RESUMEN
Peripheral plasma progesterone levels were compared in "Normandy" (6), "Charolais" (6) and French Friesian (6) heifers. The progesterone curves were fitted to mathematical models for the luteal phase (a logistic function) and for the luteolytic phase (an exponential model). 1. Discriminating and multivariate analyses showed a breed difference in luteal phase duration (17.8 +/- 1.1, 16.8 +/- 2.3, 16.1 +/- 1.06 days, respectively, for Normandy, Charolais and French Friesian heifers) and in the rate of progesterone level increase (1.1 +/- 0.4, 1.09 +/- 0.3, 1.3 +/- 0.3 ng/ml/day, respectively) (chi 2 = 22.5; P less than 0.02). 2. Progressive multiple regression showed the duration of the luteal phase was highly correlated to oestrous cycle duration (20.8 +/- 1.7, 20.5 +/- 2.8, 19.2 +/- 1.5 days, respectively) (r = 0.89). It appears that breed differences in bovine oestrous cycle duration are due to differences in luteal phase duration. 3. The rate of progesterone level increase reached a maximum at 7.2 +/- 0.8, 6.9 +/- 1.1 and 6.4 +/- 0.6 days, respectively, concomitant with maximal corpus luteum secretion. 4. The reproducibility of these parameters, summarizing the progesterone pattern during two consecutive oestrous cycles, showed that each animal had an individual secretion pattern.
