Starch-rich diet induces rumen acidosis and hindgut dysbiosis in dairy cows but the response was different among lactations

High-producing dairy cows are at risk for ruminal and hindgut acidosis due to high inclusion rates of starchy grains in their diets. Our objectives were to study the bacterial community and their fermentation end-products in feces and the susceptibility of cows of different lactations to starch-rich diet. Sixteen dairy cows (n = 5 in 2nd (2ndL), n = 6 in 3rd (3rdL), n = 5 in 4th or more (=4L) lactations) were fed one week of medium starch diet (MC, 18.8% starch and 40% concentrate), followed by four weeks of high-starch (HCwk1-4, 27.7% starch and 60% concentrate). Milk yield, dry-matter intake, ruminal pH, milk constituents, and fecal samples for short-chain fatty acids (SCFA), pH, and 16S rRNA-gene sequencing were investigated. The HC feeding caused a significant reduction in ruminal pH, fecal pH, bacterial diversity and richness, an increase in total SCFA, and a separate clustering of MC and HC samples, indicating a clear hindgut dysbiosis. The fair correlation (r = 0.5) of ruminal and fecal pH suggests that fecal pH alone is not a strong predictor of SARA. Cows in 2ndL produced less milk and had lower DMI than 3rdL or =4L cows. The DMI/kg body weight (BW) was lower for =4L than for 2ndL and 3rdL, because =4L cows were heavier. The mean ruminal pH was higher in =4L than 2ndL and 3rdL cows, but time spent below the SARA threshold pH 6.0 was highest for 3rdL and they had higher total SCFA in the feces , indicating an increased substrate flow into the hindgut in these cows. The 3rdL cows were already at the same production level as older cows, but cannot cope with the high starch level as good as the =4L cows. This can be linked to higher absorption rate and better adaptation of the microbial community due to previous adaptations in older cows. Indicator phylotypes for age were Alcaligenaceae, Anaeroplasmataceae, Erysipelotrichaceae, Succcinivibrionaceae, Coriobacteriaceae, and Peptococcaceae decreasing, and Porphyromonadaceae, Clostridiacaee, Spirochaetaceae, S247, and Barnseliellaceae increased with increasing lactation number.