Short communication: Risk of subacute ruminal acidosis affects the feed sorting behavior and milk production of early lactation cows.

The objective of this study was to determine if the risk of developing subacute ruminal acidosis, as potentially affected by dietary straw particle size, influenced the feed sorting, rumination behavior, and milk production of early lactation dairy cows. Upon calving, 41 multiparous Holstein cows were fed a total mixed ration with 9% wheat straw (dry matter basis) chopped (1) using a 2.54-cm screen (short; n = 21) or (2) using a 5.08-cm screen (long; n = 20). For the first 4 wk following calving, rumination behavior (recorded using data loggers attached to neck collars) and reticulorumen pH (recorded using wireless telemetry boluses) were monitored 24 h/d. Due to technical failures, analyses were conducted on 38 cows (n = 19 per treatment). Total mixed ration and orts samples were collected every 3 d to determine feed sorting. Based on an acidosis index, created using area under the curve data (reticulorumen pH < 5.8) normalized for dry matter intake, cows were categorized as either low (LR) or high (HR) risk for developing subacute ruminal acidosis (SARA). Low risk cows had higher mean (6.3 vs. 6.1), minimum (6.0 vs. 5.7), and maximum (6.7 vs. 6.5) reticulorumen pH than HR cows. Low risk cows also had less severe area under the curve values (1.5 vs. 49.4 pH × min/d) and spent less time below a reticulorumen pH threshold of 5.8 (21.9 vs. 309.2 min/d). Although total milk yield was unaffected by SARA risk, LR cows produced more 4% fat-corrected milk (47.2 vs. 41.9 kg/d) and milk fat (2.1 vs. 1.8 kg/d) than HR cows. A SARA risk category × treatment interaction was detected for sorting all fractions of the diet; HR cows on the long treatment sorted against the longest particles in the diet, whereas LR cows on the same treatment did not sort this fraction, suggesting that SARA risk may be mitigated by a more balanced intake of physically effective fiber. High risk cows on the short treatment consumed the longest dietary particles as predicted, and sorted in favor of the physically effective medium and short particles. A SARA risk category × treatment interaction was detected for rumination behavior; within the long treatment, HR cows tended to ruminate less than LR cows (432.3 vs. 493.2 min/d). Overall, these results suggest that LR cows consumed a more balanced diet, whereas HR cows sorted their feed more, particularly when fed the diet containing long straw particles, which may have led to reduced rumination activity and yield of milk fat and fat-corrected milk.