Location: Reproduction ResearchTitle: Metabolic status, gonadotropin secretion, and ovarian function during acute nutrient restriction of beef heifers) Author
Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/5/2013
Publication Date: 9/1/2013
Publication URL: http://handle.nal.usda.gov/10113/59665
Citation: Lents, C.A., White, F.J., Ciccioli, N.H., Floyd, L.N., Rubio, I., Keisler, D.H., Spicer, L.J., Wettemann, R.P. 2013. Metabolic status, gonadotropin secretion, and ovarian function during acute nutrient restriction of beef heifers. Journal of Animal Science. 91(9):4146-4157. Interpretive Summary: Initiation of puberty and maintenance of reproductive function in cattle are related to activity of the ovaries, which are controlled by the release of reproductive hormones from the pituitary gland. It is generally well understood that reproductive function is impacted by metabolism in animals. During normal production scenarios, beef cattle can experience fluctuations in metabolic state due to changes in their body’s demand for nutrients, and young growing heifers may be more sensitive to these acute metabolic changes. To determine how metabolism affects reproductive efficiency in cattle, scientists investigated how acute changes in nutrient flux affected metabolic and reproductive hormones in beef heifers and related these changes to differences in ovarian function. They discovered that acute changes in nutrient intake impaired ovulation. This was associated with alterations in metabolic fuels such as glucose and fatty acids. They also found that concentrations of insulin-like growth factor-I, a hormone that controls growth of somatic tissues, changed with level of nutrient intake and that size of ovarian follicles is associated with concentrations of this hormone. Furthermore, the release of reproductive hormones from the pituitary gland was altered with metabolic state. These responses were variable, indicating that improvement in the efficiency of beef cattle production can be achieved if the biological and genetic basis underlying this variability can be identified.
Technical Abstract: 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 PGF2a (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.