Changes in forage nutritive value of reduced‐lignin alfalfa during regrowth

Genetically engineered alfalfa (Medicago sativa L.) cultivars with reduced lignin (RL) concentration are commercially available but their change in nutritive value as the crop matures within a growth cycle has not been thoroughly documented and might differ from non-RL cultivars. This research documents changes in forage nutritive value of a RL cultivar (HarvXtra-008) and two non-RL cultivars (54R02 and WL355RR) within five growth cycles across 2 yr in six states in the United States. A randomized complete block design with a split-plot arrangement of treatment randomization was used, with harvest growth cycles assigned to whole plots and cultivars to subplots in field trials. Cultivars were sampled on 3- to 4-d intervals between day 20 and 37 of regrowth. HarvXtra-008 was consistently lower (P <= .05) in acid detergent lignin (ADL) by 7-10% and amylase-treated neutral detergent fiber (aNDF) by 2-10% and 4-9% greater (P <= .05) in neutral detergent fiber digestibility (NDFD) than non-RL cultivars across all sampling dates. Cultivar x regrowth duration interactions (P <= .05) were found only for ADL and aNDF in two of the five growth cycles sampled, when regression coefficients were always smaller for HarvXtra-008 than 54R02. We calculated that the interval between harvests can be extended on average from 8 to 11 d for HarvXtra-008 compared with non-RL cultivars while maintaining adequate fiber digestibility for animals with high nutritional requirements; however, the extent of that longer harvest interval varied from 2 to 20 d depending on environmental conditions.