Short communication: Noninvasive indicators to identify lactating dairy cows with a greater risk of subacute rumen acidosis

The objective of the current study was to evaluate if milk urea nitrogen (MUN) and milk fat content could be used as the noninvasive indicator to identify cows with greater or lower risk of subacute ruminal acidosis (SARA). Our hypothesis was that cows with lower MUN and milk fat content would have greater risk of SARA, whereas cows with higher MUN and milk fat content would have lower risk of SARA. In the screening study, 35 late-lactating Holstein cows (DIM = 250 ± 71.1; BW = 601 ± 45.4 kg) were fed a high-grain diet containing 35% forage and 65% concentrate mix ad libitum for 21 d. Concentration of MUN ranged from 5.7 to 13.9 Mg/dL among the 35 cows, and the average milk fat content was 3.5%. Then, 5 cows with highest MUN concentrations with milk fat higher than 3.5% were selected as animals that presumably have low risk of SARA, and 5 cows with lowest MUN concentrations with milk fat less than 3.5% were selected as animals that presumably have high risk of SARA. These 10 animals were ruminally cannulated during the subsequent dry period. As 1 low-risk cow was culled due to fatty liver, 9 animals (DIM = 122 ± 33.2; BW = 615 ± 49.1 kg) were used in the subsequent study in the following lactation. All cows were fed a high-grain diet consisting of 35% forage and 65% concentrate mix ad libitum for 21 d. Ruminal pH was measured every 30 s for 72 h. Minimum (5.75 vs. 5.30) and mean ruminal pH (6.35 vs. 6.04) was higher for low- compared with high-risk animals. In addition, duration of rumen pH below 5.8 was shorter in low-risk animals (52.5 vs. 395 min/d). These results suggested that MUN and milk fat content in late-lactating cows fed a high-grain diet may be used to identify cows that have higher or lower risk of SARA.