Genetic improvement of beef cattle in a tropical environment with special reference to the Gudali and Wakwa breeds in Cameroon

English:In an attempt to genetically Improve Gudali beef cattle JO Cameroon, two selection experiments were conducted between 1968 and 1988 at the Animal Production and Research Stations of Wakwa, Ngaoundere. The one experiment involved a two-breed synthetic beef breed, the Wakwa, obtained from inter se matings of the first filial generation of American Brahman (50%) x Gudali (50%) crosses. The other experiment involved recurrent selection of the indigenous purebred Gudali in an effort to enhance its beef production without any serious detrimental effects to its adaptational qualities.In order to assess the genetic progress of the two experiments, a study using mixed model methodology was carried out. The objectives were to quantify factors affecting growth traits, estimate (eo )variance components, predict genetic merit (breeding values) for direct and maternal performance and determine genetic progress by examining direct and maternal genetic trends for all animals. A total of 2886 records for birth weight (BWT), 2732 for average preweaning daily gain (ADG), 2899 for weaning weight (WWT), 2098 for yearling weight (YWT) and 1957 for eighteen months weight (EWT) of Gudali cattle were used in the study. Corresponding number of records for the Wakwa were 1793, 1656,1838, 1372and 1328. The results indicated that sire, sex, season (S), calf birth year (C), herd (H), HxSxC interaction, cowage group and ages at weaning (WAGE), yearling (YAGE) and eighteen months (EAGE), as well as covariates for weaning, yearling and eighteen month weights, were significant (p<0.05) sources of variation for these traits. Therefore, for reliablegenetic parameter estimations and evaluation of genetic merit of individual candidate animals for selection, these sources of variation should be taken into consideration. Hence the inclusion of these factors in the mixed model for the estimation of genetic parameters and prediction of breeding values.Estimates obtained for direct, maternal and total heritabilities were 0.37, 0.05 and 0.21 for BWT; 0.24, 0.17 and 0.07 for ADG; 0.27,0.19 and 0.11 for WWT; 0.51, 0.20 and 0.22 for YWT; and 0.18, 0.02 and 0.18 for EWT, respectively, in the Gudali. Corresponding estimates in the Wakwa were 0.55, 0.23 and 0.18 for BWT; 0.26, 0.07 and 0.12 for ADG; 0.28, 0.09 and 0.15 for WWT; 0.18, 0.00 and 0.17 for YWT and 0.14, 0.06 and 0.17 for EWT. Estimates for genetic correlations between direct and maternal effects were generally highly negative and ranged from -0.76 for ADG to -0.98 for YWT in the Wakwa and from -0.77 for WWT to - 0.88 for BWT in the Gudali. However, in both breeds the genetic correlation was nil for EWT. These estimates obtained are indicative that there are distinct possibilities of improving direct preweaning and/or postweaning growth in the both breeds through selection. However, a high selection intensity for direct performance may in the long run be detrimental to maternal performance as a result of the generally strong genetic antagonism between them. Although some estimates of genetic parameters for preweaning weight from one to seven months traits were not obtained for the Wakwa breed due to limited data, estimates obtained for the Gudali indicated that the highest, although moderate, estimate for maternal heritability (0.24) was for weight at two months of age. Therefore, an attempt to optimise direct and maternal performance in the Gudali through selection could be mosteffective at two months and at yearling, respectively. It is suggested that the apparent genetic antagonism generally found between direct and maternal ability be specifically investigated using more suitable data.An assessment of genetic progress indicated positive and significant (p<O.Ol) mean annual direct trends of 0.02 and 0.03 kg/year for BWT; 0.0005 and 0.0005 kg/year for ADG; 0.23 and 0.20 kg/year for WWT; 0.33 and 0.24 kg/year for YWT and 0.25 and 0.24 kg/year for EWT in the Gudali and the Wakwa, respectively. The difference between direct responses for all traits in both breeds were not significant. Corresponding trends for maternally influenced traits were -0.01 and -0.02; -0.1 and -0.3; -0.10 and -0.06; -0.15 and -0.00, -0.01 and 0.04. With the exception of ADG in the Gudali and EWT in the Wakwa, maternal trends were also significant (p<0.05) but negative. Considering the fact that the Wakwa breed has been shown to be more susceptible to streptothricosis and less tolerant to the tropical environment than the Gudali, it is economically justified to improve the purebred Gudali by selection rather than crossbreeding to the American Brahman or other exotic breeds.