Quantifying Morphological Development and Forage Nutritive Value of Stem and Leaf Fractions for Reduced Lignin and Reference Alfalfa Varieties.

Much of the variability in the digestibility and intake of alfalfa is associated with changes in cell-wall concentration. This study was conducted to further characterize changes in morphological development and forage nutritive value of alfalfa stem and leaf fractions under weekly sampling for both reduced lignin and reference alfalfa varieties. Reference variety ‘DKA43-22RR’ and the reduced lignin variety ‘54HVX41’ were seeded in a randomized complete block at two locations in Minnesota in 2015. Alfalfa was hand-sampled at weekly intervals for a period of five to six weeks during the summer of 2015 and the spring and summer of 2016. Samples were staged to determine maturity and then divided into stem and leaf fractions, weighed, and analyzed for forage nutritive value. To explore the relationship between alfalfa maturity and forage nutritive value, forage nutritive value parameters for stem and leaf fractions were regressed on cumulative growing degree days (GDD) using regression modeling. On average, sampling began around the mid-vegetative stage and finished after flowering. Leaf to stem ratios and stem and leaf crude protein (CP) and neutral detergent fiber (NDF) concentrations did not differ between alfalfa varieties. Concentrations of CP decreased across GDD and were greater within leaf fractions, while NDF concentrations increased across GDD and were greater within stem fractions. Stem acid detergent lignin (ADL) concentrations increased across GDD and were decreased for ‘54HVX41’ compared to ‘DKA43-22RR’. Leaf ADL concentrations remained fairly constant over time and did not differ between varieties. Neutral detergent fiber digestibility (NDFD) for both stem and leaf fractions decreased across GDD. Stem NDFD for ‘54HVX41’ was greater compared to ‘DKA43-22RR’; leaf NDFD did not differ between varieties. These results indicate that much of the change in forage nutritive value for reduced lignin alfalfa is a result of changes within the stem fraction of the plant.