Evaluation of in vitro ruminal degradation of alfalfa

Aim of the study was the evaluation of the in vitro ruminal degradation of four different cuts of alfalfa (Medicago sativa) grass dried using two different methods. The first four alfalfa cuts (collected every 28 d) were dried in a convection oven at 65°C or dehydrated at 800°C, ground and used as the substrate for a 24 h in vitro fermentation study with ruminal liquor. The volume of gas produced was recorded every 30 min. and ammonia and VFA concentrations in the fermentation liquid as well as true degradability were determined. The drying method did not affect final gas volume but the dehydrated alfalfa determined a lower rate of gas production than the oven-dried alfalfa (–6%, P < 0.05). Gas production was also influenced by the alfalfa cut that was fermented and the first cut produced a significant lower final volume of gas than the third cut (–11%; P < 0.05). Interestingly, the fermentation lag time progressively decreased from the first cut to the fourth (P < 0.05). Ammonia concentrations in fermentation liquid were strongly influenced by both the drying method and the cut. The dehydrated alfalfa produced significantly less ammonia than the oven-dried (–12%, –21%, and –20% at 4, 8, and 24 h, respectively; P < 0.05). The fermentation of the dehydrated alfalfa resulted in lower iso-acids concentrations (P < 0.05). The first alfalfa cut produced significantly less acetic, propionic and n-butyric acid than the other three cuts (P < 0.05). True degradability was highest in the fourth and lowest in the second cut (P < 0.05). Dehydration of alfalfa resulted in lower true degradability than oven-drying (–3.4%; P < 0.05). Ammonia concentration at 8 h of fermentation was significantly negatively correlated with NDF and ADL content of alfalfa (P < 0.05). Concentrations of n-butyric acid were positively correlated to the crude protein and the B2 and C fraction content of alfalfa but negatively correlated to the alfalfa NDF, ADF and ADL content (P < 0.05). Total VFA production was negatively correlated to the alfalfa ADF content (P < 0.05). True degradability of the alfalfa samples was positively correlated to their B2 and C fractions content but negatively to their NDF, ADF and ADL content (P < 0.05). The present results show that true degradability of alfalfa was slightly reduced by dehydration and this is likely to be related to the lower degradability of alfalfa protein after the heating process. The present study also shows that the in vitro fermentation procedure can be a valuable tool to predict the rumen degradation of forages.