| Sumario: | Reproductive performance, pre-weaning growth and calf mortality based on data over a 27-year period (1963-1989) were evaluated for Boran, Tanzania Shorthorn Zebu (TSZ) and their crosses maintained on-station at Livestock Research Centres Malya and West Kilimanjaro in Tanzania. The crosses were 1/2Boran and 1/2TSZ and 3/4Boran 1/4TSZ. Under ranch-type management, estimated overall means were 1338 days (44.6 months) for age at first calving; 537 days (18.0 months) for calving interval;23.3 kg for birth weight; 95.3 kg for weaning weight at 28 weeks of age and370g for average daily weight gain up to weaning. Cumulative calf mortality from birth to weaning was 1.7%, from birth to 36 weeks was 2.1% and from birth to 72 weeks was 2.9%. Year had significant effects (P < 0.01) on all reproductive traits studied, and on pre-weaning growth and calf mortality, but no general trends over years were apparent. Season showed significant effects (P < 0.01) on daily weight gain and weaning weight, wet season born calves performing better than dry season born calves. Sex of calf had significant effects (P < 0.01) on length of calving interval and on all pre-weaning growth traits. Cows nursing a male calf had, on average, 28 days longer calving interval. Bull calves were 1.3 kg and 8.6 kg heavier at birth and weaning, respectively and gained 35 g/day more than heifer calves. Age of dam had significant effects (P < 0.01) on calving interval, on all growth traits and on mortality. Cows aged between 5-10 years were the most productive ones, having the shortest calving intervals, the heaviest calves at birth and at weaning and the lowest calf mortality up to the age of 72 weeks. Genotype had significant effects (P<0.01) on all pre-weaning growth traits and also significantly (P < 0.05) affected mortality up to the age of 72 weeks. The growth performance of Boran was superior to the other genotypes. The difference between Boran crosses and TSZ was significant (P<0.01) only in birth weight, crosses being 3 kg (14.5%) heavier than straightbred TSZ calves. Calf mortality up to the age of 72 weeks was 4.5% for 1/2Boran 1/2TSZ, 3.9% for Boran, 1.7% for TSZ and 1.3% for 3/4Boran 1/4TSZ. Performance was found to be significantly (P< 0.05) better in LRC Malya than in West- Kilimanjaro. At Malya, the mean age at first calving was 165 days earlier and calving intervals were 149 days shorter. Calves born at Malya, on average, were 4 kg and 35 kg heavier at birth and weaning, respectively, and gained 157 g/day more from birth to weaning. Similarly, calf mortality at Malya was lower by 0.8%, 1.0% and 1.6% from birth to weaning, birth to 36 weeks and birth to 72 weeks, respectively. The difference could not be explained from the structure of the data. However, it is known that LRC West-Kilimanjaro is overstocked. This has certainly led to degradation of pastures, in quality and quantity, thus affecting livestock performance. Appropriate measures to alleviate this problem should be seriously considered. Heritabilities of pre-weaning growth traits were 0.36 for birth weight, 0.45 for average daily weight gain and 0.23 for weaning weight. The higher heritability for average daily weight gain suggests that selection on this trait would be more successful than the present practice of selecting on weaning weight. However, since selection based on either average daily weight gain or weaning weight combines maternal ability of the dam and genetic potential of the calf, comparative studies on post-weaning growth performance of calves should reveal the most appropriate selection criterion. It should be noted that weaning weight and pre-weaning average daily gain are genetically identical. Also, adjustments for the systematic environmental factors found to be of significant importance in the current study should be taken into consideration during selection. Estimates of additive and heterotic effects arising from crossing Boran with TSZ dams indicate significant additive effects (P < 0.05) on birth weight only. Heterotic effects for all traits were small and not significant, therefore of no practical value. These and results on calf mortality tend to suggest that crossing Boran with TSZ dams brings little or no advantage at all in the beef production system. In the current study, post-weaning weights at 36 weeks and at 72 weeks were so few that meaningful analyses could not be undertaken. Furthermore, the level of management may have been too low to allow manifestation of differences in genetic potential. Nevertheless, results from this study tend to agree with other similar studies in Tanzania (TRAIL et al., 1985) and in Uganda (STOBBS, 1966). Crossing TSZ cattle with Boran may only be of advantage in a much higher level of improved feeding and disease control, raising doubt to increased beef production by using Boran crosses as compared to straightbred TSZ or Boran cattle in Tanzania.
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