Diurnal variation in fatty acid profiles in rumen digesta from dairy cows grazing high-quality pasture

Six dairy cows receiving a once-daily high-quality pasture allocation in the afternoon were used to investigate the relationship between diurnal variation in rumen fermentation activity and diurnal variation in rumen fatty acids (FA). Rumen digesta was collected every two hours over a 24 h period, and rumen pH, volatile fatty acid (VFA) concentration and FA were measured. Cows receiving a once-daily pasture allocation in the afternoon had lower rumen pH at night than during the day, while the variation pattern of VFA concentration and rumen pH was exaggerated, but broadly similar, to previous reports of cows on once daily pasture allocation. Rumen FA profiles varied considerably over the grazing cycle, with the most dramatic changes occurred in the evening and overnight, especially for odd- and branched-chain fatty acids (OBCFA) and C18 FA. Diurnal variation in OBCFA clearly reflected the microbial colonization of the newly ingested pasture while the variation in C18 FA was consistent with known biohydrogenation reactions in the rumen. Disappearances of most C18:3 n-3 and C18:2 n-6 by 8–12 h after ingestion suggests a rapid biohydrogenation of plant-derived polyunsaturated fatty acids (PUFA), while the high proportions of unsaturated C18 intermediates (especially C18:2 t11, c15 and C18:1 t11) and low proportion of C18:0 measured in the present study suggests inhibition of the final step of biohydrogenation by the high level of C18:3 n-3. The low proportion of conjugated linoleic acid (CLA c9, t11) in rumen digesta confirms an endogenous synthesis of this CLA isomer in milk fat of pasture based cows. The principal component analysis (PCA) biplot analysis provides a good description of variation in the overall FA, as well as the main biohydrogenation pathway for C18:3 n-3. The lower rumen buffering capacity (decreased rumen pH), dramatic decline in rumen microbial population (assessed by total OBCFA) observed between 18:00 and 20:00, as well as the large increase in plant-derived PUFA implied a reduced rumen biohydrogenation activity during the evening, which might provide an approach to reduce the biohydrogenation of protected PUFA supplements by strategic feeding to coincide with reduced rumen activity