Factors related to variation in the susceptibility to subacute ruminal acidosis in early lactating Simmental cows fed the same grain-rich diet

Cow variation in the ruminal pH responses to high-grain feeding has been noted but the factors responsible for susceptibility are inconclusive. The present study investigated a broad spectrum of responses including feed intake, performance, ruminal pH and temperature, chewing activity, feed sorting, blood metabolic variables, and acute phase response (APR) of early lactating cows. Eighteen lactating Simmental cows (initial DIM = 66.6 +/- 20.45; BW = 712 +/- 101.3 kg; milk yield = 28.7 +/- 7.0 kg, mean +/- SD) were balanced by parity and DIM and randomly assigned to 2 feeding regimens: control (n = 6) and SARA diet (n = 12). The feeding trial lasted 36 d consisting of a 1-week baseline and a 4-week experimental period. All cows started at baseline being fed a 40% concentrate diet. Afterward, the control cows continued with the 40% concentrate diet for the next 29 d, whereas the SARA cows were fed a 60% concentrate diet in the first (SARA1), third (SARA2) and last week (SARA3) of the feeding trial. In the second week, they were fed the control diet. Based on the area of reticular pH below 6.0, the SARA cows were further divided into susceptible and tolerant groups (n = 6 each). Compared with tolerant, the susceptible cows had a longer duration of reticular pH < 6.0 throughout the feeding trial, especially during the first week of the high-grain feeding (927 vs. 255 min/d), but there were no differences in reticular pH between control and tolerant cows. The susceptible cows consumed on average 19.8 kg DMI/d or 13.7% of metabolic BW, with values being lower than the tolerant cows (21.8 kg/d and 15.3%, respectively). The susceptible cows produced on average 2 rumination boli less per hour, but had 9 chews more per each rumination bolus than the other groups and this balanced the total ruminating chews among groups. The susceptible cows had a longer chewing and ruminating time per kg of DMI. When fed the 60% concentrate diet, the susceptible cows had shorter eating time and lesser eating chews during post-morning feeding hours compared to the other groups. Compared with tolerant, there was a tendency for lower milk fat and protein concentration in susceptible cows. The susceptible cows had lower concentrations of serum beta-hydroxybutyrate compared to control and cholesterol compared to the other groups. Compared with the other groups, the susceptible cows sorted the high-grain diet in favor of coarse particles in SARA1 and 2, but this sorting activity diminished in SARA3. The APR variables and liver enzymes did not differ among cow groups. In conclusion, cows showed different susceptibility to high grain feeding and this corresponded to alterations of chewing activity and serum lipids but not to APR and liver health variables.