Planting Date Effects on Yield of Alfalfa in New Mexico.

Traditional recommendations for planting alfalfa (Medicago sativa, L.) in the southwestern USA are late summer or early fall seedings due to cooling temperatures and less weed competition than spring plantings. The release of glyphosate resistant alfalfa has allowed for a low-cost, broad-spectrum weed management option for producers interested in spring planting to better match water resources and early irrigation allocations, as well as recovery of establishment costs by harvesting sooner. It is uncertain how alfalfa will establish and how yield will be affected under spring/summer planting conditions. Multi-year studies were initiated at two NMSU locations in New Mexico, Los Lunas (LL, 2014-15) and Tucumcari (Tuc, 2013-14), to investigate the effect of 7 planting dates (every 3 wk from May-Sept) on: establishment yr, 1^st production yr, 2^nd production yr, and cumulative (est. yr + 1^st + 2^nd prod. yrs) yields of alfalfa. Harvests were taken 80 DAP and then every 28-35 d thereafter. Subsequent year harvests were taken on a 28- to 35-d schedule between May and November. Late-June and mid-July plantings resulted in reduced establishment. Seeding yr yields were greatest for the earliest planting date at both locations (LL, May 12; Tuc, June 5). Cumulative yields were greater (range: 3 to 6 tons/ac more) for spring-planted alfalfa than those planted in late summer, due to harvestable forage in the establishment yr for May and early-June plantings. First production-yr yields were reduced at both sites for late-June (LL and Tuc) and mid-July (Tuc) plantings, likely due to poor stand establishment. Second prod yr yields continued to be lower at LL for the late-June and July planting date. Yields continued to be lower for plantings later than June 5 at Tuc into the 2^nd prod yr. However, late-summer plantings at LL were similar to May and early-June plantings in subsequent production years.