Seeding Rate, Herbicide, and Irrigation Effects on Spring-Seeded Oat-Alfalfa Companion Crops

Small grain companion crop seeding rate recommendations for alfalfa (Medicago sativa L.) establishment are dated and inconsistent, and how a small grain companion crop seeding rates should be adjusted at different moisture levels is unknown. A study was conducted to provide clarity about oat (Avena sativa L.) companion crop seeding rates that maximize weed suppression and forage yield and minimize the effect on alfalfa stand establishment. This experiment considered oat companion crop seeding rates at various irrigation levels. Companion crop treatments consisted of oats sown at 89, 45, 22, 10, and 0 (with and without herbicide) kg ha-1. Irrigation was applied using a line-source irrigation gradient with five irrigation levels (IL). The largest amounts of water were applied at high ILs, and low ILs received lower amounts of water. At the two highest ILs in the first cut, a 2-fold increase in alfalfa stem density occurred as the 89 kg ha-1 oat rate was reduced to 11 kg ha-1, but at the lowest two ILs, the increase was 5 to 7-fold. Similarly, second cut differences in stem density were only apparent at lower ILs. First cut forage yields were lowest in 0 kg ha-1 treatments and increased with increasing oat seeding rates. These differences in yield were amplified as ILs were reduced. At high ILs, oats sown with alfalfa increased yield over the control by less than 30%, but total yield was increased by approximately 60% in the lowest ILs. Conversely, yields were highest among 0 kg ha-1 treatments in second cut. At both cuts, herbicide and untreated plots produced the largest alfalfa yields, and alfalfa yield was reduced as oat seeding rates were increased. The highest oat seeding rates (89 and 45 kg ha-1) reduced weed dry matter yield to levels comparable to the herbicide in both cuts. Generally, the presence of oats or weeds reduced crude protein. NDF was also less desirable at first cut with increasing oat seeding rates, and differences in NDF increased as irrigation was reduced. Results observed in this study suggest that the highest alfalfa density, yield, and forage quality can be achieved when alfalfa is sown alone with an herbicide. If herbicides cannot be used, the best results may be achieved when alfalfa is sown with a companion crop when irrigation is high, but without a companion crop when irrigation is low. Seeding rates between 22 and 11 kg ha-1 may be most desirable when a companion crop is used