Evaluation of beef cattle genotypes and estimation of direct and maternal genetic effects in a tropical environment. 2. Adaptive and temperament traits

Abstract

Data from a crossbreeding experiment conducted during 1992–97 involving 31 genotypes from tropically adapted British (B), Sanga-derived (S), Zebu cross (Zx), Zebu (Z), and Continental (C) beef cattle breed groups on adaptive traits such as mean tick counts (TICK), mean worm egg counts (EPG), mean rectal temperatures (TEMP), and mean coat scores (COAT), and the temperament trait mean flight time (FT) were analysed. The genotypes were grouped as Bos taurus (B, S, C)- and Bos taurus (Z, Zx)- derived to enable the comparison of direct (dD) and maternal (mD) dominance effects among indicine (II), taurine × indicine (TI), and taurine (TT) crosses. British breed group in this study consisted of Belmont Adaptaur, which is a tropically adapted selected line for tick resistance and 550-day liveweight and hence does not represent temperate British breeds. Coefficients of variation ranged between 15.1 and 55.4% and repeatability estimates varied between 0.26 and 0.45 in various adaptive and temperament traits. Significant (P < 0.01) genotype differences with better performance of Zebu and its crosses over taurine crosses were noticed in all the adaptive traits. Although genotype differences were significant for FT, no clear trend was noticed. Direct additive components of Z and Zx expressed as a deviation from the British mean were significant and negative (favourable) for all the adaptive traits, except for EPG in Zx, indicating better adaptability of Zebu and its crosses relative to the British. Unfavourable additive effect for EPG in the Sanga-derived breed group and undesirable additive effect for TICK and COAT in Continental breeds relative to the British were observed. All the adaptive traits benefited by crossing as demonstrated by significant and favourable dD effects in TI and TT crosses. Heterosis estimates were significant and favourable for all the adaptive traits in Z and B crosses. Treatment to control ticks and worms resulted in significantly increased liveweight gains (LWG) in a majority of genotypes, highlighting the negative effect of parasite burdens. As the Bos indicus proportion in the genotypes increased, the responses from treatment were reduced. The additive effect of Z in the treated group was only half that of the control group, relative to the British breed mean, indicating the advantage of Zebu in the presence of parasites. Low and insignificant phenotypic correlations among TICK, EPG, and LWG were observed. A significant positive correlation between TEMP and COAT and a negative correlation between TEMP and LWG were observed in the British purebreds.