A method to evaluate various parameters of progesterone production in beef cows, using the levels of progesterone in peripheral blood sampled on specific days of the oestrous cycle

Abstract

Progesterone concentrations were determined in blood samples taken sequentially each day during 85 oestrous cycles in six beef cows. The sum of progesterone levels in sequential daily samples for a complete oestrous cycle varied considerably between cycles within cows (range 30–150 ng), and the means of cycles differed markedly between cows (range 36–60 ng). There was a typical pattern of daily progesterone levels throughout the oestrous cycle, and marked fluctuations were evident when the sum of daily progesterone levels for each cycle were plotted sequentially against time. Significant correlations and linear regression equations are described for the relationship between the sum of daily progesterone levels for a complete cycle and the mean concentration of progesterone in various combinations of daily samples. Furthermore, significant correlations were also obtained between the sum of daily progesterone levels and the maximum daily progesterone level obtained ( r = 0.87; P < 0.001), the number of daily progesterone values greater than 2 ng/ml of blood ( r = 0.89; P < 0.001) and the mean of daily progesterone levels for days 7–15 of the cycle ( r = 0.91; P < 0.001). An ability to gain an estimate of these parameters of progesterone production from a small number of blood samples collected at strategic times during the cycle would facilitate the examination of the relationships between the environment, progesterone level, and the subsequent fertility in a greater number of animals, and so provide for a more precise measure of the effects of experimental treatments.