa yields were greatest in HW and SE5 treatments. Yields were 24 and 36% less for CS and SE15, respectively, than for SES. At Brookings, SD, yield was also greatest for HW; however, CS had the greatest yield among the conventional establishment techniques. Yield for SES was 15% less than CS. At both locations, alfalfa yield was lowest for the two typical companion crop techniques. For typical companion crop and control treatments, mean tap root length, shoot‐to‐root ratio, and shoot length were reduced compared with other treatments. Subsequent alfalfa yields in 1990 were not affected by establishment technique at either location. Chemical control of an oat companion crop provided greater seedingyear alfalfa yields than typical companion cropping and greater protection from soil erosion than clear seeding.
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