Abstract #T153

# T153
Effect of one versus two prostaglandin F injections on progesterone concentrations and luteolysis in dairy cows subjected to a 5-d CIDR-Cosynch.
J. Spencer*1, K. Carnahan1, W. Price2, B. Shafii2, A. Ahmadzadeh1, 1Animal and Vet. Sci., University of Idaho, Moscow, ID, 2Statistical Program, University of Idaho, Moscow, ID.

The objective of this study was to examine the effects of one or 2 doses of prostaglandin F (PG) on progesterone (P4) concentration profiles, and luteal function in dairy cows subjected to a 5-d CIDR-Cosynch protocol. On d 0, 43 cows received GnRH (100µg; i.m.), and a CIDR was inserted. On d 5, the CIDR was removed and cows were assigned randomly to receive one dose of PG (25mg) (1PG; n = 15), or 2 doses (25mg/dose) of PG 12 h apart (2PG; n = 19). On d 5 and after CIDR removal, serial blood samples were collected every 12 h from d 5 to 8 to measure P4 concentrations. On d 8, all cows received a second GnRH injection and were inseminated. Ovaries were examined by ultrasonography on d 0, 5 and 8, and ovarian structures were recorded. Nine cows were determined acyclic and/or did not have luteal tissue at CIDR removal, thus were removed from this study. A repeated measures, generalized linear mixed model ANOVA was used to determine differences in P4 concentrations. The model included treatment, the repeated factor time, and time by treatment interaction as fixed effects, and cow within treatment was the random effect. The P4 data were assumed to follow a lognormal distribution. On d 5, all cows had a CL or developed luteal tissue, and P4 concentrations were similar between groups (P > 0.1) before treatments. As expected, P4 concentrations decreased over time in both treatments (P < 0.01). However, there was a significant effect of treatment by time interaction on P4 concentrations (P < 0.01). By 24 h after treatments, P4 concentrations were lower (P < 0.01), and remained lower (P < 0.01) throughout the the remainder of the experiment in 2PG compared with 1PG. By 72 h post-treatment, the average P4 concentrations were 1.03 ng/mL and 0.05 ng/mL for 1PG and 2PG, respectively. These results indicate differences in P4 repsonse between 1PG and 2PG in cows subjected to a 5-d CIDR-Cosynch. Compared with 1PG, 2PG injections (12 h apart) was more effective in inducing luteolysis, as P4 concentrations were significantly less by the time of AI.

Key Words: prostaglandin F, progesterone, luteolysis