Detection of estrus was determined by walking activity recorded by the radiotelemetric pedometer (Gyuho system, ComTech), as well as visual observation heat-mount detectors (KAMAR Product Inc., Zionville, IN, USA) and plasma progesterone concentrations. Estrus based on pedometer activity was defined as a 1.5-fold increase in the number of steps over the average number of steps for the previous 2 weeks. The duration of estrus was defined as the period of increased step rate.