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Birth-weight data recorded between 1986 and 1991 on 3453 N'Dama calves from the village production systems of The Gambia were analysed to identify major factors influencing this trait and to quantify the relationship between birth weight and early calf mortality. Birth weights were measured over a range of time, from a few hours to 14 days after birth. Least-squares analysis of the fixed effects of site, year and season of birth, parity of dam, sex, age in days at which birth weight was recorded and random effects of herd and cow nested within site showed that all these factors significantly influenced this trait.
A subset of these data (no. = 2452) was used to derive adjustment factors which were applied to the larger data set to standardize birth weights to a common age measurement. Results from these adjustments showed that birth weights recorded between the 7th and 14th day after birth should be decreased by proportionately 0·12 to 0·14 to be comparable with those measured on day of birth.
Maximum likelihood analyis of cumulative calf mortality to 30, 60, 90 and 120 days of age using age-standardized data (no. = 3156), which excluded deaths of traumatic origin, showed that mortality was highly influenced by birth weight. Calves were grouped into three classes, namely below-average, average and above-average of sitemates' values. Mortality rates among the three groups during the first 30 days of life were 3·6, 0·9 and 0·3%. The corresponding values at day 120 were 13·8, 6·3 and 5·0%. The average age at death during the first 120 days in the three groups were 51, 58 and 73 days. Mortality rates among calves born in the wet season were higher than those born in the dry season (e.g. 9·9 v. 5·9% for mortality to day 120).
Examination of data on calving distribution and milk production patterns of the dams of the calves used in this study as well as the causes of deaths among the calves indicated that although dam effect was an important determinant of calf survival other factors such as the vigour of the calf at birth, weather conditions and disease environment at the tethering and grazing areas were important as well.