Male traits and herd reproductive capability in tropical beef cattle. 1. Experimental design and animal measures

2013 ◽  
Vol 53 (2) ◽  
pp. 87 ◽  
Author(s):  
B. M. Burns ◽  
N. J. Corbet ◽  
D. H. Corbet ◽  
J. M. Crisp ◽  
B. K. Venus ◽  
...  
Keyword(s):  
Experimental Design ◽  
Body Condition Score ◽  
High Stress ◽  
Male Reproduction ◽  
Research Centre ◽  
Northern Australia ◽  
Female Reproduction ◽  
Cooperative Research ◽  
Muscle Area ◽  
Eye Muscle

Research into the genetics of whole herd profitability has been a focus of the Beef Cooperative Research Centre for Beef Genetic Technologies over the past decade and it has been identified that measures of male reproduction may offer a potential indirect means of selecting for improved female reproduction. This paper describes the experimental design and provides a descriptive analysis of an array of male traits in Brahman and Tropical Composite genotypes managed under the medium to high stress, semi-extensive to extensive production systems of northern Australia. A total of 1639 Brahman and 2424 Tropical Composite bulls with known pedigrees, bred and raised in northern Australia, were evaluated for a comprehensive range of productive and reproductive traits. These included blood hormonal traits (luteinising hormone, inhibin and insulin-like growth factor-I); growth and carcass traits (liveweight, body condition score, ultrasound scanned 12–13th rib fat, rump P8 fat, eye muscle area and hip height); adaptation traits (flight time and rectal temperature); and a bull breeding soundness evaluation (leg and hoof conformation, sheath score, length of everted prepuce, penile anatomy, scrotal circumference, semen mass activity, sperm motility and sperm morphology). Large phenotypic variation was evident for most traits, with complete overlap between genotypes, indicating that there is likely to be a significant opportunity to improve bull fertility traits through management and bull selection.

Divergent genotypes for fatness or residual feed intake in Angus cattle. 2. Body composition but not reproduction was affected in first-parity cows on both low and high levels of nutrition

2018 ◽  
Vol 58 (1) ◽  
pp. 43 ◽  
Author(s):  
M. Laurence ◽  
J. M. Accioly ◽  
K. J. Copping ◽  
M. P. B. Deland ◽  
J. F. Graham ◽  
...  
Keyword(s):  
Feed Intake ◽  
South Australia ◽  
Residual Feed Intake ◽  
Pregnancy Rates ◽  
Research Centre ◽  
Breeding Cycle ◽  
Cooperative Research ◽  
Muscle Area ◽  
Eye Muscle ◽  
Selection For

This paper reports a subset of results from the Beef Cooperative Research Centre-funded Maternal Productivity Project. This research aimed to describe the response of Angus cows of different and divergent genotypes to variable nutritional environments over five breeding seasons. Cows selected for a divergence in either fat depth (HFat vs LFat) or residual feed intake (RFI: HRFI vs LRFI) based on mid-parent estimated breeding values (EBV) for those traits were allocated in replicate groups to either high or low nutritional treatments at two different sites, namely the Vasse Research Centre in Western Australia and the Struan Research Centre in South Australia. The traits reported in this paper include output traits (birth and weaning weight of calves, liveweight change of cows), change traits (change in Rib Fat, P8 fat, eye muscle area and liveweight between specified time points) and reproductive traits [pregnancy rates, percentage calves born alive and days to calving at the days to calving at the second calving opportunity (DC2)]. Having had their first calf, the vulnerability of these young cows to nutritional restriction and how it may adversely affect rebreeding was examined. HFat and HRFI cows were fatter, heavier and had greater eye muscle area than LFat and LRFI, respectively, at all times during the breeding cycle on both levels of nutrition. There was no difference in either days-to-calving or pregnancy rates after the second mating between genotypes. Equally, nutritional treatment had no effect on these traits in this cohort of cows. There was evidence for an implied genetic correlation between Rib Fat EBV, DC2 and pregnancy rates of –0.38 that suggests that selection for leanness may result in reduced fertility of the herd but the effect was not significant herein. As long as producers record the phenotype for both traits and select cows with favourable DC2 as well as low fatness, these problems can be avoided, owing to only 22% of variation in pregnancy rates being explained by DC2 and Rib Fat EBV. Producers can largely be confident that selection for leanness, or increased feed efficiency, has little impact on productivity as long as cows are in adequate body condition to remain healthy and productive.

