Distinct blood and milk 18-carbon fatty acid proportions and buccal bacterial populations in dairy cows differing in reticulorumen pH response to dietary supplementation of rapidly fermentable carbohydrates

Nine Holstein dairy cows were fed diets with increasing proportions of rapidly fermentable carbohydrates (RFCH) to investigate the effect on reticular pH, milk fat content (MFC), 18-carbon fatty acid proportions in blood plasma and milk, and bacterial community in buccal swab samples. Inter-animal variation was expected in terms of reticular pH response upon higher RFCH proportions, which would be reflected in the occurrence or not of milk fat depression (MFD). Moreover, this variation in occurrence of MFD was hypothesized to be related to differences in blood and milk fatty acid proportions and in the bacterial community in buccal samples. Cows were fed a total mixed ration throughout the experiment, which consisted of 4 periods: adaptation (d 0-4) and low (d 5-18), increasing (d 19-24), and high RFCH (d 25-28). During the increasing RFCH period, the standard concentrate (211 g of starch/kg of dry matter) was gradually and partly replaced by a concentrate high in RFCH (486 g of starch/kg of dry matter). The reticular pH was measured using a bolus and the time below pH 6.00 was calculated on a daily basis. On d 13, 14, 25, 27, and 28, plasma and milk samples were collected and analyzed for 18-carbon fatty acid proportions, and buccal swabs were collected for bacterial community analysis based on 16S rRNA gene amplicon sequencing. Inter-animal variation was observed in terms of reticular pH, which allowed us to divide the cows into 2 groups: tolerant (time below pH 6.00 ≤ 0.1 h/d) and susceptible cows (time below pH 6.00 ≥ 1.26 h/d). The lower reticular pH of susceptible cows was accompanied by lower MFC. Both groups already differed in reticular pH and MFC during the low-RFCH period. Furthermore, higher RFCH amounts did not decrease the reticular pH in either of the 2 groups. Nevertheless, MFD was observed in both groups during the high-RFCH period compared with the low-RFCH period. Lower MFC in animals with lower reticular pH or during the high-RFCH period was associated with a shift in 18-carbon fatty acids toward trans-10 at the expense of trans-11 intermediates, which was observed in plasma as well as in milk samples. Moreover, lower MFC was accompanied by shifts in the relative abundance of specific bacteria in buccal samples. Genera Dialister, Sharpea, Carnobacterium, Acidaminococcus, and uncultured genera belonging to the Betaproteobacteria were more abundant in situations with greater trans-10 proportions.