The Influence of Synovex-C Growth Implants at Weaning with Different Supplementation Levels on Ovarian Parameters in Developing Beef Heifers

Abstract Use of growth promoting implants in beef heifers in Southwestern rangelands has been reported to improve growth, however, the effect on ovarian parameters is unknown. Therefore, the objective of this study was to investigate the influence of level of supplementation when heifers were administered a second Synovex-C implant at weaning on ovarian parameters. At two months of age all heifers (n = 17) were administered a Synovex- C implant. At weaning (8-months old), heifers were stratified by body weight and antral follicle counts and assigned to receive one of two levels of supplementation (High or Low) and Implant (No Implant or Implant). Heifers assigned to Implant were administered a second Synovex-C implant at weaning, thereby creating four treatment groups: High Implant (n = 4), High No Implant (n = 4), Low Implant (n = 5), and Low No Implant (n = 4). Heifers were utilized in a complete randomized block design with a 2x2 factorial. Heifers were individually fed supplement three times a week for 26 weeks with refusals being weighed after each feeding to determine intake. For the first 4 weeks of the study, High and Low heifers were offered 4.54 kg/h/d and 2.72 kg/h/d, respectively, of 20% CP cube. After acclimation High and Low heifers were offered 2.95 kg/h/d and 1.59 kg/h/d, respectively, of 32% CP cube for the remainder of the study. At the end of the supplementation period heifers underwent bilateral ovariectomies. Weight, height, and length of each ovary was recorded. Surface antral follicles were counted on both ovaries and recorded. Height and length of the largest preovulatory follicle was recorded and follicular fluid was collected for analysis of estradiol and progesterone concentrations. Ovarian and follicular measurements were analyzed using MIXED procedures in SAS. Ovarian weight (P ≥ 0.16), ovarian length (P ≥ 0.46), or ovarian height (P ≥ 0.32) were not influenced by implant, supplement level, or their interaction. There was no difference among total surface antral follicle count between implant (No Implant: 34.2 ± 4.3 vs Implant: 40.0 ± 4.6), supplement level (High: 32.75 ± 4.6 vs Low: 41.43 ± 4.3), and their interaction (P ≥ 0.19). Preovulatory follicle diameter was not impacted by implant (No Implant: 10.82 ± 1.27 mm vs Implant: 9.42 ± 1.27 mm), supplement level (High: 10.48 ± 1.27 mm vs Low: 9.77 ± 1.27 mm), or their interaction (P ≥ 0.45). Implant, supplement level, or their interaction did not influence progesterone (P ≥ 0.43), estradiol (P ≥ 0.42), or the estradiol:progesterone ratio (P ≥ 0.41) in follicular fluid. In conclusion, administration of a second Synovex-C implant at weaning did not impact ovarian parameters or the composition of follicular fluid. USDA is an equal opportunity provider and employer.