Physiological reasons for heterosis in growth ofBos indicus×Bos taurus

Abstract

SummaryBy comparing growth rates of Brahman (B), Hereford × Shorthorn (HS), their reciprocal F1, hybrid (F1BX), their Fnhybrid (FnBX) and an F1Charolais × Brahman hybrid (FlCH x B) in environments that differed in their levels of stresses that affected growth, it was shown that heterosis for growth realized in any environment arose because of heterosis in its underlying determinants, namely growth potential and resistance to environmental stresses. Growth potential of the F1BX was similar to that of the better parent (HS) whilst resistance to environmental stresses was similar to or approached that of the more resistant parent (B). This combination of high growth potential and high resistance to environmental stresses enabled the F1BX to outgain both parents at all levels of environmental stress above zero. However, some or all of the heterosis in both growth potential and resistance to environmental stresses was lost in the FnBX. Thus, although previous selection for increased live-weight gain should have favoured the FnBX, they realized lower live-weight gains than the FlBX in all environments and lower live-weight gains than the parental breeds in all but intermediate environments.Because the breeds differed in both determinants of growth, the magnitude of estimates of heterosis for realized growth was dependent on the environment in which it was measured. A figure depicting this interaction was constructed.Comparative estimates were also made of the rate of approach to sexual maturity of bulls of each breed. The F1BX had similar values to the better parent (HS) for both scrotal circumference and plasma testosterone concentrations. However, the FnBX had values that were intermediate to those of the parental breeds.Generally, gains of the -F, CH × B exceeded those of all other breeds in all environments but their rate of approach to sexual maturity was slower than that of the F1BX.