Genetics of heifer performance in 'wet' and 'dry' seasons and their relationships with steer performance in two tropical beef genotypes

Abstract

The genetics of heifer performance in tropical ‘wet’ and ‘dry’ seasons, and relationships with steer performance, were studied in Brahman (BRAH) and Tropical Composite (TCOMP) (50% Bos indicus, African Sanga or other tropically adapted Bos taurus; 50% non-tropically adapted Bos taurus) cattle of northern Australia. Data were from 2159 heifers (1027 BRAH, 1132 TCOMP), representing 54 BRAH and 51 TCOMP sires. Heifers were assessed after post-weaning ‘wet’ (ENDWET) and ‘dry’ (ENDDRY) seasons. Steers were assessed post-weaning, at feedlot entry, over a 70-day feed test, and after ∼120-day finishing. Measures studied in both heifers and steers were liveweight (LWT), scanned rump fat, rib fat and M. longissimus area (SEMA), body condition score (CS), hip height (HH), serum insulin-like growth factor-I concentration (IGF-I), and average daily gains (ADG). Additional steer measures were scanned intra-muscular fat %, flight time, and daily (DFI) and residual feed intake (RFI). Uni- and bivariate analyses were conducted for combined genotypes and for individual genotypes. Genotype means were predicted for a subset of data involving 34 BRAH and 26 TCOMP sires. A meta-analysis of genetic correlation estimates examined how these were related to the difference between measurement environments for specific traits. There were genotype differences at the level of means, variances and genetic correlations. BRAH heifers were significantly (P < 0.05) faster-growing in the ‘wet’ season, slower-growing in the ‘dry’ season, lighter at ENDDRY, and taller and fatter with greater CS and IGF-I at both ENDWET and ENDDRY. Heritabilities were generally in the 20 to 60% range for both genotypes. Phenotypic and genetic variances, and genetic correlations, were commonly lower for BRAH. Differences were often explained by the long period of tropical adaptation of B. indicus. Genetic correlations were high between corresponding measures at ENDWET and ENDDRY, positive between fat and muscle measures in TCOMP but negative in BRAH (mean of 13 estimates 0.50 and –0.19, respectively), and approximately zero between steer feedlot ADG and heifer ADG in BRAH. Numerous genetic correlations between heifers and steers differed substantially from unity, especially in BRAH, suggesting there may be scope to select differently in the sexes where that would aid the differing roles of heifers and steers in production. Genetic correlations declined as measurement environments became more different, the rates of decline (environment sensitivity) sometimes differing with genotype. Similar measures (LWT, HH and ADG; IGF-I at ENDWET in TCOMP) were genetically correlated with steer DFI in heifers as in steers. Heifer SEMA was genetically correlated with steer feedlot RFI in BRAH (0.75 ± 0.27 at ENDWET, 0.66 ± 0.24 at ENDDRY). Selection to reduce steer RFI would reduce SEMA in BRAH heifers but otherwise have only small effects on heifers before their first joining.

Importance of adaptation and genotype × environment interactions in tropical beef breeding systems

“…The Barwick et al (2009b) study also indicated that genetic correlations declined as measurement environments became more different, with the rates of decline sometimes differing between Brahmans and Tropical Composites. This occurred to varying extents for the traits and breeds examined, also suggesting the different breeds were sensitive to varying environments.…”

“…This emphasises the importance of thoroughly defining measurement protocols, including animal age and condition at time of assessment, for live animal traits that are to be included in a genetic evaluation analysis. Barwick et al (2009b) reported that Brahman heifers grew faster in wet seasons and slower in dry seasons than Tropical Composites, whereas Brahman steers grew slower than Tropical Composites throughout grow-out and finishing in different (sub)tropical and temperate feedlot environments. Brahman heifers were fatter and had higher condition scores at the end of wet and dry seasons than Tropical Composites, whereas Brahman steers had less fat and lower condition scores at the end of feedlot finishing.…”

Genetics of meat quality and carcass traits and the impact of tenderstretching in two tropical beef genotypes

“…At each location, heifers of the same year of birth (defined as a cohort) were managed as a single group. Barwick et al (2009b) described the measurements collected at two times: (1) at the end of the animals' first wet season following weaning (ENDWET) when BRAH and TCOMP heifers averaged 518 and 555 days of age and 288 and 314 kg liveweight, respectively; and (2) at the end of the second dry season following weaning (ENDDRY) when BRAH and TCOMP mean age and liveweight were 713 and 749 days and 320 and 354 kg, respectively. ENDWET and ENDDRY measurements examined for this paper were: LWT, HH, CS, SEMA, SIMF, SP8, SRIB, and IGF-I.…”

“…Heifers were weaned at the same time as steers and allocated, while maintaining genetic linkage, to one of four locations (Barwick et al 2009b). The harshest of the four environments was occupied only by BRAH, as it was considered unsuitable for TCOMP animals.…”

“…Breeding and management of animals, and experimental treatments imposed on the steer and heifers have been described by Barwick et al (2009aBarwick et al ( , 2009b. Briefly, animals of two tropically adapted genotypes, Brahman (BRAH) and Tropical Composite (TCOMP) were bred over a 4-year period (2000 to 2003) on eight co-operating properties (subsequently referred to as herds of origin), using a combination of artificial insemination (AI) and natural mating.…”