To increase intramuscular fat accumulation, Japanese Black cattle are commonly fed a high-grain diet from 10 to 30 months of age although it can result in the abnormal accumulation of organic acids in the rumen. We explored the effect of long-term high-concentrate diet feeding on ruminal pH and fermentation, and its effect on the rumen bacterial community in Japanese Black beef cattle during a 20-month fattening period. Nine castrated and fistulated Japanese Black beef cattle were housed with free access to food and water throughout the study period (10–30 months of age). The fattening stages included Early, Middle, and Late stages (10–14, 15–22, and 23–30 months of age, respectively). Cattle were fed high-concentrate diets for the experimental cattle during fattening. The body weight of the cattle was 439 ± 7.6, 561 ± 11.6, and 712 ± 18.5 kg (mean ± SE) during the Early, Middle, and Late stages, respectively. Ruminal pH was measured continuously during the final 7 days of each stage, and rumen fluid and blood samples were collected on day 4 (fourth day during the final 7 days of the pH measurements). The 24-h mean ruminal pH during the Late stage was significantly lower than that during the Early stage. Total volatile fatty acid (VFA) during the Late stage was significantly lower than during the Early and Middle stages, but no changes were noted in individual VFA components. The lactic acid concentration during the Late stage was significantly higher than that during the Early and Middle stages. The bacterial richness indices decreased significantly during the Late stage in accordance with the 24-h mean ruminal pH. Among the 35 bacterial operational taxonomic units (OTUs) shared by all samples, the relative abundances of OTU8 (Family Ruminococcaceae) and OTU26 (Genus Butyrivibrio) were positively correlated with the 24-h mean ruminal pH. Total VFA concentration was negatively correlated with OTU167 (Genus Intestinimonas), and lactic acid concentration was correlated positively with OTU167 and OTU238 (Family Lachnospiraceae). These results suggested that long-term high-grain diet feeding gradually lowers ruminal pH and total VFA production during the Late fattening stage. However, the ruminal bacterial community adapted to feeding management and the lower pH during the Late stage by preserving their diversity or altering their richness, composition, and function, to enhance lactic acid production in Japanese Black beef cattle.

to increase intramuscular fat accumulation, Japanese Black beef cattle are commonly fed a high-grain diet from 10 to 30 months of age. Castrated and fistulated cattle (n = 9) were fed a high-concentrate diets during the early, middle, and late stages consecutively (10-14, 15-22, 23-30 months of age, respectively). Ruminal pH was measured continuously, and rumen epithelium and fluid samples were collected on each stage. The 24-h mean ruminal pH during the late stage was significantly lower than that during the early stage. Total volatile fatty acid (VFA) and lactic acid levels during the late stage were significantly lower and higher, respectively, than those during the early and middle stages. In silico analysis of differentially expressed genes showed that "Oxidative Phosphorylation" was the pathway inhibited most between the middle and early stages in tandem with an inhibited upstream regulator (PPARGC1A, also called PGC-1α) but the most activated pathway between the late and middle stages. these results suggest that mitochondrial dysfunction and thereby impaired cell viability due to acidic irritation under the higher VFA concentration restored stable mitochondrial oxidative phosphorylation and cell viability by higher lactic acid levels used as cellular oxidative fuel under a different underlying mechanism in subacute ruminal acidosis. A high-grain diet promotes the growth, productivity, and quality of meat or milk production in beef and dairy cattle. On a high-grain diet, organic acids such as volatile fatty acids (VFAs) and lactic acid accumulate in the rumen 1,2. Ruminal pH is critical in the maintenance of normal, stable fermentation, microbial populations, and absorptive function 2-4 , and is determined by the balance between acid production by microbes and acid removal by absorption, neutralization, and clearance 1,5,6 , in a host-microbiome interaction 7. The occurrence of subacute ruminal acidosis (SARA) defined by a rumen pH <5.6 is due to the non-physiological accumulation of VFAs, while ruminal acidosis defined by a rumen pH <5.0 is associated with accumulated lactic acid in the rumen 2. Several short-(days) or mid-term (weeks) studies have shown that SARA or rumen acidosis challenges affected the rumen epithelial structure, gene expression, and transcriptomes 8-10. For example, in one study, 3 weeks of a high-grain diet (65% grain) compromised the structural integrity and led to the appearance of undifferentiated cells near the stratum corneum of the rumen papillae in non-lactating dairy cattle 8. In another, the rumen papillae in dairy cattle fed a total mixed ration had increased epithelial desquamation and sloughing scores during early lactation, as well as upregulation of genes encoding desmosome assembly (despoglein1 and corneodesmosin), epidermal growth factor (EGF) signaling (epiregulin), transforming growth factor β (TGFB)