Variable-Rate Irrigation Effects On Alfalfa Yields and Quality in Southeast Idaho.

Recent drought and reduced irrigation water allotments may reduce alfalfa yields and quality.  Alfalfa requires 50-100% available moisture at all times, 50% being the permanent wilting point, and 100% being field capacity. Alfalfa requires approximately 14 cm of water to produce 1 Mg of alfalfa biomass.  The study examined the effect of different irrigation rates on alfalfa yields in southeast Idaho to determine the minimal amount of irrigation to maintain optimum yields. Alfalfa was planted in 2009, but data were collected in 2010-2012 after the alfalfa was established. Treatments were planted at a density of 70 plants m^-2.  Irrigation rates were based on evapotranspiration rate (ET), and irrigation was applied when ET rate reached a threshold of 15 cm at a 90 cm root depth. Before irrigation, soil samples were taken in 15 cm increments to a depth of 90 cm for measuring gravimetric and volumetric. Gravimetric soil moisture was determined as the difference of wet soil weight and 24 hr dried weight, and volumetric weight was calculated by multiplying gravimetric soil moisture by 1.35 g cm^-1 (the bulk density of our silt loam soil). Alfalfa stand counts were measured in the spring and fall of each year. Lower irrigation rates have resulted in lower alfalfa stands. Quality samples were taken from the low and high irrigation treatments for each cutting. In 2012 the first cutting maximum yield of 1.1 Mg ha^-1 was obtained with 8.2 cm of irrigation water and 2^nd cutting maximum yield of 1 Mg ha-¹ required 11.2 cm of irrigation water.  Treatments did respond to different irrigation rates, and the treatments receiving the lowest amount of irrigation water had the lowest yield.  Lower irrigation has resulted in increased quality. The irrigation requirement to achieve maximum yield for this study (<11.2 cm) was lower than the University of Idaho recommended rate (14 cm).