M. L. Wolcott

Genetics of adaptive traits in heifers and their relationship to growth, pubertal and carcass traits in two tropical beef cattle genotypes.

Genetic analyses of tropical adaptive traits were conducted for two tropically adapted genotypes, Brahman (BRAH) and Tropical Composite (TCOMP). Traits included tick scores (TICK), faecal egg counts (EPG), buffalo fly-lesion scores (FLY), rectal temperatures under hot conditions (TEMP), coat scores (COAT), coat colour on a light to dark scale (COLOUR), navel scores (NAVEL) and temperament measured as flight time (FT). The data comprised adaptive measures recorded at specific times on 2071 heifers comprising 966 BRAH and 1105 TCOMP. The genetic correlations of these adaptive traits with heifer growth, scanned carcass, pubertal measures and steer growth and carcass traits were estimated. BRAH recorded significantly (P 50%). In general, phenotypic correlations between these adaptive traits were low and genetic correlations were non-significant, implying trait independence. Genetic correlations between EPG and weight traits (0.29 to 0.44) indicated a positive relationship, implying no deleterious effect of worms on the growth at a genetic level, especially in TCOMP. The negative genetic correlations between COAT and body-condition score across genotypes (–0.33 to –0.48) indicated genetic advantage of sleek coats in tropics. A positive genetic correlation between COAT and the age at the first-observed corpus luteum (0.73) in BRAH indicated that BRAH with sleeker coats were genetically early maturing. Further, sleeker coats were genetically indicative of lower weights and lower fat cover at puberty in BRAH. The scanned fat measures at rump and rib sites for feedlot steers showed strong genetic correlation (0.50–0.58) with heifer TEMP, indicating genetically fatter animals had genetically lower heat tolerance. In BRAH, a positive genetic association between heifer COLOUR and scanned fat measures in steers (0.50–0.54) implied increased fatness in genetically darker animals. Further, in BRAH, a strong negative genetic correlation (–0.97) was observed between steer retail beef yield and heifer TEMP, indicating a favourable genetic association. In general, genetic correlations between adaptive traits and other economic traits were genotype specific. Further, it can be concluded that selection for productive and pubertal traits in tropical beef cattle genotypes would not adversely affect their tropical adaptability.

Genetics of steer daily and residual feed intake in two tropical beef genotypes, and relationships among intake, body composition, growth and other post-weaning measures

Genetic parameters for Brahman (BRAH) and Tropical Composite (TCOMP) cattle were estimated for steer production traits recorded at weaning (WEAN), 80 days post-weaning (POSTW), feedlot entry (ENTRY) and after ∼120 days feedlot finishing (EXIT). The TCOMP was 50% Bos indicus, African Sanga or other tropically adapted Bos taurus, and 50% non-tropically adapted Bos taurus. Data involved 2216 steers, comprising 1007 BRAH by 53 sires and 1209 TCOMP by 50 sires. Individual daily feed intake (DFI) and residual feed intake (RFI) were assessed on 680 BRAH and 783 TCOMP steers over an ~70-day feedlot test. Other traits were liveweight (LWT), average daily gain (ADG), ultrasonically scanned rump (SP8) fat depth, rib (SRIB) fat depth, M. longissimus area (SEMA) and intra-muscular fat % (SIMF), body condition score (CS), hip height (HH), flight time (FT) and serum insulin-like growth factor-I concentration (IGF-I). BRAH were significantly (P < 0.05) lighter at ENTRY and EXIT, and had lower DFI (10.8 v. 13.2 kg/day) and RFI (–0.30 v. 0.17 kg/day), greater SP8 (5.8 v. 5.1 mm) but similar SRIB at ENTRY, lower SRIB (8.2 v. 8.9 mm) but similar SP8 at EXIT, and greater HH than TCOMP. Heritabilities for DFI, RFI, LWT, ADG, scanned body composition, HH and IGF-I measures, across measurement times, were generally in the 20 to 60% range for both genotypes. Genetic variance for RFI was 0.19 (kg/day)2 in BRAH and 0.41 (kg/day)2 in TCOMP, suggesting a clear potential to genetically change RFI in both genotypes. Trait variances and genetic correlations often differed between the genotypes, supporting the use of genotype-specific parameters in genetic evaluation. The genotype differences may be associated with evolutionary changes that have occurred in B. indicus as a part of their adaptation to tropical environments. Measures with potential to be used as genetic indicators of DFI were LWT measures in BRAH and TCOMP, ADG at ENTRY in TCOMP, and SP8 and SIMF at ENTRY in BRAH. Measures with potential to be genetic indicators of RFI were HH and ADG at ENTRY in BRAH, and IGF-I in both genotypes. Taller and faster-growing BRAH steers at ENTRY had genetically lower RFI. IGF-I was negatively genetically correlated with RFI whether IGF-I was measured at POSTW, ENTRY or EXIT. SRIB fatness at EXIT was strongly positively genetically correlated with RFI in TCOMP but only lowly correlated in BRAH. Fatness at ENTRY was lowly and negatively genetically correlated with RFI. The results emphasise the need for a population-specific understanding of trait relationships and of trait differences between measurement times if genetic indicator traits are to be utilised in genetic evaluation of RFI.

