The effects of dietary protein on age and weight at the onset of puberty in Brown Swiss and Zebu heifers in the tropics.

The effects of dietary protein level on age and weight at onset of puberty were studied in the Mexican tropics using 12 Brown Swiss and 12 Zebu heifer calves in a completely randomized block design. The calves were weaned at 3 mo of age. They were confined in individual pens and fed either an adequate protein (12.8%) or a high protein (16.4%) diet ad libitum through the onset of puberty, which was defined as the first ovulation. To detect the first ovulation, heifers were palpated at 8 mo of age and then every 2 wk. At the detection of a developing follicle or any behavioral sign of estrus, they were palpated daily. Ovulation was confirmed by palpation of a corpus luteum. A blood sample for the determination of plasma progesterone concentration was taken with each palpation. There was a significant effect of breed on age at the onset of puberty, which was 9.4 and 12.3 mo for the Brown Swiss and Zebu heifers, respectively. Weight at puberty was 233 kg, independent of breed. Dietary protein had no effect on the onset of puberty. Brown Swiss heifers reach puberty earlier than Zebu heifers. Heifers in the tropics fed according to NRC recommendations reach puberty at ages comparable to those in temperate climate. This suggests that, if managed properly, calving age for Brown Swiss and Zebu heifers can be 24 and 27 mo, respectively. This is a considerable improvement from 36 to 48 mo of age now common in the area of study.