Crown moisture and prediction of plant mortality in drought-stressed alfalfa

Withholding alfalfa (Medicago sativa L.) irrigations during the summer, a practice referred to as summer irrigation termination (SIT), can conserve substantial amounts of water in long-season desert environments; however, plant mortality associated with SIT may be substantial. Proper timing of re-irrigation is critical for minimizing mortality and yield reductions following SIT. Procedures that would permit probable mortality prediction during drought stress would improve management efficiency with SIT. This study was conducted to determine (1) whether plant mortality occurs once the moisture content of the plant woody stem portions (crown) falls below some critical threshold and (2) if such a threshold could be used to predict the likelihood of plant mortality during SIT. Crown samples were taken from single, spaced, field-grown plants in Tucson, Arizona, at the end of a 84-day SIT period in 1994. A crown moisture content of about 42% was identified as a likely threshold critical for crown tissue SIT survival. This value was then used to predict whole-plant mortality of alfalfa grown in solid-seeded plots comparable to commercial fields. Crown samples were taken at five locations within the field along a solid gradient that was related to plant mortality. At each sampling location, the proportion of samples with less than 42% crown moisture was used to predict plant mortality. Predicted mortality slightly overestimated actual mortality but differences between predicted and observed mortality were significant for only one of five sampling locations. Alfalfa growers may be able to use this simple method of crown moisture determination to prevent permanent yield reductions by initiating irrigation before substantial portions of crowns fall below the threshold moisture content of 42%.