Implications of ovarian and uterine maturity evaluated in replacement heifer candidates
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Profitability across the reproductive lifetime of a cow depends on reproductive success in the first breeding season and requires heifers to attain puberty prior to this time. Pre-breeding evaluations may be used to determine pubertal status of heifers via reproductive tract evaluation. Disparities in ovarian and uterine maturity may be detected at the time of evaluation in peripubertal heifers. Two experiments were conducted to characterize the incidence and implications of disparate ovarian and uterine development during the peripubertal period. Reproductive tract development was assessed, and an individual ovarian score (OS) and uterine score (US) were assigned. Pregnancy outcomes were evaluated on the basis of OS, US and reproductive tract score (RTS: first digit = OS; second digit = US) and relationships between reproductive and physiologic maturity were investigated. In Experiment 1, duplicate pre-breeding evaluations (PBE) were conducted on 469 heifers approximately 40 and 30 days prior to the start of breeding seasons from 2019-2021. Scores for ovarian (1 = no ovarian development, pea sized; 2 = very few follicles < 8 mm; 3 = several follicles 8-10 mm; 4 = large preovulatory follicle > 10 mm; 5 = corpus luteum present) and uterine (1 = infantile, undeveloped, difficult to palpate; 2 = poorly developed; 3 = distended, moderately developed; 4 = coiled, well-developed, toned; 5 = distended, well-developed) development were assigned following assessment via transrectal palpation and ultrasonography. Uterine horn diameter (UHD), antral follicle count (AFC), largest follicle diameter (LFD), weight, body condition score (BCS), age, and pelvic area (PA) were recorded. Heifers were subjected to the 14-day CIDR-PG estrous synchronization protocol. Split-time artificial insemination was performed and followed by exposure to bulls 14 days later. Pregnancy diagnosis was performed via transrectal ultrasonography 75-90 days after artificial insemination In Experiment 2, data from 22,173 heifers custom developed by Heartland Cattle Company from 2014-2018 were analyzed retrospectively. Pre-breeding evaluations were conducted 35-45 days prior to breeding and 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 were assigned following assessment via transrectal palpation. Weight, hip height, and PA were recorded, and average daily gain (ADG) was calculated for the development period. Heifers were subjected to the 14-day MGA-PG protocol and artificial insemination (AI) were performed based on detected estrus. Pregnancy diagnosis was performed using transrectal ultrasonography 45 days after the end of the breeding season. The incidence of disparate ovarian score (OS) and uterine score (US) was 33.7 percent (n = 158/469) in Experiment 1 and 16.3 percent (n = 3,622/22,174) in Experiment 2. Heifers assigned a RTS of less than 3-3 (Experiment 1 = 4.3 percent (n = 20/469); Experiment 2 = 0.6 percent (n = 135/22,174)) demonstrated poor reproductive performance as greater proportions failed to become pregnant in Experiment 1 (P = 0.03) and conception to first service AI was decreased in Experiment 2 (P < 0.01). Reproductive performance did not differ (P > 0.10) between heifers assigned disparate or non-disparate scores of greater than RTS = 3-3. In both experiments, heifers achieving greater physiologic maturity as indicated by age, weight, pelvic area (PA), average daily gain (ADG), or hip height (HH) exhibited greater reproductive maturity as measured by OS and US, respectively. We propose that disparate ovarian and uterine development is the result of rapid and asynchronous growth of the reproductive tract during the peripubertal period. Consequently, independent assessment of ovarian and uterine maturity may increase precision in characterizing the reproductive maturity of heifers, predicting proximity to puberty attainment, and identifying prepubertal heifers that are unlikely to exhibit satisfactory reproductive performance in the first breeding season.