Lignification and Foliar Senescence of Lolium perenne and Festuca arundinacea Between Growing Seasons Affect Fermentative Processes and Ruminal Microbiota

Cell wall lignification in grass leaf tissue occurs during plant development and senescence, reducing the availability of fermentable carbohydrates in the rumen. However, these seasonal effects on cellular aging, lignin aggregation, and nutrient availability remain unknown. Therefore, the objective of this research was to evaluate the effect of foliar senescence and harvested season (winter 2012 vs. summer 2013) of Lolium perenne and Festuca arundinacea Schreb. on leaf lignin aggregation, in vitro fermentation, and microbial adhesion. The experiment was a split-split-plot; the growing seasons were the whole plots, the grass species were the subplots, and the senescence stages (0%, 30%, and 60%) were the sub-subplots. Lignin accumulation occurred (p ≤ 0.05) in the fibers of vascular bundle sheath extension in both grasses and was higher for Festuca during winter. The senescence stage significantly affected (p ≤ 0.05) the chemical composition, digestibility (640.0–767.8 g kg−1 DM), and total volatile fatty acid (VFA) production (31.7–43.4% of total VFAs). Furthermore, the rumen microbiota developed extracellular structures for adherence, which decreased as leaf tissues showed higher lignin accumulation compared with non-senescent leaves (0%). In vitro dry matter degradability, VFA production, chemical composition and content, and rumen microbiota adhesion were affected by senescence stage in the winter of 2012 and the summer of 2013.