Abstract
Longevity traits and reasons for disposal of 546 cows of different genotypes, over a period of 18 years, in a dual-purpose (meat and milk) herd were studied. The effects of genotype groups (N'Dama, Brown Swiss, F1s, F2s, F1-N'Dama backcross, and F1-Brown Swiss backcross) on longevity traits were examined along with reasons for disposal. Survival curves were plotted to show the probability for a cow that entered the breeding herd at two years old to remain in the herd until a particular age. The overall mean length of productive life in the herd, disposal age, and average number of calvings were 4.85 ± 0.18 years, 7.14 ± 0.13 years, and 4.12 ± 0.17, respectively. All traits were significantly (P < 0.05) affected by genotype group, while age of cow's dam only significantly (P < 0.05) affected disposal age. The scale parameters (?) generated from the survival analysis and based on Weibull distribution, were generally higher among the purebred, indicating that the purebred cows were initially removed at a faster rate. The percentages of cows disposed at each age class showed that more cows were disposed at four years compared to other age classes. The major reason for cow disposal, was reproductive failure accounting for about 40% of the total number of cows disposed. One-third of the cows removed for calving and calf survival problems were culled, because their calves were either delivered by caesarean section or they were hand-pulled. Death losses accounted for 9.8% of total cows disposed, while unsoundness and other reasons accounted for 15 and 8%, respectively. The reasons for disposal that were significantly (P < 0.05) affected by genotype group were death losses, calving and calf survival problems, and unsoundness. The highest percentage of death losses (16.3%) was recorded among the N'Dama group probably due to calving difficulty when mated to Brown Swiss bulls. On the average, young cows were removed more for reproductive failure, calving or calf survival problems, and death-related causes. At 10 years of age and above, less than 15% of culled cows died, indicating that the probability of obtaining a salvage value for culled cows increased with age.