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

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.

Male traits and herd reproductive capability in tropical beef cattle. 2. Genetic parameters of bull traits

A total of 4063 young bulls of two tropical genotypes (1639 Brahman and 2424 Tropical Composite) raised in northern Australia were evaluated for a comprehensive range of production and reproduction traits up to 24 months of age. Prior to weaning, peripheral blood concentrations of luteinising hormone (LH) and inhibin were measured at 4 months of age. At weaning (6 months) blood insulin-like growth factor-1 (IGF-I) and flight time were recorded. Body composition traits of fat depth and eye-muscle area were determined by ultrasonography at 15 months of age when additional measurements of liveweight, hip height and body condition score were recorded. Bull breeding soundness was evaluated at ~12, 18 and 24 months of age when measurements of scrotal circumference, sheath score, semen mass activity, progressive motility of individual sperm and percent morphologically normal sperm were recorded. Magnitude of heritability and genetic correlations changed across time for some traits. Heritability of LH, inhibin, IGF-I and of 18-month scrotal circumference, mass activity, progressive motility and percent normal sperm was 0.31, 0.74, 0.44, 0.75, 0.24, 0.15 and 0.25, respectively, for Brahmans and 0.48, 0.72, 0.36, 0.43, 0.13, 0.15 and 0.20, respectively, for Tropical Composites. Inhibin and IGF-I had moderate genetic association with percent normal sperm at 24 months in Brahmans but low to negligible associations in Tropical Composites. Body condition score in Brahmans and sperm motility (mass and individual) traits in both genotypes had moderate to strong genetic correlation with percent normal sperm and may prove useful candidates for indirect selection. There is scope to increase scrotal circumference by selection and this will be associated with favourable correlated responses of improved semen quality in both genotypes. The lack of genetic antagonism among bull traits indicates that selection for improved semen quality will not adversely affect other production traits.

Genetic correlations of young bull reproductive traits and heifer puberty traits with female reproductive performance in two tropical beef genotypes in northern Australia

