Influence of Steroids and Gonadotropins on Reproduction in Beef Cattle

The objective of this thesis was to evaluate the influence of steroids and gonadotropins on reproduction in beef cattle. In experiment 1, beef heifers were used to determine the influence of growth-promoting implants on growth, reproductive development, estrous behavior, and pregnancy rate. Heifers were assigned to 1 of 4 implant treatment groups: control (CON); trenbolone acetate (TBA); trenbolone acetate plus estradiol (TBA+E2) or zeranol (ZER). Heifers were implanted once, A.I. and exposed to bull during this experiment. Body weight, BCS, HH, RTS, estrous behavior and pregnancy data were collected throughout this experiment. Average daily gain of heifers was greater for TBA+E2 heifers. Fewer heifers treated with ZER were classified with a cyclic RTS on d 106 than CON and TBA treated heifers while heifers treated with TBA+E2 were similar to all treatments. Heifers treated with TBA had increased mounts during estrus compared with all other treatments. Overall and A.I pregnancy rates did not differ among treatments.In experiment 2, superovulated beef donors were utilized to determine the feasibility of performing a cow-side LH assay (PrediBov®) on superovulated donors, with emphasis on determining how to use the results in a commercial program. Donors were subjected to superstimulation; blood samples were collected starting at CIDR removal, continuing every 6 h until a positive test was acquired or 36 h after CIDR removal. Whole blood (0.5 mL) was submitted to the assay and donors were inseminated approximately 12 and 24 h after a positive test or onset of estrus. The majority of positive LH tests occurred within 12 to 24 h after CIDR removal. Forty-four percent of the positive tests occurred 0 to 6 h after the onset of estrus. Donors that were inseminated 6 to 10 h after a positive LH test produced more viable and grade 1 embryos than donors inseminated either \u3c 6 or 10 to 14 h after a positive test. There were no differences in embryo production between insemination times from the onset of estrus or between donors inseminated approximately 12 and 24 h after a positive test or the onset of estrus