Monitoring of plasma and milk progesterone for evaluation of postpartum estrous cycles and early pregnancy in mares.

Plasma and milk progesterone concentrations in 13 mares were determined 3 times a week for 5 months, beginning at parturition. The estrous cycle was divided into 2 phases. Estrus was considered to occur when the plasma progesterone concentration was less than 1 ng/ml, with diestrus occurring when plasma progesterone content was greater than or equal to 1 ng/ml. Based on this classification, the period of estrus averaged 8.9 days, diestrus averaged 13.9 days, and the estrous cycle averaged 22.8 days. During estrus, the progesterone concentration in plasma averaged 0.4 ng/ml and in milk averaged 2.2 ng/ml. During diestrus corresponding values were 6.8 and 6.9 ng/ml. In the early pregnant mare, the average progesterone concentration was 5.4 ng/ml for plasma and 4.7 ng/ml for milk. The progesterone pattern was similar in milk and in blood plasma, and the correlation between 362 paired values was 0.7 (P less than 0.01). It was concluded that the estrous cycle of the mare can be mapped accurately by measuring plasma or milk progesterone content at 2- to 3-day intervals. A breeding program based on monitoring blood progesterone was outlined.