Genetic correlations of young bull and heifer puberty traits with measures of early and lifetime female reproductive performance were estimated in two tropical beef cattle genotypes. Heifer age at puberty was highly (r(g) = -0.71 +/- 0.11) and moderately (r(g) = -0.40 +/- 0.20) genetically correlated with pregnancy rate at first annual mating (mating 1) and lifetime annual calving rate, respectively in Brahman (BRAH). In Tropical Composite (TCOMP), heifer age at puberty was highly correlated with reproductive outcomes from the first re-breed (mating 2), mainly due to its association with lactation anoestrous interval (r(g) = 0.72 +/- 0.17). Scrotal circumference were correlated with heifer age at puberty (r(g) = -0.41 +/- 0.11 at 12 months in BRAH; -0.30 +/- 0.13 at 6 months in TCOMP) but correlations were lower with later female reproduction traits. Bull insulin-like growth factor-I was correlated with heifer age at puberty (r(g) = -0.56 +/- 0.11 in BRAH; -0.43 +/- 0.11 in TCOMP) and blood luteinising hormone concentration was moderately correlated with lactation anoestrous interval (r(g) = 0.59 +/- 0.23) in TCOMP. Semen quality traits, including mass activity, motility and percent normal sperm were genetically correlated with lactation anoestrus and female lifetime female reproductive traits in both genotypes, but the magnitudes of the relationships differed with bull age at measurement. Preputial eversion and sheath scores were genetically associated with lifetime calving and weaning rates in both genotypes. Several of the early-in-life male and female measures examined were moderately to highly genetically correlated with early and lifetime female reproduction traits and may be useful as indirect selection criteria for improving female reproduction in tropical breeds in northern Australia.

The prediction of retail beef yield from real time ultrasound measurements on live animals at three stages through growout and finishing

Analyses were performed to test the relationship between retail beef yield percentage (RBY) and real time ultrasound measurements taken at weaning, entry to finishing and preslaughter for animals finished under pasture and feedlot conditions to meet domestic, Korean and Japanese market specifications. The first analysis tested the power of live animal measurements (scanned P8 fat depth, scanned eye muscle area and liveweight) to predict RBY and contrasted this with a model containing these live animal measurements plus a term (HERD × KILL ) which accounted for all known classification variables. This indicated that scanned P8 fat depth, measured at slaughter, was the most useful predictor of retail beef yield, accounting for 52% of the variation in RBY for the equation containing live animal measurements alone. The power of live animal measurements to predict RBY decreased as the time between scanning and slaughter increased. Models which included HERD × KILL predicted RBY accurately (accounting for 82–86% of the variation in RBY), but live animal measurements contributed little to this result, accounting for only 8% of the variation in RBY for measurements at slaughter in the presence of the HERD × KILL term. A second analysis examined whether market category, finishing regime or breed classifications consistently influenced the relationship between the measured traits and RBY at the 3 scanning times. The magnitude of the variation between significantly different coefficients (for scanned P8 fat depth, scanned eye muscle area and liveweight) was generally small, though the results suggested that in some instances, developing separate equations for animals of different classifications would marginally improve the accuracy of RBY prediction. The final analysis investigated the improvement in RBY prediction when measurements from entry to finishing were included with those taken before slaughter. HERD × KILL was included in the model to account for all known classification variables. Measurements of both P8 fat depth and EMA from the earlier measurement time were significant predictors of RBY in the presence of the corresponding measurement at slaughter, but accounted for an increase in R 2 of only 0.0007. It was concluded that a single scan and liveweight measurement, close to slaughter, would provide the best live animal measurements for RBY prediction, and that no improvement in accuracy would be achieved by additional scans taken earlier in an animal’s life.

The genetics of cow growth and body composition at first calving in two tropical beef genotypes

