ALFALFA YIELD AND PLANT WATER RELATIONS WITH VARIABLE IRRIGATION

In the arid and semiarid western USA, alfalfa (Medicago sativa L.) grown for forage requires large amounts of water for high productivity. Managing alfalfa to achieve the best possible water-use efficiency (WUE) is essential for the crop to remain competitive for water supplies both within and outside agriculture. This study was conducted i n the San Joaquin Valley of California, to define alfalfa forage yield and plant water relation responses of three alfalfa cultivars (CUF 101, Moapa 69, and WL 318) to contrasting irrigation intensities and establish critical plant water-status values for irrigation scheduling. A single line-source sprinkler system provided a variable water supply. The soil was a Hanford sandy loam, silty substratum (coarse-loamy, mixed, nonacid, thermic typic xerorthent). Maximum total season crop evapotranspiration (ET(c)) of 1000 mm gave 26.3 Mg ha-1 of hay yield that was similar for the three cultivars. A linear hay yield (Y(h))-ET(c) relationship was defined as Y(h) = -0.212 + 0.0265ET(c) (r2 = 0.82). Water-use efficiency, 23.1 Mg ha-1 dry matter per meter of water used as ET(c), was comparable with other C3 species. As midday plant water potential (psi(w)) declined below -1 MPa, yield reductions were observed for all cultivars. Increased crop water stress index (CWSI) was correlated (r = 0.84) with declining psi(w) below the -1 MPa yield-limiting psi(w) threshold. Absolute values of psi(w) or CWSI associated with a given yield reduction were cultivar-dependent; Moapa 69 and WL 318 responded alike, but each differed from CUF 101. Plant-based water-status measurements provided a practical and reliable index for assessing the adequacy of supplied irrigation water.