EFFECTS OF POSTFEEDING INTERVAL ON BLOOD CONSTITUENTS RELATED TO ENERGY METABOLISM IN NONPREGNANT ANGUS AND HEREFORD HEIFERS

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Abstract
Ten Angus heifers and 10 Hereford heifers were fasted for 48 h to study changes in blood glucose, plasma nonesterified fatty acids (NEFA), and plasma ketone concentrations and their relationships to prefast feeding levels and breed. Prior to fasting, five heifers of each breed were fed for 140 days on one of two rations, which produced significantly different average rates of gain (0.16 vs. 0.47 kg/day) and average depths of subcutaneous adipose tissue (5.0 vs. 10.3 mm). Blood was sampled at 1, 24, and 48 h postfeeding. Blood glucose concentrations were higher (P < 0.01) at 24 h postfeeding but did not change from 24 to 48 h postfeeding. Plasma NEFA concentrations increased during the 48-h fast. Plasma ketones were lower (P < 0.01) at 24 h postfeeding but at 48 h were higher (P < 0.01) than at 1 h postfeeding. Packed cell volume (PCV) increased during 24 and 48 h of fasting. Blood concentrations of glucose, NEFA, and ketones at either 24 or 48 h of fasting were not influenced by prefast feeding level. Angus heifers had higher (P < 0.05) blood glucose and higher (P < 0.05) PCV than Hereford heifers. Correlations between the blood constituents at the three sampling times were generally low. Only the correlations of plasma NEFA with ketone concentrations at 24 h postfeeding (.55) and glucose with PCV at the initial sampling (−.52) reached significance (P < 0.05).