A study was conducted to quantify the separate and combined impacts on selection for economic merit of including residual feed intake (RFI) traits in beef cattle breeding objectives and of having records available. RFI is a trait of interest in numerous livestock species. It was defined here for young animals at pasture (RFI-P), in the feedlot (RFI-F), and in cows (RFI-C). Results showed selection response in total economic merit increased by up to 65% for breeding objectives where RFI-P, RFI-F, and RFI-C were all included. A large proportion of the benefit (more than 50%) came from being able to include RFI traits in the breeding objective, suggesting major benefits may be realised even where a suitable industry measure is not yet available. Residual feed intake should be considered in breeding objectives and selection where parameter estimates are available. Estimates of genetic variance are among those most needed for RFI-C, and are likely to need to be understood in cows that are approximately maintaining or even losing weight.

Improved female reproductive performance (FRP) of beef breeds in northern Australia is an important means to increase profitability. However selection to improve FRP has proved difficult due to low heritabilities and late expression of traits. One trait that may influence maiden calving performance is age at puberty. Several studies have shown age at puberty is heritable in beef cattle, particularly in 'Bos taurus' breeds (e.g. Gregory et al. 1995), but limited studies (and numbers) exist for 'Bos indicus' genotypes. Vargas et al. (1998) reported, from a small study, a heritability for age at puberty in Brahmans of 0.42. However the relationship between age at puberty and subsequent measures of female fertility are inconclusive. Some studies show a favourable relationship between improved pregnancies and earlier age at puberty (Morris et al. 2000). However several others (e.g. Dow et al. 1982) have observed no relationship. The aim of this study was to estimate the heritability of FRP traits and the genetic correlation with age at puberty in 2 tropically adapted genotypes raised in northern Australian environments.

The CRC for Cattle and Beef Quality was established in 1993 to identify the genetic and non-genetic factors affecting beef quality and other production traits of economic importance. Providing feed for cattle is the single largest input cost in beef production enterprises. Advances in computing and electronics has allowed the development of reliable automatic feed intake recorders making it easier to measure feed intake, and this in association with increased producer demand for a means of selecting on the basis of efficiency, has led to increased research in feed efficiency. Understanding genetic variation in feed efficiency and providing the tools for genetic improvement in the efficiency of feed utilization can have a major impact on profitability. Progress of research within the CRC, and internationally, for beef cattle feed efficiency has been regularly reviewed and published (Archer et al. 1999; Herd et al. 2003a; Arthur et al. 2004; Arthur and Herd 2005, 2006). This paper summarises the CRC's major contributions to new knowledge and applications for genetic improvement of growth and feed efficiency.

The theory relating genetic difference between the sexes, genotype x sex interaction and thegenetic correlation between the sexes is described by Eisen and Legates (1966), wheregenotype x sex is considered a special case of genotype x environment interaction. It followsfrom this that a genetic correlation of less than one between the sexes is evidence both ofgenotype x sex interaction and of genetic difference between the sexes. By extension, this alsoholds for similar traits in different environments. Here, we present genetic correlations forcorresponding measures of steer and heifer growth and live body composition for two tropicalgenotypes that are important to beef production in tropical and sub-tropical Australia. The dataare from a project described by Burrow et al. (2003). The correlations provide insight into thepossible magnitude of genotype x environment and genotype x sex interactions whenenvironments differ markedly for the sexes. As in industry, steers were run in a more benignenvironment and finished in a feedlot, while heifers were run in typically harsher, tropicalenvironments.

