237 - Incidence and Implications of Disparate Uterine and Ovarian Development Observed Among Heifers Evaluated During the Peripubertal Period

Rapid development in the reproductive tract occurs in the peripubertal period, with variable rates of development of reproductive tissues potentially resulting in perceived disparities in ovarian and uterine development at pre-breeding examination. Data collected from 22,173 heifers evaluated over five consecutive breeding seasons (2014-2018) were analyzed retrospectively to assess relationships between observed disparities in uterine and ovarian development and the timing of conception within the breeding season. Pre-breeding evaluations were conducted 35-45 days prior to breeding. Weight, hip height, and pelvic area were recorded. A single veterinarian conducted all reproductive evaluations via rectal palpation, and heifers were assigned individual scores for ovarian (2=infantile, 3=no significant structures, 4=large follicle and/or corpus luteum) and uterine (2=infantile, 3=mid-sized, distended tract, 4=well-vascularized, distended or coiled tract) development respectively. Heifers received artificial insemination (AI) based on detected estrus following the 14-day MGA-PG protocol. Pregnancy diagnosis was performed via transrectal ultrasonography. Pregnancy rates to AI (P/AI) were greater for scores 4-4 (n = 14,567) in comparison with 3-3 (n=3,941; P < 0.0001) or 2-2 (n = 44; P < 0.0001) and likewise were greater (P < 0.0001) for scores 3-3 in comparison with 2-2. Heifers with disparate scores of 2-3 (n = 38) did not differ (P = 0.53) from those with disparate scores of 3-2 (n = 53) with respect to P/AI, but either disparity was associated with greater (P < 0.01) P/AI relative to scores of 2-2 and reduced P/AI relative to scores of 3-3 (P < 0.01). Scores of 4-3 (1,698) and 3-3 did not differ (P >0.44) in P/AI, but P/AI tended (P = 0.06) to be greater among scores 3-4 (1,832) in comparison with 3-3. Similar (P > 0.33) P/AI was observed among heifers scored 4-4, 4-3, or 3-4. In summary, disparate ovarian and uterine development impacted timing of conception within the breeding season among heifers with lower tract scores. Such disparities were observed among 0.4% of heifers evaluated.