In vitro ruminal fermentation characteristics of alfalfa silages in response to different pre-ensiling treatments

The present study investigated the in vitro ruminal fermentation of alfalfa silages (AS) that had been produced using different pre-ensiling treatments, i.e., by changing the wilting intensity and dry matter concentration, and adding sucrose, and therefore differed in silage quality and their N fractions. The data were obtained using an in vitro rumen-simulation technique (Rusitec) system, in which the AS were incubated isonitrogenously in quadruplicate. Samples were taken after 2 days (first time point) and 7 days (second time point) of AS incubation, and a variety of fermentation characteristics as well as the degradability of fiber fractions and organic matter were determined. Sucrose addition substantially raised the propionate concentration during both sampling time points from an average of 17.8 to 29.7 mmol/L (P < 0.001), which might be explained by microbial utilization of residual sugars and lactate from the AS. The extraordinary high concentrations of isovalerate and ammonia-N with all AS point to enhanced deamination activity. At the second time point, the n-butyrate concentration increased during the incubation of high-intensity wilted AS (P = 0.007), which might have been caused by the higher hemicellulose degradability that was also observed for these silages (P = 0.002). However, the organic matter degradability decreased (P = 0.035), indicating a lower degradability of other feed fractions. The gas production (P < 0.001) and degradability of organic matter (P = 0.002) and fiber fractions (P < 0.001) decreased from first to second time point, whereas the concentrations of ammonia-N (P = 0.004), acetate (P < 0.001), and isovalerate (P < 0.001) increased. Thus, it seemed that alterations in the Rusitec system and the microbial community occurred, yet it is unclear why the acetate concentration increased, whereas the fiber degradability decreased. The beneficial effects of combining all three pre-ensiling treatments on silage quality, i.e., higher acidification and increased true protein preservation, were not fully transferred to the in vitro ruminal fermentation system, and comprehensive research on pre-ensiling treatments will pave the way for an optimized ruminal N utilization from AS in the future.