Variability in chewing, ruminal fermentation, digestibility and bacterial communities between subacute ruminal acidosis-susceptible and acidosis-tolerant sheep

Sorting behaviour is a common phenomenon observed in ruminants when they are provided with a total mixed ration, which contributes to variations in the severity of subacute ruminal acidosis (SARA). Pelleted total mixed ration (PTMR) reduces sorting, but high-grain content increases acidosis risk. However, whether the variability in the severity of SARA exists in sheep fed the same high-grain PTMR is less understood. This study aimed to investigate SARA variability among individual sheep offered a high-grain PTMR, considering chewing activity, ruminal fermentation, bacterial communities and nutrient digestibility. Twenty ruminally cannulated male Hu sheep were individually housed in cages and fed a PTMR comprising 80% concentrate mix and 20% roughage. A 14-day adaptation period to the diet and facilities was provided before a 10-day sample collection period. Continuous monitoring of ruminal pH was conducted for 48 h, during which time chewing activity was also recorded. Ruminal fluid samples were collected for analysis of volatile fatty acid and microbial DNA extraction. Faecal samples were collected to measure nutrient digestibility. Based on their acidosis index, the sheep were classified into two groups: SARA-susceptible group (n = 6) and SARA-tolerant group (n = 6). The SARA-susceptible sheep exhibited a lower ruminal mean pH and minimum pH than the SARA-tolerant sheep (P < 0.05). Additionally, the SARA-susceptible group increased the acidosis index, duration and areas of pH below 5.8 and 5.6 compared to the SARA-tolerant group (P < 0.05). The SARA-susceptible group also exhibited a longer ruminating time than the SARA-tolerant group (P < 0.05). The SARA-susceptible group exhibited a tendency to increase the relative abundance of Firmicutes (P = 0.089), while simultaneously decreasing the copy number of Fibrobacter succinogenes in the rumen, as well as the digestibility of NDF and ADF compared to the SARA-tolerant group (P < 0.05). The acidosis index was found to be positively correlated with ruminating time (min/kg DM intake (DMI)) and total chewing time (min/kg DMI), but negatively correlated with the copy number of Fibrobacter succinogenes and Ruminococcus albus in the rumen. These findings indicate that there exists variability in the SARA severity among sheep when fed a high-grain PTMR, as evidenced by varied chewing activity, bacterial communities and nutrient digestibility. Ruminating time, total chewing time per kilogram of DMI as well as the copy number of Fibrobacter succinogenes and Ruminococcus albus in the rumen hold potential as indicators for assessing the severity of SARA.