Relationship of glucocorticoids and hematological measures with feed intake, growth, and efficiency of finishing beef cattle

The objective of this experiment was to determine the association of glucocorticoids and markers for immune status in finishing beef steers and heifers with DMI, growth, and efficiency. Steers (n = 127) and heifers (n = 109) were individually fed a finishing ration for 84 d with BW measured every 21 d. Blood samples were collected via jugular venipuncture for metabolite (glucose and lactate) and cortisol analysis and rectal grab samples of feces were collected for corticosterone analysis on d 83 of the experiment. Plasma cortisol was not correlated to DMI (r = -0.08, P > 0.05) or fractional DMI (g DMI/kg BW; r = -0.03; P > 0.05), but was negatively correlated with ADG (r = -0.17, P < 0.01) and G:F (r = -0.20; P < 0.01) and positively correlated to residual feed intake (RFI; r = 0.14, P < 0.05). Fecal corticosterone was positively correlated to fractional DMI (r = 0.15, P < 0.05) and RFI (r = 0.23, P < 0.01) and negatively correlated to G:F (r = -0.18; P < 0.01). Using a mixed model analysis, none of the metabolites or hormones were associated with DMI (P > 0.05), but fecal corticosterone was positively associated with BW-adjusted DMI in heifers only (P = 0.04). Plasma lactate (P < 0.01) was and plasma cortisol (P < 0.10) tended to be negatively associated with ADG. Plasma cortisol (P < 0.05) and fecal corticosterone tended (P < 0.10) to be negatively associated with G:F. Fecal corticosterone was positively associated with RFI in heifers (P < 0.04). In a mixed model analysis, total leukocyte count was positively associated with ADG (P < 0.04) and tended to be positively associated with G:F (P < 0.06). Amongst leukocyte subtypes, neutrophil count was positively associated with ADG in steers (P < 0.02) and monocytes were positively associated with ADG in heifers (P < 0.03). Lymphocyte counts (LY) in steers were negatively associated with DMI (P = 0.03) and fractional DMI (P < 0.03). In heifers, LY tended to be positively associated with DMI (P < 0.09) and BW-adjusted DMI (P < 0.06). Lymphocyte count was also positively associated with ADG (P < 0.01) and G:F (P = 0.05) in heifers. The association of production traits with immune status seems to be different between steers and heifers. There was a stronger relationship of cortisol than fecal corticosterone to feed efficiency measures, suggesting that an cortisol concentrations could be a better marker for feed efficiency traits than fecal corticosterone concentrations.