Temporal fermentation characteristics and microbial community dynamics in rumens of sheep grazing a ryegrass-based pasture offered either in the morning or in the afternoon

Eight ruminally-fistulated wethers were used to examine the temporal effects of afternoon (PM; 1600 h) vs. morning (AM; 0800 h) allocation of fresh spring herbage from a perennial ryegrass (Lolium perenne L.)-based pasture on fermentation and microbial community dynamics. Herbage chemical composition was minimally affected by time of allocation, but ammonia concentrations tended to be greater for the PM group. The 24-hour pattern of ruminal fermentation (i.e. time of sampling relative to time of allocation), however, varied considerably for all fermentation variables (P ≤ 0.001). Most notably amongst ruminal fermentation characteristics, ammonia concentrations showed a substantial temporal variation; concentrations of ammonia were 1.7-, 2.0- and 2.2-fold greater in rumens of PM wethers at four, six, and eight hours after allocation, respectively, compared with AM wethers. Fourteen bacterial taxa were detected at relative abundances of >1%, based on amplified 16S rRNA gene sequences. The relative abundances of members of Prevotella 1 (the most abundant bacteria present) (P = 0.04), Bacteroidales RF16 group (P = 0.005), and Fibrobacter spp. (P = 0.008) were greater for the AM group, whereas the relative abundance of Kandleria spp. was greater (P = 0.04) for the PM group. Of these taxa, only Prevotella (P = 0.04) and Kandleria (P < 0.001) showed a significant interaction between time of allocation and time of sampling relative to feed allocation. Relative abundances of Prevotella were greater at two (P = 0.05), four (P = 0.003), and six (P = 0.01) hours after AM allocation of new herbage, whereas relative abundances of Kandleria were greater at two (P = 0.003) and four (P < 0.001) hours after PM allocation. Early post allocation rise in ammonia concentrations in PM rumens occurred simultaneously with sharp increases in the relative abundance of Kandleria spp. and with a decline in the relative abundance of Prevotella. All measures of fermentation and most microbial community composition data showed highly dynamic changes in concentrations and population dynamics, respectively, with substantial temporal changes occurring within the first eight hours of allocating a new strip of herbage. The dynamic changes in the relative abundances of bacterial groups, Kandleria in particular, and to a lesser extent Prevotella, in synchrony with the substantial diurnal variation in ammonia concentrations, has potential effects on the efficiency by which N is utilized by the grazing ruminant, with flow-on environmental consequences.