Bedding surface does not alter circulating patterns of cortisol, corticosteroid-binding globulin, or free cortisol index in preweaned Jersey calves

Abstract Text: Previous research found no significant treatment differences in behavior or performance of Jersey calves residing at one farm when housed individually in hutches bedded with gravel (n=11), sand (n=12), or rubber mat (n=11) from birth to 10 wk of age. The aim of the present study was to further examine the effects of these bedding surfaces on plasma total cortisol (CORT) and corticosteroid binding globulin (CBG) concentrations, and free cortisol index (FCI) analyzed from blood samples (10 ml) collected weekly from each of the calves during this earlier study beginning within 24 h of birth. Calves were provided 4 L of waste milk 1x/d from birth to 6 wk of age, and 2 L of waste milk 1x/d with calf starter and water ad libitum during the remaining 4 wk. Serum CORT was analyzed using a commercial RIA kit procedure and bovine CBG by an indirect ELISA developed and validated in our laboratory. The FCI was calculated using the ratio of serum CORT (nmol/L) to CBG (mg/L) concentrations. Data were analyzed by MIXED procedures in SAS 9.3 for repeated measures. No differences were found (P > 0.10) in CORT, CBG or FCI among treatment groups at any time point measured. Age-related changes (P < 0.01) among the three variables were found such that CORT was greatest (132.7 ± 32.0 nmol/L) at birth, reached a nadir (2.1 ± 0.5 nmol/L) at 6 wk of age, and increased (P < 0.05) to 8.0 ± 2.0 nmol/L by 9 wk of age. Serum CBG increased (P < 0.01) between birth and 1 wk of age (1.1 vs. 1.4 mg/L; SE = 0.1) and remained unchanged until the calves reached 6 wk of age at which time CBG was at its lowest (0.8 ± 0.1 mg/L; P < 0.05). Subsequently, CBG increased (1.4 ± 0.1 mg/L; P< 0.05) at 7 wk of age where it persisted thereafter. The FCI reflected the changes observed in CORT. The data are consistent with our previous results indicating that any of these bedding types may be used without compromising the welfare of preweaned Jersey calves. Whether the changes in CORT, CBG and calculated FCI noted here are age and/or diet related await further study.

Keywords: dairy calf, free cortisol index, bedding