Changes in Alfalfa Yield and Nutritive Value within Individual Harvest Periods

Understanding the relationship between alfalfa (Medicago sativa L.) yield and nutritive value throughout the growing season will permit optimum timing of harvest. Our objective was to determine the rate at which alfalfa yield and fiber components change during each of four harvest periods. In spring, early summer, late summer, and fall of 2004 and 2005 at Pennsylvania, Wisconsin, and Idaho, primary growth of three alfalfa cultivars was initially harvested at late vegetative stage and every 5 d thereafter for 20 d. Forage dry matter (DM) yield, neutral detergent fiber (NDF), and in vitro neutral detergent fiber digestibility (NDFD) were measured at each harvest. Rate of DM production of all cultivars was greatest both years during the spring at Pennsylvania (222.5 and 702.2 kg ha(-1) d(-1), respectively) and early summer at Wisconsin (83.4 and 278.8 kg ha(-1) d(-1), respectively), and in early and late summer at Idaho (198.4 and 194.4 kg DM ha(-1) d(-1), respectively, in 2005 only). The rate of increase in NDF was generally greatest during spring at Pennsylvania and Wisconsin, and during early summer at Idaho, while the rate of decrease in NDFD was generally greatest during early summer at all locations. The results suggest that the negative association between yield and nutritive value has greatest impact on timing of harvests made in spring and early summer in humid environments, and in early and late summer in more arid regions.