Osorio-Arce et al., 2010. Livest. Res. Rural Dev., 22

The objective of this study was to determine the effect of some breed and environmental factors on age at first calving (AFC) and calving interval (CI) of Brahman cows and their crosses with European-breed bulls in a production system in the humid tropics of Mexico. Cows were fed on paddocks of Jaragua grass (Hyparrenia rufa) in bad condition. Services were given all year using artificial insemination or natural mating. Information for AFC was obtained for 446 cows and 1849 CI from 645 cows. Cows belonged to the breed groups (BG): Brahman (Br), Charolais x Brahman (Chx), Brown Swiss x Brahman (BSx) and Simmental x Brahman (Six). The statistical model for AFC included the fixed effects of year of birth (YB), season of birth (SB), BG, and the  BG*SB and YB*SB; whereas that for CI included the effects of year of calving (YC), season of calving (SC), parity number (PN), BG and cow within BG.
The overall means and standard error for AFC was 1472 + 15.9 days and was affected (P<0.05) by YB, YB*SB and BG*SB. The Br cows had the greatest AFC means in the three seasons with a general mean of 1600+24 days. The overall mean and standard error for CI was 558+2.65 days and was affected (P<0.05) by YC, SC, and PN. The cows calving in the rainy season had shorter CI than those cows calving in the dry or rainy and windy seasons. The first calving cows took longer to get pregnant than multiparous cows. The Chx cows had smaller CI than the SPx and Six crosses. The repeatability for CI was 0.08 ± 0.07. In conclusion, under the present conditions of management and feed of the cow-calf production systems in southeastern of Mexico, the AFC was slightly longer than 4 years and he CI longer than 450 days. Therefore the environmental conditions do not allow that some crosses or breed perform better.