Costs associated with feeding represent one of the major inputs for beef cattle producers.Arthur et al. (2001) and Robinson and Oddy (2004) demonstrated that net feed intake (NFI: defined as feed eaten, less what is expected based on the animal’s growth rate and body weight) was heritable (h 2 = 0.18 to 0.51). Robinson and Oddy (2004) estimated geneticcorrelations of daily feed intake (FI) and NFI with body composition traits and reported that animals with lower NFI produced leaner carcases. Given the high costs of FI measurement, there is interest in indirect means of predicting genetic merit for NFI. Moore et al. (2005) reported a moderate positive correlation (r g = 0.54) between NFI and blood plasma levels of IGF-I (IGF-I) measured post-weaning in Angus seedstock cattle. Johnston et al. (2002) found a genetic correlation of 0.56 between IGF-I concentration and NFI in feedlot tested cattle. These experiments examined NFI in predominantly Bos taurus breeds. As part of a larger study described by Burrow et al. (2003), genetic parameters for FI and NFI were estimated for two genotypes of tropically adapted cattle. This paper presents the initial findings of this research, including the genetic relationships of FI and NFI with steer production traits and IGF-I measurements, taken from weaning to feedlot exit.

This thesis analysed carcass records from 2180 tropically adapted, steers (986 Brahman (BRAH) and 1194 Tropical Composite (TCOMP)) describing weight, eye muscle area, P8 and 12/13th rib fat depth, percent intramuscular fat and retail beef yield, with tenderness assessed as shear force. All steers were feedlot finished with a subset (680 BRAH and 783 TCOMP) recorded for individual feed intake. Female reproductive performance in the half-sib sisters of these steers (1007 BRAH and 1108 TCOMP) was evaluated as outcomes of their first (Mating 1: when females averaged 27 months of age) and second (Mating 2) annual matings, and averaged over up to 6 matings (termed 'lifetime' reproduction traits). Heifer and cow weight, eye muscle area, P8 and 12/13th rib fat depth, body condition score and hip height were recorded at 18 months of age, immediately prior to first calving and at Mating 2. The maternal genetic component of weaning weight (Maternal WWT) was estimated based on weaning weight records available for these steers and females and the progeny of females (N = 12528).

Female and male measures with potential to be practical, early-in-life genetic indicators of female reproduction in Brahmans were chosen from earlier reports and compared, along with genomic measures, for multi-trait use to improve Brahman lifetime annual weaning rate ('LAWR'). Results suggested substantial genetic gains in 'LAWR' may be possible in 10 years using these measures, but need confirming in other data. Female hip height and coat score and male preputial eversion and liveweight were measures that could warrant wider recording for 'LAWR' improvement. Recording of pregnancy test outcomes from matings 1 and 2 should also be encouraged. A genomic EBV in combination with other measures added to the gain in 'LAWR', but needed an accuracy approaching 60% to be the most important contributor to gains in the combinations of measures studied.

Early-in-life female and male measures with potential to be practical genetic indicators were chosen from earlier analyses and examined together with genomic measures for multi-trait use to improve female reproduction of Brahman cattle. Combinations of measures were evaluated on the genetic gains expected from selection of sires and dams for each of age at puberty (AGECL, i.e. first observation of a 'corpus luteum'), lactation anoestrous interval in 3-year-old cows (LAI), and lifetime annual weaning rate (LAWR, i.e. the weaning rate of cows based on the number of annual matings they experienced over six possible matings). Selection was on an index of comparable records for each combination. Selection intensities were less than theoretically possible but assumed a concerted selection effort was able to be made across the Brahman breed. The results suggested that substantial genetic gains could be possible but need to be confirmed in other data. The estimated increase in LAWR in 10 years, for combinations without or with genomic measures, ranged from 8 to 12 calves weaned per 100 cows from selection of sires, and from 12 to 15 calves weaned per 100 cows from selection of sires and dams. Corresponding reductions in LAI were 60-103 days or 94-136 days, and those for AGECL were 95-125 or 141-176 days, respectively. Coat score (a measure of the sleekness or wooliness of the coat) and hip height in females, and preputial eversion and liveweight in males, were measures that may warrant wider recording for Brahman female reproduction genetic evaluation. Pregnancy-test outcomes from Matings 1 and 2 also should be recorded. Percentage normal sperm may be important to record for reducing LAI and scrotal size and serum insulin-like growth factor-I concentration in heifers at 18 months for reducing AGECL. Use of a genomic estimated breeding value (EBV) in combination with other measures added to genetic gains, especially at genomic EBV accuracies of 40%. Accuracies of genomic EBVs needed to approach 60% for the genomic EBV to be the most important contributor to gains in the combinations of measures studied.

