Alfalfa yields in establishment and subsequent years after herbicide and phosphorus application during establishment

Phosphorus and weed control treatments were applied to assess their effect on establishment and long-term yields of alfalfa (Medicago sativa L.) and weeds. Experiments were established on soil with less than 5 kg available P2O5 ha−1 at two dryland locations near Lethbridge, Alberta. Wild oats (Avena fatua (L.) Beauv.) and volunteer barley (Hordeum vulgare L.) at location 1 and Russian thistle (Salsola pestifer A. Nels) at location 2 yielded more than 100 g m−2 in the establishment year where herbicides were not applied; however, control of these weeds during establishment year had no effect on long-term alfalfa yields or weed infestation following establishment. Phosphorus applications ranging from 80 to 320 kg P2O5 ha−1 prior to alfalfa establishment caused large increases in alfalfa yield for several years. Weed infestations were minimal following alfalfa establishment. Ten years after alfalfa was established at location 2 there was a significant linear increase in weed yield (mainly dandelions) with rate of P2O5 applied prior to seeding alfalfa. Thus, P2O5 applications had little effect on weed infestation in established alfalfa.