Alfalfa Response to Soil Water Deficits. I. Growth, Forage Quality, Yield, Water Use, and Water-Use Efficiency

Although many researchers have reported yield increases due to alfalfa irrigation when soil water has been limited, little information is available on alfalfa plant and soil water relationships which might be used to improve water management strategies. Our objective was to determine effects of plant water deficits on alfalfa seasonal growth patterns, forage quality, forage yield, water use, and water-use efficiency. During 1981, established alfalfa grown on a sandy soil was subjected to four water supply levels designated: high irrigation (H), medium high irrigation (MH), medium low irrigation (ML), and unirrigated, rainfall only (U). Soil water depletions occurred for all treatments to at least 1.9 m. Alfalfa midday plant water potentials in H and MH treatments ranged from −0.7 to −1.3 MPa throughout the season. For alfalfa subjected to ML and U treatments, ψₘₚ reached −2.7 and −4.0 MPa, respectively. Dry weight (DW) accumulations for alfalfa in H and MH treatments increased with successive harvests during the summer while harvest DW for alfalfa in ML and U treatments declined. Relative growth rate (RGR) declined sharply as ψmp decreased. At moderate plant water stress (ψₘₚ of −1.5 to −2.0 MPa), little growth occurred and under severe water stress, (ψₘₚ < −2.5 MPa), RGR were negative due to leaf loss. Reduced soil water availability resulted in increased leaf:stem weight ratios and in vitro dry matter disappearance (IVDMD), but only under severe and long-term plant water stress. Crude protein (CP) concentration was unaffected by plant water status. It appeared that irrigation water could be used efficiently on coarse-textured soils by moderate water application to alfalfa at 50% depletion of extractable soil water, an apparent threshold for maintenance of favorable plant water status. A close association between forage yield of individual harvests and cumulative indicates potential for initiation of irrigations when plant measurements indicate soil water depletions to a threshold level.