Spring and Summer Planting Date Effects on Yield of Alfalfa in New Mexico.

Traditional recommendations for planting alfalfa (Medicago sativa, L.) in the southwestern USA have been late summer or early fall seedings due to cooling temperatures and less weed competition than spring plantings. However, 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 limited 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 Agricultural Science Center locations in New Mexico, Los Lunas (2014) and Tucumcari (2013), to investigate the effect of seven planting dates (every 3 wk from May-Sept) on: establishment-year, total multi-year, and average annual yields of alfalfa. All treatments were seeded at 20 lb/ac of one glyphosate resistant cultivar per site. Seeding year harvests were taken 80 DAP and then every 28-35 d thereafter. Subsequent year harvests were taken on a regular 28- to 35-d schedule between May and November. Late-June and mid-July plantings resulted in reduced establishment. Total 2- and 3-yr yields were greater (4 and 5 tons/ac greater for Los Lunas and Tucumcari, respectively, P<0.05) for spring planted alfalfa than those planted in late summer, due to harvestable forage availability in the establishment year for May and early-June plantings. First production-year yields were reduced at both sites for late-June and mid-July plantings. Yields continued to decrease for later plantings at Tucumcari. However, late-summer plantings at Los Lunas were similar to May and early-June plantings in subsequent production years. Alfalfa weevil (Hypera postica, Gyllenhal) damage was greater for earlier plantings in the year following establishment (2015) at Los Lunas.