Interference of annual weeds in seedling alfalfa (Medicago sativa).
The summer annuals Echinochloa crus-galli, Amaranthus retroflexus and A. powellii sown with Medicago sativa in mid-Aug. severely suppressed M. sativa. Although frost killed the weeds in Oct. and Nov., interference from E. crus-galli killed 30% of M. sativa seedlings and the influence of all 3 weed species reduced the yield of M. sativa forage the following year. Two winter annuals Bromus tectorum and Sisymbrium altissimum suppressed M. sativa sown in Aug. or Sep. reducing forage yields the following year. Yield of M. sativa sown on 27 Aug. was unaffected if weeds were removed by 36 days after M. sativa emergence. These winter annuals sown 65 or more days after M. sativa emergence had little or no impact on M. sativa yields, though they sometimes produced enough biomass to reduce the quality of the 1st hay cut.