The genetics of cow growth and body composition traits, measured before first calving (pre-calving: in females before calving following their first 3-month annual mating period, at an average age of 34 months) and at the start of the subsequent mating period (Mating 2: on average 109 days later), were evaluated in 1016 Brahman (BRAH) and 1094 Tropical Composite (TCOMP) cows. Measurements analysed were liveweight, ultrasound-scanned measurements of P8 and 12/13th rib fat depth and eye muscle area, body condition score and hip height. Traits describing the change in these from pre-calving to Mating 2 were also included in the analysis. The maternal genetic component of weaning weight was estimated from weaning-weight records on these cows, their steer half-sibs and their progeny generated from up to six matings (n = 12 528). Within pregnant cows at pre-calving, BRAH were significantly lighter, leaner at the P8 site and taller than their TCOMP contemporaries, and these differences were also significant at Mating 2. There was a genetic basis for variation in growth and body composition traits measured at pre-calving and Mating 2 in BRAH (h2 = 0.27–0.67) and TCOMP (h2 = 0.25–0.87). Traits describing the change from pre- calving to Mating 2 were also moderately heritable for both genotypes (h2 = 0.17–0.54), except for change in hip height (h2 = 0.00 and 0.10 for BRAH and TCOMP, respectively). Genetic correlations between measurements of the same trait at pre-calving and Mating 2 were consistently positive and strong (rg = 0.75–0.98) and similar for both genotypes. In lactating cows, genetic correlations of growth and body composition traits with their change from pre-calving to Mating 2 showed that when animals had low levels of P8 and rib fat at Mating 2, change in eye muscle area was an important descriptor of genetic body condition score (rg = 0.63). This was supported by moderate genetic relationships, which suggested that lactating cows that were genetically predisposed to lose less eye muscle area were those that ended the period with higher P8 fat (rg = 0.66), rib fat (rg = 0.72) and body condition score (rg = 0.61). Change in liveweight, body condition score and, in particular, eye muscle area was significantly related to the maternal genetic component of weaning weight (rg = from –0.40 to –0.85) in both genotypes, suggesting that cows with higher genetic milk-production potential were those with the propensity for greater loss of these traits over the period from pre-calving to Mating 2. These results showed that for tropically adapted cows, the change in eye muscle area from pre-calving to Mating 2 was a more important descriptor of body condition at Mating 2 than was change in fat depth, and that higher genetic milk-production potential, measured as maternal weaning weight, was genetically related to higher mobilisation of muscle, and therefore body condition, over this period.

Effects of early weaning on growth, feed efficiency and carcass traits in Shorthorn cattle

This study aimed to examine the impact of early weaning on residual feed intake, and other production and carcass traits, in a group of cattle subjected to early or conventional weaning treatments, but otherwise managed as contemporaries. Shorthorn (n = 140) calves were randomly allocated by sex and sire to early and conventional weaning treatments. Early weaned animals (n = 69) were weaned at an average of 123 days of age and 145 kg liveweight, while conventionally weaned steers and heifers (n = 71) were 259 days old at weaning and 273 kg. Following conventional weaning, animals were managed as contemporaries through backgrounding, and entered feedlot finishing at a mean age of 353 and 408 days for heifers and steers, respectively, for finishing and feed intake testing. At the conclusion of feed intake testing hip height was measured, and animals were ultrasound scanned to assess fat depth, eye muscle area and percent intramuscular fat. Early weaned animals were significantly lighter (P < 0.001) than their conventionally weaned contemporaries, when weighed at conventional weaning. The weight difference observed at conventional weaning of 19.4 kg between treatment groups persisted throughout the experiment, with significant (P < 0.05) differences of 17.1, 15.6 and 15.8 kg at feedlot entry, and the start and end of the feed intake test period, respectively. Weaning treatment also approached significance for daily feed intake (P = 0.06), with early weaned animals tending to eat less than their conventionally weaned contemporaries (daily feed intake = 11.6 and 12.0 kg, respectively). Weaning treat\ment did not significantly affect feed efficiency whether measured as residual feed intake (P = 0.64) or feed conversion ratio (P = 0.27). None of the other traits measured were significantly affected by weaning treatment. These data showed that early weaning, as implemented for this experiment, resulted in animals that were lighter than their conventionally weaned contemporaries through backgrounding and finishing. Weaning treatment did not, however, influence feed efficiency or the post-weaning growth and carcass composition traits measured for this experiment.

