Persistence of subacute ruminal acidosis across first and second lactations in dairy cows based on reticular pH metrics and microbiota, and systemic health responses.

The aim of this study was to determine the associations between the subacute rumen acidosis (SARA) severity in the first lactation with that in the second lactation, based on reticular pH metrics, fermentation, microbiota and systemic health variables. The experiment included 17 lactating Holstein cows (second lactation, 635 ± 56 kg BW, BCS 3.2 ± 0.2, DIM 87 ± 57 d). Cows were first fed a 40%-concentrate (LC) diet for 2 wk, then switched to a 65%-concentrate (HC) diet (DM basis) for 4 wk. While inflammation, liver health and reticular pH metrics variables were measured in all cows, ruminal and fecal fermentation and microbiota were measured only in 11 rumen cannulated cows. Data showed that the first lactation classified SARA severity type (that is, LOW or HIGH) had no effect on the second lactation SARA indices (time reticular pH below threshold, area under the curve, dry matter intake related time and area below the pH threshold). All these SARA indices increased at the start of HC feeding but showed no more difference to LC feeding after 3 wk in HC feeding. Likewise, the severity of SARA did not affect liver enzymes aspartate aminotransferase, gamma-glutamyl transferase and glutamate dehydrogenase, which, however, increased and remained high during the entire HC feeding period. Furthermore, first lactation LOW SARA cows showed a tendency for greater serum albumin and lower haptoglobin, indicating a lower inflammatory state. Total VFA concentration increased with the HC diet in free rumen fluid, particle-associated rumen liquid (PARL) and feces and switched from acetate and n-butyrate towards propionate and n-valerate. Furthermore, first lactation HIGH SARA cows displayed a lower total VFA concentration during HC feeding in the second lactation trial, with greater molar proportion of acetate and lower molar proportion of propionate and valerate compared with LOW cows. Alpha diversity indices for PARL microbiota decreased during HC feeding when compared to LC feeding. Cows' first lactation classified SARA severity type mainly affected microbiome diversity indices as well as the VFA profile. Cows classified as HIGH SARA revealed greater alpha diversity indices during HC feeding. Concluding, cows increased their overall ability to adapt to an HC diet in their second lactation, independent of their first lactation SARA severity type. New findings of our study indicate that the individual SARA severity based on reticular pH metrics does not persist from the first to the second lactation.