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

TL;DR

This study shows that social rank affects how dairy heifers respond hormonally to repeated regroupings, with medium-ranked heifers showing higher IGF-1 levels.

Contribution

The study reveals that social rank and changes in rank influence endocrine responses to repeated regroupings in grazing dairy heifers.

Findings

Medium-ranked heifers had higher IGF-1 concentrations than high- and low-ranked heifers at multiple regrouping events.

Heifers that lowered their social rank had greater IGF-1 concentrations than those maintaining their rank.

Agonistic interactions decreased and stabilized after the first two regroupings.

Abstract

Summary Social regroupings (SR) are known social stressors that affect animal performance, while social rank may determine differences in response to repeated SR. We wanted to know if repeated SR influenced social behavior and if social rank and possible changes of social rank during SR determined differences in growth rate and in some metabolites and hormones associated with energy status in dairy heifers under grazing conditions. Fourteen “resident” heifers (remained in the same paddock; shown as black in the graph) were regrouped with 5 new heifers (“exchanged”; shown as red in the graph) every 21 days. The total number of interactions between resident heifers decreased and stabilized after the first 2 SR. Insulin-like growth factor 1 (IGF-1) concentrations in serum were greater in medium-ranked than in low- and high-ranked heifers in SR1, SR4, and SR9, and heifers that lowered social rank presented greater serum IGF-1 concentrations compared with heifers that maintained social rank across SR. Social rank influences the endocrine response to repeated SR in heifers.

Summary Social regroupings (SR) are known social stressors that affect animal performance, while social rank may determine differences in response to repeated SR. We wanted to know if repeated SR influenced social behavior and if social rank and possible changes of social rank during SR determined differences in growth rate and in some metabolites and hormones associated with energy status in dairy heifers under grazing conditions. Fourteen “resident” heifers (remained in the same paddock; shown as black in the graph) were regrouped with 5 new heifers (“exchanged”; shown as red in the graph) every 21 days. The total number of interactions between resident heifers decreased and stabilized after the first 2 SR. Insulin-like growth factor 1 (IGF-1) concentrations in serum were greater in medium-ranked than in low- and high-ranked heifers in SR1, SR4, and SR9, and heifers that lowered social rank presented greater serum IGF-1 concentrations compared with heifers that maintained social rank across SR. Social rank influences the endocrine response to repeated SR in heifers.

•Social regroupings triggered the display of agonistic interactions.•The number of agonistic interactions decreased and stabilized after the first SR.•Medium-ranked presented greater IGF-1 concentrations than high- and low-ranked ones at SR1, SR4, and SR9.•IGF-1 concentrations were greater in heifers that lowered than in those that maintained social rank.•Endocrine response to repeated SR depends on the individual position in the hierarchy.

Social regroupings triggered the display of agonistic interactions.

The number of agonistic interactions decreased and stabilized after the first SR.

Medium-ranked presented greater IGF-1 concentrations than high- and low-ranked ones at SR1, SR4, and SR9.

IGF-1 concentrations were greater in heifers that lowered than in those that maintained social rank.

Endocrine response to repeated SR depends on the individual position in the hierarchy.

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 height was measured, and blood samples for nonesterified fatty acids, glucose, and IGF-1 were taken. Concentrations of nonesterified fatty acids, glucose, and IGF-1 were taken. Social behavior was registered by continuous sampling and 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 (HRA), medium-ranked (MRA), and low-ranked (LRA). In addition, the social rank in the first and the last 5 SR 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, whereas the number of interactions decreased after the first 2 SR. Medium-ranked heifers presented greater IGF-1 concentrations than HRA heifers at SR1, SR4, and SR9, and than LRA heifers at SR3 and SR4, whereas IGF-1 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.

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