Seeding Year Forage Yield and Nutritive Value of Reduced Lignin and Conventional Alfalfa Cultivars Grown in Diverse Environments.

Lignification of secondary cell walls with advancing maturity reduces alfalfa (Medicago sativa) digestibility. Forage Genetics International, Samuel Roberts Noble Foundation, USDA-ARS-Dairy Forage Research Center, and Monsanto Company collaborated to genetically modify alfalfa lignin content and composition and released the trait as HarvXtra^TM.  Alfalfa with this transgenic reduced lignin trait should maintain acceptable nutritive value longer than conventional alfalfa cultivars, thus allowing a longer harvest interval.  We compared forage yield and nutritive value of HarvXtra^TM alfalfa to two conventional cultivars (high yield potential cv. 54R02 and high nutritive value cv. WL355RR) at three harvest intervals (28, 33, or 38 days) in six diverse environments (CA, KS, MI, OH, PA, WI) during the seeding year. Within sites, experimental design was a RCBD (n=4) using a split-plot treatment arrangement with harvest interval as main plot and alfalfa cultivar as subplot. First growth of the seeding year was clipped uniformly and discarded. In the second and third growth cycles, plots were harvested to 5-cm stubble and samples analyzed for nutritive value using near infrared reflectance spectroscopy. When HarvXtra^TM was compared with the mean of 54R02 and WL355RR across all sites and cuttings in the seeding year, it averaged 14% less acid detergent lignin (ADL, 43.1 vs. 50.2 g/kg, respectively, P<0.05), 7% greater neutral detergent fiber digestibility (NDFD, 524 vs. 487 g/kg, P<0.05), and 10% greater relative forage quality (RFQ, 262 vs. 239, P<0.05). Cultivar differences were reduced in fall cuttings. ADL increased with harvest interval, while NDFD and RFQ decreased (P<0.05).  Across sites and cuttings, seeding year alfalfa yield increased with harvest interval (P<0.05), but averaged 7% less for HarvXtra^TM than for the other cultivars (P<0.05).  Transgenic reduced lignin alfalfa maintained a reduced lignin content and improved nutritive value in most harvest environments up to a 38-day harvest interval in the seeding year.