Dietary protein and energy restriction in mares: rapid changes in plasma metabolite and hormone concentrations during dietary alteration

Two diets consisting of bermudagrass hay and a corn-cottonseed hull-based supplement were formulated to provide either 100% (control) or 50% (restricted) of the protein and energy requirements for maintenance for mature mares. Twelve light horse mares were fed the control diet for 7 d, and then at 0800 on d 0, six mares were switched to the restricted diet. All diets were fed as two equally sized meals at 0800 and 1600. At 0800 on d 7, mares receiving the restricted diet were switched back to the control diet. Relative to control mares, mares switched to the restricted diet had reduced plasma concentrations of glucose (P = 0.005) and insulin (P = 0.09) in response to the two restricted meals on d 0. However, concentrations of both glucose and insulin returned to control levels (P 0.1) within 1 h after the consumption of the control diet on d 7. Dietary restriction increased (P = 0.009) plasma NEFA concentrations within the first 24 h, and NEFA concentrations remained elevated (P 0.001) in restricted mares until the mares were returned to the control diet on d 7. Meal-induced increases (P 0.05) in plasma concentrations of glucose, insulin, urea N, glucagon, and thyroxine were observed. These results emphasize the importance of early sampling when monitoring plasma constituents during nutrient alterations and indicate that prefeeding responses of plasma constituents alone may not fully explain the metabolic consequences of nutrient restriction