517. RELATIONSHIPS OF LUTEAL PHASE VARIABLES (PRIOR TO AI) WITH FOLLICULAR WAVES IN DAIRY COWS

Abstract

This study was done to consider relationships of luteal phase variables (prior to AI) with follicular waves. The estrous cycles of 10 cows were synchronized with 2 im injections of prostaglandin F2α given 11 d apart. The cows were randomly assigned to 1 of 2 treatments. Cows in the control treatment received no treatment, whereas GnRH6 cows received a GnRH injection on d 6 of the estrous cycle (estrus = d 0). Daily, from estrus d 0 to the next estrus d 23, cows had their ovaries scanned by ultrasound. Blood samples were collected by tail following each ultrasound examination from estrus until next estrus (estrus = d 0). Concentrations of plasma progesterone were determined by radioimmunoassay kit. The limit of detection of the assay was 0.125 ng/mL and the intra- and interassay coefficients of variation were 8.4% and 10.2%, respectively. Data that were analyzed by using PROC GLM of SAS. For comparisons between groups, the 2-sample t-test was used for continuous traits, such as size of cl or hormone concentrations. Prospective comparisons of indices of progesterone indicated that the length of luteal lifespan was longer in three-wave than in two-wave cows (P<0.01). Plasma progesterone concentrations were similar at peak and measured as area under the curve on day 5 through 17 preceding insemination in two-wave (6.70±0.30 ng/ml) and three-wave cows (7.30±0.50 ng/ml). Length of the luteal phase (defined as from the day of estrus until the last day on which plasma progesterone remained greater than 2 ng/ml) was <2 days shorter in two-wave cows than in three-wave cows (16.7±0.30 vs. 18.40±0.50 d; P<0.05). In addition, the day of peak progesterone occurred earlier in two-wave cows (13.50±0.40 vs. 16.30±0.70 d; P<0.05).