Body weight, tick burden (Boophilus microplus), physiological parameters and reproductive efficiency of crossbred zebu cattle.

: Body weight at calving, degree of tick load (Boophilus microplus), rectal temperature, respiratory frequency and heart rate were evaluated and correlated to reproductive efficiency in 63 F1 Brown Swiss x Nellore milked animals reared in the tropical region of Peru. Cows were bred by free natural service, and first ovulation was estimated by progesterone measurements. The physiological parameters were registered at morning milking (04.00 to 06.00) at an average air temperature of 22.4 +/- 0.9 degrees C. The intervals from parturition to first ovulation and conception were significantly affected when the body weight at calving was lower than 400 kg (p less than 0.01). The degree of tick burden, rectal temperature (38.4 +/- 0.2 degrees C, mean +/- s.e.), respiratory rate (25.2 +/- 0.3 breaths/min) and heart rate (66.7 +/- 0.4 beats/min) were not related to post-partum reproductive efficiency. The average intervals from parturition to first ovulation and to conception were 56.3 +/- 55.5 and 77.7 +/- 46.0 days mean +/- s.d.), respectively. The fertility rate was 85.7%. The results indicate the F1 cross to be well adapted to the given environment. It remains to be determined whether higher levels of temperate blood can be used without an accompanying decrease in reproductive performance.