Short communication: Relationship between competitive success during displacements at an overstocked feed bunk and measures of physiology and behavior in Holstein dairy cattle.

The objective of this study was to evaluate how behavioral and physiological parameters are affected based on a cow's level of success at displacing others at an overstocked feed bunk. Forty Holstein nonlactating, late-gestation dairy cattle were housed in an overstocked pen [5 stalls/10 cows and 0.34 m of linear feed bunk (FB) space/cow] in groups of 10 (4 heifers and 6 multiparous cows) for 14 d. Plasma nonesterified fatty acids, glucose, and fecal cortisol metabolites (11,17-dioxoandrostanes) were measured in blood and feces sampled every 2d. A glucose tolerance test and an ACTH challenge were conducted on all cows on d 13 and 14, respectively to further explore the effects of competitive success on energy metabolism and stress physiology. Feeding behavior and displacements at the FB were recorded between d 7 to 10 of the observation period. A competition index (CInd) was calculated for each cow by dividing the number of times the cow displaced another at the FB by the total number of displacements the cow was involved in, either as an actor or reactor. Cows were then divided into 3 subgroups based on their CInd: high success (HS: CInd ≥0.6), medium success (0.4 ≤ CInd <0.6), and low success (LS: CInd <0.4). Heifers accounted for 7, 36, and 79% of the total number of animals in the HS (n=15), medium success (n=11), and LS (n=14) groups, respectively. No differences were observed in daily feeding time, total number of displacements, and time to approach the FB following fresh feed delivery between the 3 CInd groups; however, cows in the LS group had greater daily nonesterified fatty acid and 11,17-dioxoandrostane concentrations relative to cows in the HS group. No differences existed in cortisol response to an ACTH stimulation test between CInd categories. During the glucose tolerance test, glucose response curves were the same between all 3 CInd categories; however, the peak insulin response of LS cows was 130 μIU/mL greater than the peak HS response, indicating that LS cows may have decreased tissue responses to insulin or increased pancreatic responses to glucose. In an overstocked environment, dairy cattle physiology is associated with a cow's level of success at displacing other individuals at the feed bunk.