Social rank affects the endocrine response to frequent regroupings in grazing dairy heifers

Abstract

The objectives of this study were to determine the social behavior response to repeated social regrouping (SR), and if social rank and the changes of social rank affect the growth rate and some metabolites and hormones linked to energy status, in “resident” replacement dairy heifers subjected to SR for 205 d. Fourteen dairy heifers (153.3 ± 16.1 kg; 9 to 11 mo-old), maintained under grazing conditions, were regrouped every 21 d with 5 new animals (total = 10 SR). Two days after each SR, heifers were weighed and their withers heigh was measured, average daily gains for each period was calculated, and blood samples for nonesterified fatty acids, glucose and insulin-like growth factor type 1 (IGF-1) were taken. Concentrations of NEFA and glucose were determined in every SR, while IGF-1 concentrations were determined only at SR 1, 3, 4, 7, and 9. Social behavior was registered by continuous sampling 2 d before SR, the day of SR, and 7 d later. Social status was calculated by the ETlog software, considering the total number of agonistic interactions on 2 d before and 7 d after each SR. A dominance scale was constructed in each SR, and heifers were categorized as high-ranked heifers (HRA), medium-ranked heifers (MRA), and low-ranked heifers (LRA). In addition, the social rank in the first and the last 5 SRs were compared, resulting in 3 categories: heifers that remained in the same social rank (Mai), and heifers that lowered (Low) and raised (Rai) their social ranks. Variables were analyzed by repeated measures using a generalized lineal mixed model, considering the social rank in each SR or the change in social rank, the number of SR and their interaction as main effects, and the farm of origin and animal as random effects. Heifers performed more agonistic (2.6 vs 1.8 ± 0.6, day of SR and 7 d later, respectively) and total (3.8 ± 1.2 vs 2.5 ± 0.7, day of SR and 7 d later, respectively) interactions on the day of each SR than 7 d after, while the number of interactions decreased after the first 2 SRs. Medium-ranked heifers presented greater IGF-1 concentrations than HRA-ranked heifers at SR1, SR4, and SR9, and than LRA heifers at SR3 and SR4, while IGF concentrations were greater in HRA heifers than MRA- heifers at SR3 and SR7. In addition, LRA heifers had greater IGF-1 concentrations than MRA ones at SR3 and compared with HRA heifers at SR9. Heifers that lowered their social had greater IGF-1 concentrations than Mai heifers along the SR (132.5 ± 17.1 vs 97.8 ± 11.2 ng/mL, Low and Mai heifers, respectively) and at SR1, SR3, and SR9. In conclusion, although regrouping animals according to their characteristics can facilitate farm handling, it should be considered that endocrine profile may be affected according to heifers' position in the social hierarchy.