In Vitro Ruminal Fermentation and Methane Output of Monospecific, Binary, and Multispecies Pastures Under Two Defoliation Frequencies

Pastures are the dietary basis of grazing systems in regions such as southern Chile, and their botanical composition and management can influence ruminal fermentation and associated by-products. The objective of this study was to evaluate the effects of pasture type and defoliation frequency (DF) on in vitro ruminal fermentation kinetics and methane and ammonia production. Four pasture types were evaluated: Bromus valdivianus Phil. monoculture (Bv), Lolium perenne monoculture (Lp), a binary mixture of both species (LpBv), and a multispecies pasture (Msp). All pastures were managed under two defoliation frequencies (150 and 300 growing degree days [GDDs]) and sampled across four seasons (summer, autumn, winter, and spring). Samples were dried at 60 °C, ground, and incubated for 48 h at 39 °C using the ANKOM RF automated gas production system, with ruminal inoculum collected from rumen-cannulated cows. Total gas production, methane production, and concentrations of volatile fatty acids (VFAs) and ammonia (NH3) were evaluated. The CH4 proportion was affected by the DF × season interaction, and CH4 intensity was affected by the pasture type × DF interaction. Total gas production was greater for Lp than for Bv and Msp and was greater at 300 than at 150 GDD. Total VFAs showed a pasture type × DF × season interaction. Across pasture types, NH3 concentration was lower at 300 than at 150 GDD. Overall, no treatment consistently reduced methane output. Extending the defoliation interval may reduce ruminal NH3, but its environmental benefit should be evaluated together with seasonal methane responses and potential trade-offs in forage nutritive value and animal performance.