Maternal body composition in seedstock herds. 4. Genetic parameters for body composition of Angus and Hereford cows

2018 ◽  
Vol 58 (1) ◽  
pp. 145 ◽  
Author(s):  
K. A. Donoghue ◽  
S. J. Lee ◽  
P. F. Parnell ◽  
W. S. Pitchford
Keyword(s):  
Body Composition ◽  
Body Condition Score ◽  
Genetic Relationships ◽  
Genetic Correlations ◽  
Fat Percentage ◽  
Muscle Area ◽  
Eye Muscle ◽  
Life Traits ◽  
Condition Score ◽  
Over Time

The genetics of body composition traits measured before calving and at weaning in the first and second parities were evaluated in 5975 Angus and 1785 Hereford cows. Traits measured were liveweight, body condition score and hip height and ultrasound scanned measurements of subcutaneous P8 and 12/13th rib fat depth, loin eye muscle area and intramuscular fat percentage. Corresponding yearling measures on these animals were obtained for analyses of relationships between yearling information with later-in-life traits. There was moderate genetic variation in all body composition traits measured at pre-calving and weaning in Angus (h2 = 0.14–0.59) and Hereford (h2 = 0.14–0.64) cows. Genetic correlations between measurements of the same trait at pre-calving and weaning were consistently positive and high in both parities for both breeds, indicating animals were ranking similarly for the same trait measured over time. Genetic correlations between measurements of different traits were generally consistent over time (pre-calving and weaning) in both breeds, indicating genetic relationships between traits were not changing significantly over time. Genetic correlations with corresponding yearling measures of body composition were consistently positive and high for the first parity, and lower for the second parity. The results of this study indicate that genetic improvement in body composition traits in cows is possible, and that body composition information recorded at yearling age is a reasonably good predictor of later in life performance for these traits.

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

2014 ◽  
Vol 54 (1) ◽  
pp. 60 ◽  
Author(s):  
M. L. Wolcott ◽  
D. J. Johnston ◽  
S. A. Barwick
Keyword(s):  
Body Composition ◽  
Genetic Relationships ◽  
Genetic Correlations ◽  
Female Reproduction ◽  
Muscle Area ◽  
Eye Muscle ◽  
Weaning Weight ◽  
Igf I ◽  
Age At Puberty ◽  
Tropical Adaptation

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.

Designing complex research projects to estimate genetic parameters plus treatment and other effects - optimising the experimental design

2008 ◽  
Vol 48 (8) ◽  
pp. 1110 ◽  
Author(s):  
D. L. Robinson
Keyword(s):  
Experimental Design ◽  
Genetic Parameters ◽  
Genetic Parameter ◽  
Genetic Correlations ◽  
Research Centre ◽  
Parameter Estimates ◽  
Cooperative Research ◽  
Number Of Factors ◽  
Integrated Research ◽  
One Year

