Persistence and single-plant productivity in nondormant alfalfa

Plant mortality is generally substantial in alfalfa (Medicago sativa L.) fields both at establishment and during the productive life of the stand. Most research has examined the effects of mortality on alfalfa yield per unit area. Consequences of mortality within a population on single-plant productivity have received relatively little attention. Understanding the basis for this mortality could affect management practices and the breeding and testing of alfalfa cultivars. The objective of this study was to describe agronomic, morphological, and physiological traits associated with persistence in nondormant alfalfa. This was accomplished by comparing two populations of S₁ progenies derived from 60 surviving 6-yr-old plants from a commercial field of 'CUF-101' (= field-derived [FD] population) and 60 greenhouse-grown CUF-101 plants (= greenhouse-derived [GHD] population) in field and greenhouse trials at Tucson, AZ. In a field trial, crown mortality, as measured by the percent live stems, did not differ significantly between the populations. Total forage yield, yield in winter and spring, and stem elongation rates were significantly greater in the GHD population. Root weight, total plant weight, root/shoot weight ratio, and concentration of total nonstructura) carbohydrates in the root were significantly greater in the FD population in a greenhouse trial. Differences observed between the two populations are consistent with the elimination of presumably less competitive plants with smaller root systems and plants intolerant of grazing in the winter and early spring. This research suggests that mortality within stands of nondormant alfalfa is less likely for plants with larger roots and generally slower growth patterns that are typically associated with increased fall dormancy.