Inoculant Effects on Alfalfa Silage: In Vitro Gas and Volatile Fatty Acid Production

Abstract

Alfalfa silages from 2 similar trials were analyzed for in vitro ruminal gas production. In both trials, there were 15 treatments: alfalfa treated at ensiling with 1 of 14 lactic acid bacterial inoculants or untreated alfalfa. First-cut (477g of dry matter/kg) and second-cut (393g of dry matter/kg) alfalfa were ensiled in glass jars for a minimum of 35 d at room temperature (∼22°C). At opening, a portion of each silage was wet-ground with a mixer. Each silage was then assessed for in vitro ruminal gas production in 3 replicate runs with the wet-ground silage, 1 on the fresh silage and 2 on frozen and thawed silage. In vitro gas production was measured in 160-mL sealed serum vials incubated at 39°C. One gram of silage was incubated with 17.1mL of nutrient solution, 0.9mL of reducing solution, and 12mL of ruminal inoculum (1:2vol.vol mixture of rumen fluid and buffer). Gas production was measured manually by using a pressure gauge at 3, 6, 9, 24, 48, and 96h. At 96h, the rumen fluid was analyzed for pH and volatile fatty acids. In the 2 trials, the untreated control silage produced either numerically the highest or one of the highest levels of gas production per unit of dry matter incubated. In first-cut silage, 9 of the inoculant treatments at 9h and 4 treatments at 96h had reduced gas production compared with the control. In second-cut silage, 10 inoculant treatments at both 9 and 96h had reduced gas production compared with the control. Furthermore, in first-cut silage, the fraction of total gas production at 3, 6, and 9h was numerically the highest for the control, and only 4 treatments were not significantly lower than the control at 9h. In second-cut silage, 2 of 14 inoculated treatments produced faster fractional rates of gas production than the control, but most inoculated treatments had numerically slower fractional rates (4 significant) in the first 9h. The in vitro fermented wet-ground control silages had one of the highest acetate:propionate ratios in both trials, significantly higher than 12 and 8 of the inoculated treatments in first- and second-cut silage, respectively. The response in acetate:propionate ratio in both cuts was similar, even though the control silage was highest in lactic acid in one trial and lowest in the other. Overall, inoculation of crops at ensiling appears to affect in vitro ruminal fermentation of wet-ground silages, even in the absence of large effects during silage fermentation.