There is an increasing trend towards integrated research, in which several individuals or institutions pool their expertise and make use of common resources, collaborating towards a common set of scientific goals. Integrated research enables a larger number of factors to be investigated, and the most influential or important ones identified, providing information on how the different factors interact or fit together. Good experimental design is, however, required to ensure the aims can be achieved and resources spent wisely. Issues involved in the experimental design of the Australian Beef Cattle Cooperative Research Centre for Meat Quality are discussed. Theoretical results and simulation studies were used to determine optimal numbers of progeny per sire for estimating genetic parameters. For heritabilities of 0.2 and 0.5, the optima are respectively 21 and 9 progeny with recorded measurements. The curves surrounding the optima are quite flat, so aiming for 10–15 progeny with measurements per trait should provide reasonable accuracy in many situations. Estimates of heritabilities, genetic correlations and phenotypic variances have lower sampling correlations than genetic variances and covariances, suggesting that when results are pooled over different breeds or trials, it is better to pool estimates of heritabilities and genetic correlations than (co)variances. Using sires in more than one year increases the robustness of estimated sire effects and increases the accuracy of genetic parameter estimates for hard-to-measure traits (e.g. feed efficiency) that are not recorded on all animals. Unless sires can be chosen as a true random sample of the population, arrangements of link sires (and other effects such as treatments) should be chosen to provide accurate estimates when all terms in the model are fitted as fixed.

scholarly journals Can sustainable cotton production systems be developed for tropical northern Australia?

2013 ◽  
Vol 64 (12) ◽  
pp. 1127 ◽  
Author(s):  
S. J. Yeates ◽  
G. R. Strickland ◽  
P. R. Grundy
Keyword(s):  
Crop Production ◽  
Production Systems ◽  
Climatic Factors ◽  
Research Effort ◽  
Deep Understanding ◽  
Research Centre ◽  
Northern Australia ◽  
Cotton Production ◽  
Cooperative Research ◽  
Funding Model

This article reviews research coordinated by the Australian Cotton Cooperative Research Centre (CRC) that investigated production issues for irrigated cotton at five targeted sites in tropical northern Australia, north of 21°S from Broome in Western Australia to the Burdekin in Queensland. The biotic and abiotic issues for cotton production were investigated with the aim of defining the potential limitations and, where appropriate, building a sustainable technical foundation for a future industry if it were to follow. Key lessons from the Cotton CRC research effort were: (1) limitations thought to be associated with cotton production in northern Australia can be overcome by developing a deep understanding of biotic and environmental constraints, then tailoring and validating production practices; and (2) transplanting of southern farming practices without consideration of local pest, soil and climatic factors is unlikely to succeed. Two grower guides were published which synthesised the research for new growers into a rational blueprint for sustainable cotton production in each region. In addition to crop production and environmental impact issues, the project identified the following as key elements needed to establish new cropping regions in tropical Australia: rigorous quantification of suitable land and sustainable water yields; support from governments; a long-term funding model for locally based research; the inclusion of traditional owners; and development of human capacity.

scholarly journals 236 Comparison of ultrasound scanned loin eye muscle characteristics and back-fat depth measured in lean muscle selection and control Katahdin sheep flock

2020 ◽  
Vol 98 (Supplement_4) ◽  
pp. 173-173
Author(s):  
Tumen Wuliji ◽  
Christopher Baughman ◽  
Jessica Epple-Farmer ◽  
Jeremiah Markway ◽  
Luke Wilbers ◽  
...  
Keyword(s):  
Production Efficiency ◽  
Body Condition Score ◽  
Meat Product ◽  
Correlation Coefficients ◽  
Live Weight ◽  
Muscle Area ◽  
Vertical Column ◽  
Live Animal ◽  
Eye Muscle ◽  
And Control

