Seasonal variations in plasma progesterone concentrations in cyclic Taiwan yellow cattle (Bos indicus)

Plasma concentrations of progesterone were measured by radioimmunoassay in plasma samples obtained from 11 mature cyclic cows of native Taiwan Yellow cattle during the warm (March-April) and cool (November-December) seasons. Progesterone levels began to increase after Day 4 (Day 0 = the day of estrus) and reached maximum values on Day 10 and Day 14 for cool and warm seasons, respectively. Progesterone concentrations in plasma during the estrous cycle were significantly higher (p < 0.001) in the cool season (2.66 +/- 0.19 ng/ml) than in the warm season (1.98 +/- 0.15 ng/ml).