Changes in concentrations of hormones, metabolites, and amino acids in plasma of adult horses relative to overnight feed deprivation followed by a pellet-hay meal fed at noon.

Experiment 1 was conducted to characterize the concentrations of prolactin, growth hormone (GH), cortisol, insulin, glucagon, glucose, nonesterified fatty acids (NEFA), urea N, and 10 indispensable amino acids in the plasma of mares (n = 8) and stallions (n = 8) during the last 4 h of a 19-h period of feed deprivation and for 8 h after a noon meal. Experiment 2 was similar to Exp. 1 except that only stallions (n = 8) were used, and they were either fed (n = 4) or not fed (n = 4) at noon in a 2 x 2 Latin square design conducted over two sampling days 7 d apart. In Exp. 1, increases (P < .01) after feeding were observed for plasma concentrations of prolactin, cortisol, insulin, glucagon, glucose, urea N, and all amino acids except methionine; NEFA concentrations decreased (P < .01) after feeding. Episodic increases in GH concentrations were observed for most horses but were not associated with either feeding or gender (P > .1). Plasma urea N concentrations were higher (P < .025) overall in stallions than in mares, and the rise in prolactin concentrations after feeding was greater (P < .01) in stallions than in mares. In Exp. 2, meal-associated increases (P < .01) were observed for plasma concentrations of prolactin, insulin, glucagon, and glucose; NEFA concentrations decreased (P < .01). Except for cortisol, no hormone or metabolite varied with time across days when the stallions were not fed (P > .1), indicating that there was no inherent diurnal or feeding schedule-associated fluctuations in their concentrations. Cortisol concentrations varied (P < .02) over time but did not differ (P > .1) between fed and nonfed stallions. Again, GH concentrations were episodic but did not differ (P > .1) between fed and nonfed stallions. The lack of feeding effects on GH secretion in horses is similar to the response in pigs but differs from that in ruminants, in which GH concentrations generally decline after feeding.