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* Department of Animal Science, University of Minnesota, St. Paul 55108-6118
Northwest Research and Outreach Center, University of Minnesota, Crookston 56716-5000
North Central Research and Outreach Center, University of Minnesota, Grand Rapids 55744-3361
4 Corresponding author: crook001{at}umn.edu
Multiparous cows (n = 59) were blocked by expected calving date and previous 305-d mature-equivalent milk yield and assigned randomly to a 2 x 2 factorial design to determine the effects of bovine somatotropin (bST; Posilac, Monsanto Animal Agricultural Group, St. Louis, MO) and dietary fat on ovarian activity during the first 90 d in milk (DIM). Diets that included whole, high-oil sunflower seeds [SS; 10% of dietary dry matter; rich in linoleic acid (18:2)] or a mixture of Alifet-High Energy and Alifet-Repro [AF; Alifet USA, Cincinnati, OH; 3.5 and 1.5% of dietary dry matter, respectively; protected source of linolenic (18:3), eicosapentaenoic, and docosahexaenoic fatty acids] were provided from calving. Diets were isocaloric at equal intakes, but AF provided more net energy for lactation at actual intakes (1.54 vs. 1.66 Mcal/kg of dry matter). Cows received 0 or 500 mg of bST (N, Y) every 10 d from 12 to 70 DIM and at 14-d intervals from 70 to 280 DIM. Breeding was initiated after 90 DIM. Follicular dynamics, luteal growth and development (15 to 90 DIM), and plasma progesterone concentrations (1 to 90 DIM) were evaluated (3 times per week). Days to first ovulation (33.6 ± 1.4) and incidence of anovulation at 45 or 70 DIM did not differ among treatments. Interovulatory intervals were similar among treatments (22.1 ± 0.9 d). Incidence of estrous cycles with 2 follicular waves was greater for SSY (71.0%) and AFN (80.0%) than for other groups, but more 3-wave cycles occurred with AFY (83.3%). Growth rate of the ovulatory follicle was greater for AF than SS (1.9 vs. 2.2 ± 0.11 mm/d) and diameter of ovulatory follicles was larger for AFN than the other treatments (17.9 vs. 15.7 ± 0.7 mm). Area under the progesterone curve was reduced for SSY (63.2, 48.1, 55.5, and 61.4 ± 5.1 ng·d/mL for SSN, SSY, AFN, and AFY, respectively). The number of class 1 (3 to 5 mm) follicles was decreased and the number of class 2 (6 to 9 mm) follicles was increased by bST. The number of class 2 follicles was reduced by AF. Initiation of bST administration at 12 DIM and dietary n-3 fatty acids altered ovarian activity during the first 90 DIM and could benefit reproductive performance. Dietary n-3 fatty acids interacted with bST administration in early lactation to increase the incidence of estrous cycles with 3 follicular waves. Although these changes could benefit reproductive performance, evaluation with a larger number of cows is needed to determine if these alterations improve fertility.
Key Words: somatotropin • n-3 fatty acid • follicular dynamic
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