Deficit Irrigation for Optimum Alfalfa Seed Yield and Quality

Received for publication August 7, 2006. While alfalfa (Medicago sativa L.) seed yield is known to respond positively to moderate deficit irrigation, the effects of deficit irrigation on seed viability and seed size are not known. Minimum seed viability for certification in Oregon is 85% germination plus hard seed. Two alfalfa cultivars (‘Tango’ and ‘Accord’) were planted in 2000 and were subjected to four subsurface drip irrigation treatments in 2001, 2002, and 2003 in eastern Oregon. Before flowering, the field was irrigated uniformly to replace approximately 65% of the accumulated alfalfa evapotranspiration (ETc). After the onset of flowering, four irrigation levels (80, 60, 40, and 20% of the accumulated ETc) were applied every 3 to 4 d to plots arranged in a randomized complete block design with five replicates. Seed yield showed a quadratic response to ETc replacement level and was maximized by an ETc replacement of 50% over the 3 yr. While germination decreased with increasing ETc replacement, hard and abnormal seed increased with increasing ETc replacement. The highest viable seed was achieved with an ETc replacement level of 32%, over the 3 yr. Seed weight increased with increasing ETc replacement level, but the largest increase occurred from 20 to 60% ETc replacement. With an ETc replacement of 50% that maximized yield, seed quality met or exceeded the minimum certification standard of 85% viable seed each year. The results suggest that alfalfa seed production can be optimized with 50% ETc replacement using drip irrigation.