Effects of rumen fluid pH on rumen activity and quality and technological properties of milk

Excess fermentation and accumulation of short chain fatty acids (SCFA) in the rumen is a frequent result of feeding dairy cows with high-grain diets. This allows to sustain increasing milk yield but has risen the risk of rumen acid- osis. This study investigated the effects of rumen pH on milk composition traits, coagulation properties, cheese yield, and rumination activity (RA) of dairy cows. Rumen fluid samples were taken through rumenocentesis from 100 Holstein Friesian cows (5 to 75 days in milk - DIM) without clinical signs of disease kept in 2 herds, to determine pH and con- centration of SCFA. Within the day of rumenocentesis individ- ual milk samples were also taken from each cow. Further, RA was monitored using a microphone-based monitoring system (Ruminact, Milkline) fitted on cows, to record the time spent ruminating from 2 days before to 2 days after rumenocente- sis. Data were analyzed using a linear mixed model including the fixed effects of parity and DIM class, the quartile of rumen fluid pH (QpH) and the random effect of herd-test date and also of cow when repeated observations per cow were analyzed. Rumen pH (5.98±0.37) averaged 5.52 to 6.46 in the first to fourth quartile, respectively, with 32% of cows with rumen pH <5.8. The concentration (mmol/l) of total ruminal SCFA and the proportion (%) of propionic acid on rumen fluid linearly increased (p<.05) as rumen pH declined, whereas that of acetic acid linearly decreased. Whereas the average time spent ruminating during the day was similar in cows of different QpH (on average 26%, but with a clear cir- cadian variation), the circadian rumination pattern was influ- enced by rumen fluid pH, and the RA linearly increased (p<.05) in the daily interval 06:00 to 12:00 as rumen pH declined. Cows of different QpH did not differentiate for body condition score and test day milk yield (36.6 ± 8.0 kg/d). Likewise, QpH did not affect (p<0.05) milk pH, somatic cell count (SCC), fat, protein, lactose and casein milk content. Cows of different QpH provided milk with similar coagulation properties (rennet coagulation time, curd firming rate and curd firmness after 30 minutes) and cheese yield. Conversely, curd firmness and cheese yield linearly decreased as SCC content of milk (100; 100 to 400; >400 x 103 cells/ml) increased. In conclusion, QpH altered SCFA content and pro- portion in rumen fluid and rumen activity, but not milk qual- ity traits, even in cows with rumen pH <5.8. Acknowledgements Research funded by University of Padova, CPDA134009/13 and DOR1787424/17.