Effect of different separation protocols between mares and foals on plasma cortisol and cell-mediated immune response

Abstract

The objective was to compare effects of weaning in pairs or singly on physiological indicators of stress (plasma cortisol [F] and lymphocyte proliferation response to mitogens expressed as a stimulation index [S.I.] in mares and foals. Twenty pairs of Standardbred mares and foals were assigned to treatment groups as follows: non-weaned (control), n=6; weaned singly (WS), n=6; weaned pairs (WP), n=8. Blood sampling to determine cortisol was performed via jugular venipuncture prior to weaning and at 4, 8, 16, 24, 32, 40 and 48h post-weaning. Blood samples for lymphocyte blastogenesis in response to Concanavalin A (Con A) or Phytohemagglutinin (PHA) were taken prior to, 4 and 28h post-weaning. There was a trend toward increased F concentrations post-weaning in both mares and foals. WS and WP foals displayed increased F concentrations for up to 40 hours post-weaning when compared to control foals (48.8, 59.0, 22.1 ng/ml, respectively). Weaned mares displayed elevated F concentrations compared to controls for up to 24 hours post-weaning (65.9 and 40.5 ng/ml, respectively). There was less (p<0.05) lymphocyte proliferation in weaned mares compared to controls in response to stimulation by both Con A and PHA. There was also lower (P<0.05) lymphocyte proliferation response to mitogens in WP foals compared to control foals and WS foals. There was no difference (p>0.05) between WS or control foals. While weaning caused an increase in plasma F concentration, there was no difference in that increase due to the effect of separation protocol. Lymphocyte proliferation in response to mitogen was lessened in weaned mares and WP foals. The traditional management practice of paired weaning appears to be more stressful than weaning singly, perhaps through aggressive behavior by a dominant partner.