Factors affecting growth of calves and pregnancy rate of Santa Gertrudis cows in Morocco.

The aim of this study is to estimate the effect of early weaning on calf gr owth and pregnancy rate in 227 primiparous Santa Gertrudis cows managed und er extensive conditions in Morocco. Calving season started in September thr ough February and females were exposed to bulls from January 16 to June 20, 1997. Pregnancy diagnosis was carried out by tranrectal palpation 2 months after the end of breeding season. Calf weights were recorded at birth, wea ning, May and July 1997 to characterise calf growth. Weaning at different a ges allowed to allocate the animals into 3 age groups : 50-95 days, 96-120 days and 121-170 days. Statistical analysis was carried out using GLM and C ATMOD models (SAS). The average weaning weight was 150.99 +/- 12.3 kg. It was significantly aff ected by birth weight (p < 0.01), sex (p < 0.05) and calving date (p < 0.01 ). On the other hand, preweaning growth rate was affected by birth weight ( p < 0.01). The DAG and weaning weight were positively correlated (r = 0.72) to weaning date (p < 0.01) and age at weaning (p < 0.01). Growth rate was significantly (p < 0.01) higher in calves born at the end of season (952 +/ - 167 g/d) than in those born early in the season (744 +/- 152 g/d). Growth rate was lower in calves; weaned at 96-120 days of age (707 +/- 137 g/d) ( p < 0.01) than those weaned earlier (732 +/- 224 g/d) or later (724 +/- 137 g/d). Post weaning growth decreased when weaning was late (p < 0.01). Mean adjusted weight at 205 days was significantly (p < 0.01) affected by calf sex, birth date, weight and age at weaning. In this study, the overall mean pregnancy rate was 69 %. This parameter was higher in females that calved early (79.3 %) than those that calved late ( 40.4 %) in the calving season (p < 0.01). When calves were weaned at 96-120 days of age, dams were more likely to become pregnant than in other age gr oups (p = 0.05). In addition, pregnancy rate was significantly (p < 0.05) h igher (84 %) in cows exposed to bulls after 90 days postpartum than those e xposed to males before 60 days post partum (41 %). In conclusion, early weaning at 96-120 days of age seems to allow an optimu m pregnancy rate when early calving occured. Adequate feeding of weaned cal ves is required to improve growth performance.