Influence of Insulin and Energy Intake on Ovulation Rate, Luteinizing Hormone and Progesterone in Beef Heifers

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Abstract
Ovarian and gonadotropin responses to insulin and energy restriction were investigated in a 2 × 2 factorial experiment using 2-yr-old Brangus heifers. Thirty heifers were paired by weight and body condition, then assigned to treatment groups receiving 75 (LE) or 180% (HE) of NRC recommendations for dietary energy for maintenance. Diets were adjusted weekly to maintain daily .25 to .5 kg weight loss or 0 to .25 kg weight gain, respectively. On d 10 of the first estrous cycle subsequent to the initial 45 d of feeding, heifers within each dietary group were allocated to receive twice daily infusions of either 40 U insulin (I) or saline (C). Infusions began at 5 and 10 h postprandial and were given in six boluses, 20 min apart. Infusions continued daily until d 20 or estrus, whichever occurred first. On d 11, blood samples were collected at 15-min intervals for 12 h to determine luteinizing hormone (LH) and insulin concentrations. On d 16 to 20, twice daily im injections of 1 mg follicle stimulating hormone (FSH) were administered. Heifers were ovariec-tomized on d 11 after estrus. Number of corpora lutea (CL) in LE-I heifers was greater (P<.05) in LE-C, HE-C or HE-I. Total CL weight (g) per heifer was greater (P<.05) in HE-C and LE-I heifers than in LE-C. Individual CL wt was heavier in HE than in LE heifers (P<.05). Corpus luteum content (µg/CL) was greatest (P<.05) in HE heifers and total CL progesterone per heifer tended (P<.1) to be higher in HE-C and LE-I. Concentration of CL progesterone (µg/g) did not differ (P>.1). Time of treatment at ovariectomy was 77 ± 2 and 79 ± 2 d for HE and LE, respectively. Endogenous insulin concentrations were greater in the HE-C group than in LE-C (P<.05). Infusion of insulin increased serum insulin concentration in LE-I more than in HE-I (P<.05). Serum LH in frequent interval samples and serum progesterone in samples taken weekly before insulin treatment, daily during insulin or saline infusion and prior to ovariectomy did not differ by treatment group (P>.1). Conclusions are that energy restriction reduced CL weight and decreased insulin secretion or metabolism but not LH or progesterone in intact heifers. Exogenous insulin increased ovulation rate in energy-deprived beef heifers. Copyright © 1986. American Society of Animal Science. Copyright 1986 by American Society of Animal Science.

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