246 Incorporation of chorionic gonadotropin (hCG) into the presynchronization portion of the 7 & 7 Synch protocol for postpartum beef cows.

Abstract Data were collected to evaluate the incorporation of chorionic gonadotropin (hCG) into the presynchronization portion of the 7 & 7 Synch protocol for postpartum beef cows. Cows (n = 455) of varying parities across locations within the University of Missouri Agricultural Experiment Station, University of Tennessee Agricultural Experiment Station, and producer locations were assigned to one of three treatments. The standard CIDR-based 7 & 7 Synch protocol (administration of prostaglandin F2α [PG; 500 µg cloprostenol sodium] coincident with CIDR insertion on Day -17, gonadotropin-releasing hormone [GnRH; 100 μg gonadorelin] on Day -10, and PG coincident with CIDR removal on Day -3) served as a reference control treatment (7 & 7 treatment). The other two treatments followed the same treatment schedule but with chorionic gonadotropin (hCG) administered on Day -17, either coincident with PG administration and CIDR insertion (hCG + CIDR treatment) or coincident with PG administration with no CIDR (hCG treatment). On Day 0, fixed-time artificial insemination (FTAI) was performed at 66 h after PG administration. At a single location, transrectal ovarian ultrasonography was performed on Days -17, -10, and -3 of the treatment schedules to characterize presence and number of CL as well as diameter of the largest follicle present. Blood samples were collected on the same days and analyzed for serum progesterone and estradiol concentrations. Pregnancy diagnosis data was collected 75-90 days post FTAI. Cows in the hCG and hCG + CIDR treatments were more likely to have a CL and/or multiple CL present on Day -3 compared to control cows (P = 0.01). Largest follicle diameter (mm) did not differ when measured on Day -3 among treatments (P = 0.2; 7 & 7 = 12.8 ± 0.4; hCG + CIDR = 12.3 ± 0.4; hCG = 13.4 ± 0.5). There was a tendency for the proportion of cows expressing estrus by 66 hours post PG administration to be greater among cows that received a CIDR (P = 0.09; 7&7 and hCG + CIDR = 71.9% [220/306]; hCG = 44.3% [66/149]). Pregnancy rates tended to be greater among cows receiving a CIDR when performed at a fixed time of 66 h (P = 0.06; 7&7 and hCG + CIDR = 69% [211/306]; hCG = 54% [80/148]). In summary, chorionic gonadotropin can be effectively used to induce CL formation in beef cows and has potential for application in the presynchronization portion of estrus synchronization protocols. However, further work may be required to optimize results with hCG when exogenous progestins are not included in the protocol.