Factors influencing gestation length, birth weight and calf survival of Angus, Zebu and Zebu cross beef cattle.

Day of birth in a calendar year (DB), gestation length (GL), birth weight (BW) and survival rate of calves (SR) were recorded for 4 years for single-sire matings of (1) Angus (A) and Brahman (B) sires to A, B, Brangus (BR) and Africander-Angus (AF) dams; (2) BR sires to BR dams, and (3) AF sires to AF dams, a total of four straightbred and six crossbred breed groups. Sex (S) differences were found for GL, BW and SR (P less than .05 to P less than .01), but not for DB (P greater than .05). Age of dam had no effect on GL but influenced DB (P less than .01), BW (P less than .01) and SR (P less than .05). Precalving weight of the dam influenced BW (P less than .01) but not GL. Birth weight was influenced by GL (P less than .01). Differences among breed groups were found for all traits. Purebred A had the shortest GL and the earliest BD, while purebred B had the longest GL, the latest BD, th lightest BW and the lowest SR of any of the breed groups. The BR and AF had higher BW but were generally intermediate to the A and B in other traits. Combined breed of sire and dam effects acccounted for most of the breed differences. The response and percentage heterosis obtained for such traits as DB and SR depended upon characteristics of the breeds involved and environmental factors such as date of breeding season and weather at calving. Heterosis calculated as a percentage of the A and B average was not significant for DB or GL but was 15% for BW and 14% for SR.