Increases in Cortisol due to Weaning Stress and the Subsequent Alterations to Immune Function in Beef Calves

Abstract

Weaning is defined as the physical separation of the cow-calf pair and the end of milk feeding. Natural weaning occurs between 7 and 14 months and is a gradual process. However, domestic weaning occurs between 6 and 8 months and occurs rapidly. Calves that are abruptly separated from their dam respond with increased vocalization and walking, and decreased eating and resting. The psychological stress the calf undergoes during weaning causes elevated glucocorticoid and catecholamine hormone concentrations that may predispose to increased morbidity and/or mortality from infectious diseases such as Bovine Respiratory Disease Complex. As an attempt to counter these changes, alternative weaning methods have been implemented and normally occur in two stages. Two-stage weaning begins with the cessation of milk feeding for approximately one week with the calf maintaining some contact with their dam and then permanent separation occurs. One of these methods uses a single fence to separate the cow-calf pair; this process allows the calf to see, hear and smell their dam, but does not allow the calf to suckle from its dam. Increases in cortisol, a glucocorticoid, have been linked to immunological alterations. Most notably, elevated cortisol concentrations decrease neutrophil function by down regulating the gene expression of CD62L and Fas. Cortisol also alters lymphocyte phenotype by decreasing ?δ T cells and increasing°? T cells in the circulation. Lastly, increases in cortisol can modify T cell cytokine production. The cytokines IL-12 and IFN? are secreted from T helper 1 cells while T helper 2 cells secrete IL-4 and IL-10; these T cells subsets also inhibit one another. During higher cortisol concentrations, these T cells are biased toward T helper 2 cytokine production. All these changes in immune function can lead to increased susceptibility to disease around the time of weaning. Therefore, two trials were conducted to test the hypotheses that abrupt weaning results in elevated concentrations of cortisol and subsequently alters immunological functions, and that fenceline weaning alleviates the increase in cortisol and alterations to immune function associated with weaning. In the fall of 2008, 12 Angus and Angus-X heifers (186°21 kgs; 174°16 days of age) were blocked by age and weight and randomly allotted into two groups, fenceline and abrupt. Blood samples were taken on day -7, 0, 7, 14, 21, and 42; fecal samples were taken on day -7, 0, and 3. All calves were weighed on day -7, 0, 7, 14, and 42. On day -1 all calves were separated from their dam and transported for 2 hours to another facility. On day 0 all calves were vaccinated with Brucella abortus (strain RB51). Serum was analyzed for IFN? and IL-4 as well as IgG1 and IgG2 specific antibodies to RB51. Fecal samples were analyzed for cortisol metabolites. Both IgG1 and IgG2 antibodies to RB51 increased from day 0 to day 14 (P<0.05), however no differences were detected between treatment groups. Fecal cortisol metabolites were higher on day 0 in abruptly weaned calves (P< 0.001) but did not differ between groups on day -7 or day 3. Fenceline calves had higher concentrations of IFN? in the serum on day -7 and day 0 as compared to the abruptly weaned calves (P<0.04). In the fall of 2009, forty-four Angus and Angus-X calves (19 heifers and 25 steers; 181°27 kgs; 148°17 days old) were blocked by age and gender and randomly allotted within block into two treatment groups, fenceline (FL) and abrupt (AB). Approximately half the fenceline calves were separated from their dams by a single fence at day -7 and the rest of the fenceline group at day -6; all calves were removed from their dam at day 0. Calves were vaccinated with Histophilus somni on day 1. Blood samples were taken at day -6, 1, 3, 8, 15, and 22. Fecal samples were taken on day -7, -6, 1 and 3. All calves were weighed on day -7, 0, 8, and 22. Serum samples were analyzed for IgG1 and IgG2 specific-H. somni antibodies, white blood cells were analyzed for lymphocyte phenotypes, and gene expression using 18S as the housekeeper gene. Fecal samples were analyzed for cortisol metabolites. Abruptly weaned calves had higher concentrations of cortisol metabolites in the feces than fenceline calves at day 1 (P<0.0001). No difference in average daily gain or H. somni specific antibodies between treatment groups was detected. There was a treatment*date interaction in lymphocyte and neutrophil populations (P<0.05); neutrophils from fenceline calves dropped from day -6 to day 1, but increased from day 1 to day 3, while abrupt calves decreased from day -6 to day 3. Lymphocytes from fenceline calves increased from day -6 to day 1, but decreased from day 1 to day 3, while lymphocytes from abrupt calves increased from day -6 to day 3. No difference in treatment groups was detected for lymphocyte phenotypes or gene expression; however, a date effect was detected. The CD4 and CD8 cell populations increased over time (P<0.0001) and WC1 and TcR1 decreased over time (P=0.0243 and P=0.0027 respectively) for both treatment groups. A decrease was detected over time for expression of GAPDH and CD62L (P<0.0001). The gene expression for the cytokines IFN?, IL-4 and IL-10 had no change over time. Results from the two studies suggest that fenceline weaning decreases the cortisol response associated with cow-calf separation, but does not have a significant effect on immunological parameters measured in this study.