Metabolic status, gonadotropin secretion, and ovarian function during acute nutrient restriction of beef heifers

The effect of acute nutritional restriction on metabolic status, gonadotropin secretion, and ovarian function of heifers was determined in 2 experiments. In Exp. 1, 14-mo-old heifers were fed a diet supplying 1.2 × maintenance energy requirements (1.2M). After 10 d, heifers were fed 1.2M or were restricted to 0.4 × maintenance requirements (0.4M; d 0). Heifers received PGF2α (25 mg, intramuscularly) on d -10, 0, and 10 to synchronize ovulation. After 30 d, 1.2M and 0.4M heifers were realimented to 1.2 M for 100 d. Blood samples were collected every other day from d 0 to 14 then 3 times weekly thereafter. Heifers in Exp. 2 were managed as in Exp. 1 except that animals were fitted with an indwelling jugular catheter and blood samples were collected at 10-min intervals for 8 h on d 9, 10, and 11. Concentrations of progesterone in plasma were used to quantify ovarian luteal function. All 1.2M heifers ovulated, whereas only 30% of 0.4M heifers ovulated in Exp. 1. Concentrations of NEFA were greater and concentrations of thyroxine and IGF-I were less (P < 0.05) in plasma of 0.4M heifers compared with 1.2M heifers. The size of dominant follicles in Exp. 1 was reduced (P < 0.05) in 0.4M compared with 1.2M heifers. Concentrations of IGF-I were increased and anovulatory heifers resumed ovarian cycles an average of 35 d after realimentation. Concentrations of insulin were greater (P < 0.05) in plasma of 1.2M compared with 0.4M heifers in Exp. 2. The frequency of LH pulses was reduced (P < 0.05) in 0.4M heifers on d 9, and FSH in plasma on d 11 was not influenced by treatment. Reduced concentrations of IGF-I in plasma of nutrient-restricted heifers were associated with the reduced size of dominant follicles and indicated a local effect of growth factors on follicles. The decreased LH pulse frequency of 0.4M heifers before luteolysis indicates that restriction of nutrients decreased LH support of follicle growth. A preovulatory increase in estradiol in plasma and an ovulatory surge of LH were not detected in nutrient restricted heifers that did not ovulate. It is concluded that restricting beef heifers to 0.4 × maintenance energy requirements reduced the availability of metabolic fuel and decreased metabolic hormones, resulting in changes within the reproductive neuroendocrine-ovarian axis that compromised the ability of the dominant follicle to secrete sufficient concentrations of estrogen to stimulate an ovulatory surge of LH.