NATURAL AND PROSTAGLANDIN-F (PG)-SYNCHRONIZED ESTROUS-CYCLE IN BROWN SWISS AND SIMMENTAL HEIFERS IN THE HIGHLAND OF PERU

The objectives of this study were to evaluate the length of the estrous cycle, duration of estrus, time of ovulation and progesterone patterns during prostaglandin F (PG)-synchronized (PG-S) and natural estrous cycles in Brown Swiss (BS) and Simmental (SIM) heifers reared in the Peruvian Highland (elevation 4250 m). Two- to three-year-old Brown Swiss (n = 8) and SIM (n = 4) heifers that had shown at least three estrous cycles were synchronized with two injections of Tiaprost (150 mg per 5 ml) 11 days apart. The same heifers were evaluated during their third estrous cycles after PG synchronization, which were considered natural cycles. Progesterone concentrations were determined in serum collected at daily intervals; estrus was detected visually. The ovaries of each heifer were examined 30 h after onset of PG-S and fourth estrus following PG treatment to determine the occurrence of ovulation. Percentages of BS and SIM heifers that exhibited a decrease in progesterone concentrations to levels lower than 1 ng ml-1 and behavioral estrus after PG treatment (responding heifers) did not differ (P > 0.10). The interval from PG injection to the onset of estrus and progesterone concentrations for 4 days after PG for responding heifers did not differ (P > 0.10) between breeds. The duration of estrus did not differ (P > 0.10) between breeds or type of cycle. More (P < 0.05) ovulations occurred after 30 h from the onset of estrus than before 30 h. Estrous cycle lengths were longer (P < 0.05) in PG-S heifers compared to their natural cycles (21.8 and 20.7 days, respectively). Breed, type of cycle (PG-S or natural) or interaction of breed by type of cycle did not influence (P > 0.10) progesterone patterns during estrous cycles. However, there was an interaction (P < 0.05) of day by type of cycle for progesterone concentrations on Days 3-10, indicating that corpus luteum function was delayed during PG-S cycles in heifers of the Peruvian Highland.