Reproduction records from 2137 cows first mated at 2 years of age and recorded through to 8.5 years of age were used to study the genetics of early and lifetime reproductive performance from two genotypes (1020 Brahman and 1117 Tropical Composite) in tropical Australian production systems. Regular ultrasound scanning of the reproductive tract, coupled with full recording of mating, calving and weaning histories, allowed a comprehensive evaluation of a range of reproductive traits. Results showed components traits of early reproductive performance had moderate to high heritabilities, especially in Brahmans. The heritability of lactation anoestrous interval in 3-year-old cows was 0.51 ± 0.18 and 0.26 ± 0.11 for Brahman and Tropical Composite, respectively. Heritabilities of binary reproductive output traits (conception rate, pregnancy rate, calving rate and weaning rate) from first and second matings were generally moderate to high on the underlying scale. Estimates ranged from 0.15 to 0.69 in Brahman and 0.15 to 0.34 in Tropical Composite, but were considerably lower when expressed on the observed scale, particularly for those traits with high mean levels. Heritabilities of lifetime reproduction traits were low, with estimates of 0.11 ± 0.06 and 0.07 ± 0.06 for lifetime annual weaning rate in Brahman and Tropical Composite, respectively. Significant differences in mean reproductive performance were observed between the two genotypes, especially for traits associated with anoestrus in first-lactation cows. Genetic correlations between early-in-life reproductive measures and lifetime reproduction traits were moderate to high. Genetic correlations between lactation anoestrous interval and lifetime annual weaning rate were -0.62 ± 0.24 in Brahman and -0.87 ± 0.32 in Tropical Composite. The results emphasise the substantial opportunity that exists to genetically improve weaning rates in tropical beef cattle breeds by focusing recording and selection on early-in-life female reproduction traits, particularly in Brahman for traits associated with lactation anoestrus.

Meat quality and carcass traits were measured for 2180 feedlot finished Brahman (BRAH) and Tropical Composite (TCOMP) steers to investigate genetic and non-genetic influences on shear force, and other meat quality traits. Genetic and phenotypic correlations were estimated between carcass and meat quality traits, and with live animal measurements collected in steers from weaning to feedlot exit, and their heifer half-sibs up to their first mating, which were managed in Australia’s tropical or subtropical environments. Left sides of carcasses were tenderstretched (hung by the aitch-bone) while right sides were conventionally hung (by the Achilles tendon). Tenderstretching reduced mean shear force by 1.04 kg, and phenotypic variance by 77% of that observed in conventionally hung sides. Genotype differences existed for carcass traits, with TCOMP carcasses significantly heavier, fatter, with greater eye muscle area, and lower retail beef yield than BRAH. TCOMP had lower shear force, and higher percent intramuscular fat. Meat quality and carcass traits were moderately heritable, with estimates for shear force and compression of 0.33 and 0.19 for BRAH and 0.32 and 0.20 for TCOMP respectively. In both genotypes, estimates of heritability for carcass traits (carcass weight, P8 and rib fat depths, eye muscle area and retail beef yield) were consistently moderate to high (0.21 to 0.56). Shear force and compression were genetically correlated with percent intramuscular fat (r'g' = –0.26 and –0.57, respectively), and meat colour (r'g' = –0.41 and –0.68, respectively). For TCOMP, lower shear force was genetically related to decreased carcass P8 fat depth (r'g' = 0.51). For BRAH steers and heifers measured at pasture, fatness traits and growth rates were genetically correlated with shear force, although the magnitude of these relationships varied with time of measurement. Net feed intake was significantly genetically correlated with carcass rib fat depth (r'g' = 0.49), eye muscle area (r'g' = –0.42) and retail beef yield (r'g' = –0.61). These results demonstrate that selection to improve production and carcass traits can impact meat quality traits in tropically adapted cattle, and that genotype specific evaluations will be necessary to accommodate different genetic relationships between meat quality, carcass and live animal traits.