| Summary: | The objective of this study was to analyze the effect of regrouping on Holstein heifer salivary cortisol concentrations and behavior. Eighteen heifers (192.8 ± 13.6 days of age) were used during this study. Each of these heifers was introduced into a pen of older existing heifers. The heifers were assigned to four groups that corresponded to each of the four regroupings. Saliva samples were collected the day before regrouping (baseline; pre-regrouping), the day of regrouping, and the day after regrouping (post-regrouping). Video cameras continuously recorded from the hour before each regrouping through one day after each regrouping. Salivary cortisol concentrations were higher than the baseline for novel and existing heifers on the day of regrouping and one day post-regrouping (<i>p</i> = 0.01). More aggressive and agnostic behaviors occurred during Regroupings 1 and 4 than during Regroupings 2 and 3. Novel heifers spent more time standing (<i>p</i> = 0.05) and drinking (<i>p</i> = 0.05) than the existing heifers and less time lying (<i>p</i> = 0.05), but no other differences were observed between the behavior of existing and novel heifers. The salivary cortisol results of this study demonstrate that regrouping is a stressful event for both novel and existing dairy heifers.
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