Fermentation quality and in vitro digestibility of first and second cut alfalfa (Medicago sativa L.) silages harvested at three stages of maturity

To investigate the suitable harvest stage of alfalfa for ensiling, first cut or second cut alfalfa were harvested at the budding stage (BS), the initial flowering stage (IFS) and the full flowering stage (FFS) in 2016 and 2017, and they were then ensiled for 45 days. The nutritional value and fermentation characteristics were assessed; subsequent in vitro gas production, digestibility, ruminal fermentation parameters, and cellulolytic bacteria populations and their activities per alfalfa silage were also determined. The results showed that the silage dry matter (DM), neutral detergent fibre (NDF) and acid detergent fibre (ADF) contents increased (P < 0.01), and crude protein content decreased (P = 0.005) with advancing maturity of the alfalfa. In first cut silage, the BS had the lowest (P < 0.05) pH and the highest (P < 0.05) asymptotic gas volume, dry matter digestibility (DMD), neutral detergent fibre digestibility (NDFD) and total volatile fatty acid (VFA) production in vitro among the three treatments. In second cut silage, the silage pH, ammonia nitrogen (NH3-N), acetic acid and butyric acid contents decreased (P < 0.05) with advancing maturity, but the IFS had the highest asymptotic gas volume, DMD and NDFD among the three treatments; the proportions of Fibrobacter succinogenes, Butyrivibrio fibrisolvens and Ruminobacter flavefacien and carboxymethyl-cellulase and β-glycosidase activities of in vitro incubation fluid were higher for the IFS compared with those of the BS. In conclusion, based on the nutritional value, fermentation characteristics and subsequent in vitro rumen digestibility of alfalfa silage, the best harvest stage of the first cut and second cut alfalfa were at BS and IFS, respectively. At the same maturity stage, the first cut alfalfa silage was better than the second cut silage.