PS5-14. Rapid Resynchronization of the Bovine Estrous Cycle to Facilitate Multiple Services with Artificial Insemination.

Abstract While estrous synchronization for artificial insemination (AI) is well established, no standardized U.S. protocol exists to identify and re-inseminate beef females failing to conceive after the first AI. This study evaluated progestin treatment initiated 13 d after fixed-time AI (FTAI) to synchronize estrus return and Color Doppler ultrasonography for early identification of non-pregnant heifers based on corpus luteum (CL) functionality. Thirteen days after FTAI (D0), 1,447 Angus-influenced replacement heifers were randomly assigned to oral melengestrol acetate for 6 d (MGA; n = 485), insertion of a once-used intravaginal progesterone-releasing device for 7 d (CIDR; n = 483), or no progestin treatment (Control; n = 479). Reused CIDRs had been used in one previous 7-d synchronization protocol. From D14 to D23, heifers were monitored twice daily for estrus using ESTROTECT™ patches and re-inseminated 9–14 h after standing estrus or > 50% patch activation. On D24, heifers not detected in estrus underwent subjective Color Doppler evaluation of total area of CL with blood flow (CLBF), CL area measurement, and blood collection. Heifers with CLBF <25% were classified as non-pregnant, immediately re-inseminated, and administered GnRH. Conventional B-mode ultrasonography confirmed pregnancies after the first and second AI on D60. Progestin treatment delayed but synchronized estrus return (n = 663). Compared with controls, the hazard of estrus expression was reduced by 39% in CIDR-treated heifers (hazard ratio = 0.61) and 65% in MGA-treated heifers (hazard ratio = 0.35; P < 0.0001), extending the median interval to estrus by 2 d without affecting overall estrous response (69%, 67%, and 62% for Control, CIDR, and MGA, respectively; P = 0.31). Because MGA further delayed estrus response, more heifers remained eligible for Doppler evaluation and re-insemination on D24 (Control: 8b%, CIDR: 14ab%, MGA: 19a%; P = 0.006). Consequently, progestin treatment increased the overall re-insemination rate (Control: 84b%, CIDR: 94a%, MGA: 90ab%; P = 0.006), with Color Doppler ultrasonography increasing re-insemination by approximately 13 percentage points. First-service pregnancy per AI diagnosed by Color Doppler (66%, 63%, and 62%; P = 0.36), confirmed by B-mode ultrasonography (56%, 54%, and 53%; P = 0.72), and the false-positive Doppler diagnosis (16%, 14%, and 14%; P = 0.51) were similar among Control, CIDR, and MGA treatments, respectively. False-positive diagnoses represented missed re-insemination opportunities because heifers incorrectly classified as pregnant were not re-inseminated. False- and true-positive heifers also had similar CL area (3.1 cm2) and serum progesterone concentrations (7.7 ng/mL) on D24, suggesting potential embryonic loss, which will be further evaluated using pregnancy-associated glycoproteins. Although progestin-treated heifers had lower second-service pregnancy per AI (Control: 65a%, CIDR: 51ab%, MGA: 48b%; P = 0.055), cumulative pregnancy by D24 remained similar among treatments (76%, 79%, and 74%, respectively; P = 0.37) because of higher re-insemination rates. In conclusion, progestin treatment initiated 13 d after FTAI synchronized estrus return, increasing the number of non-pregnant heifers identified for re-insemination by Color Doppler assessment of corpus luteum functionality. However, despite higher re-insemination rates, progestin treatment did not improve cumulative pregnancy after two AI services within 24 d.