Factors associated with calf mortality in tropically adapted beef breeds managed in extensive Australian production systems

Data from 9296 calves born to 2078 dams over 9 years across five sites were used to investigate factors associated with calf mortality for tropically adapted breeds (Brahman and Tropical Composite) recorded in extensive production systems, using multivariate logistic regression. The average calf mortality pre-weaning was 9.5% of calves born, varying from 1.5% to 41% across all sites and years. In total, 67% of calves that died did so within a week of their birth, with cause of death most frequently recorded as unknown. The major factors significantly (P < 0.05) associated with mortality for potentially large numbers of calves included the specific production environment represented by site-year, low calf birthweight (more so than high birthweight) and horn status at branding. Almost all calf deaths post-branding (assessed from n = 8348 calves) occurred in calves that were dehorned, totalling 2.1% of dehorned calves and 15.9% of all calf deaths recorded. Breed effects on calf mortality were primarily the result of breed differences in calf birthweight and, to a lesser extent, large teat size of cows; however, differences in other breed characteristics could be important. Twin births and calves assisted at birth had a very high risk of mortality, but <1% of calves were twins and few calves were assisted at birth. Conversely, it could not be established how many calves would have benefitted from assistance at birth. Cow age group and outcome from the previous season were also associated with current calf mortality; maiden or young cows (<4 years old) had increased calf losses overall. More mature cows with a previous outcome of calf loss were also more likely to have another calf loss in the subsequent year, and this should be considered for culling decisions. Closer attention to the management of younger cows is warranted to improve calf survival.

Genetic relationships of female reproduction with growth, body composition, maternal weaning weight and tropical adaptation in two tropical beef genotypes

The genetic relationships of female reproduction with growth and body composition, tropical adaptation traits and maternal weaning weight (descriptive of genetic potential milk production) were estimated in 1027 Brahman (BRAH) and 1132 Tropical Composite (TCOMP) females. Female reproduction was evaluated at puberty, as outcomes of the first and second annual mating periods (Mating 1 and Mating 2, which commenced when females averaged 27 and 39 months of age, respectively), as well as annual averages over up to six matings. Traits evaluated included age at puberty, Mating 1 and 2 pregnancy rate, weaning rate and days to calving, and lifetime annual calving and weaning rate. Traits describing growth and body composition (liveweight, hip height, ultrasound-scanned P8 fat depth and eye muscle area, subjective body condition score and blood IGF-I concentration) were measured in the animals as heifers (at ~18 months of age), and again at the start of Mating 2. Traits describing tropical adaptation included coat-length scores in both genotypes and, in BRAH, buffalo fly lesion scores. Previously reported analyses of these data identified heifer IGF-I and coat and buffalo-fly-lesion scores as potential genetic indicators for age at puberty in BRAH. The results of the present study found that exploiting these relationships would have no unfavourable genetic consequences for later female reproduction and, in some cases, may be indicators of female reproduction, when evaluated as outcomes of Matings 1 or 2, or as lifetime annual calving or weaning rates. For BRAH, heifer liveweight was a genetic indicator for Mating 1 weaning rate (rg = 0.70), and, while standard errors were high, there were also positive genetic correlations of heifer hip height, eye muscle area and blood IGF-I concentration with Mating 1 weaning rate (rg = 0.61, 0.58 and 0.43, respectively). For TCOMP, significant genetic relationships of heifer growth, body composition and tropical adaptation traits with female reproduction were virtually absent, suggesting that there is less opportunity to identify earlier in life measures as genetic indicators of reproduction for this genotype. Higher maternal weaning weight was significantly genetically related to lower lifetime annual weaning rate (rg = –0.50) in BRAH, and with lower Mating 2 calving and weaning rate (rg = –0.72 and –0.59, respectively) in TCOMP, which will need to be considered when making selection decisions that affect genetic milk in these genotypes. Importantly, the results presented revealed no strong genetic antagonisms of heifer growth and body composition traits with female reproduction, suggesting that selection could be undertaken to improve these simultaneously.