Abstract Sheep farming is an important enterprise for many farmers in US, and meat is a viable protein resource for human nutrition. However, increasing concern for health aspects of meat consumption has led consumers to demand a higher ratio of lean to fat tissues. Application of ultrasound carcass scanning in meat animal selection and marketing as a performance prediction will enhance the selection efficiency, accuracy and profitability. One hundred and sixty two mixed-age Katahdin ewes were evenly divided into a high lean muscle selection (n = 81) and control (n = 81) flock based on the prescreened ultrasound measures, including loin eye muscle area (LEA), loin eye muscle width (LEW), and depth (LED), and back-fat depth (BFD). The LEA is measured between the 12th and 13th ribs lateral and perpendicular to the vertical column and parallel to rib covering the longissimus dorsi muscle. Live animal measurements were conducted using ultrasound scanner fitted with a 3.5 MHz linear transducer (ExaGo, France) prior to fall breeding schedules. Live weight (LWT), body condition score (BCS) and FAMACHA score (FAM) were recorded. Data were analyzed using GLM procedures of SAS statistics. There was no difference between selection and control flocks for LWT, BCS and FAM; however, LEA, LEW, LED and BFD were significantly (P < 0.05) greater for the lean selection over control flock (Table 1). The correlation coefficients (P < 0.01) were found as moderate (r = 0.6) to high (r = 0.8) among LED, LEA, LEW and BFD, whereas these traits showed lower (r = 0.4) but significant (P < 0.01) correlations with LWT. The results indicate that lean muscle selection in sheep with ultrasound scanning of longissimus muscle characteristics will enhance the production efficiency and genetic gain and facilitate producing a high value meat product.

Effect of late gestation bodyweight change and condition score on progeny feedlot performance

2016 ◽  
Vol 56 (12) ◽  
pp. 1998 ◽  
Author(s):  
J. T. Mulliniks ◽  
J. E. Sawyer ◽  
F. W. Harrelson ◽  
C. P. Mathis ◽  
S. H. Cox ◽  
...  
Keyword(s):  
Nutrient Intake ◽  
Body Condition Score ◽  
Late Gestation ◽  
Research Centre ◽  
Nutrient Restriction ◽  
Feedlot Performance ◽  
Muscle Area ◽  
Condition Score ◽  
Fat Thickness ◽  
And Performance

Inadequate nutrient intake during late gestation can cause cow bodyweight (BW) loss and influence cow reproductive performance and subsequent productivity of steer progeny. Therefore, a 7-year study with a 3 × 3 arrangement of treatments was conducted at Corona Range and Livestock Research Centre, Corona, New Mexico to evaluate the effects of cow BW change and body condition score (BCS) during late gestation on subsequent cow pregnancy rates, progeny steer feedlot performance, and health. Cows were retrospectively classified to 1 of 3 BW change groups: (1) cows that lost BW during late gestation (LOSS; mean –26 ± 2 kg); (2) cows that maintained BW during late gestation (MAIN; mean –1 ± 1 kg); or (3) cows that gained BW during late gestation (GAIN; mean 25 ± 2 kg). Cows were also classified to 1 of 3 BCS groups: BCS of 4 (mean BCS = 4.0 ± 0.02; range 4.0–4.5), 5 (mean BCS = 5.0 ± 0.02; range 5.0–5.5), or 6 (mean BCS = 6.0 ± 0.02; range 6.0–6.5). After weaning each year, steers were preconditioned for 45 days and were received and treated as custom-fed commercial cattle at a feedlot in mid-November. Calf weaning BW, initial feedlot BW, final BW, and hot carcass weight were unaffected (P ≥ 0.22) by dam’s prepartum BW change or BCS. However, steers from GAIN and MAIN tended (P = 0.06) to have increased ADG in the feedlot. Twelfth-rib fat thickness, longissimus muscle area, and days on feed were not influenced (P ≥ 0.18) by late gestation BW change or BCS. Calves from BCS 6 cows tended (P = 0.10) to have greater yield grades at harvest in the feedlot. Percentage of steers grading Choice or greater was increased (P < 0.01) in steers from LOSS cows and cows in BCS 4 during late gestation compared with other groups. These data suggest that modest nutrient restriction during late gestation can have a minimal effect on growth and performance of steer progeny from birth through the finishing phase.

scholarly journals Liveweight prediction from hip height, condition score, fetal age and breed in tropical female cattle

2013 ◽  
Vol 53 (4) ◽  
pp. 275 ◽  
Author(s):  
Geoffry Fordyce ◽  
Angela Anderson ◽  
Kieren McCosker ◽  
Paul J. Williams ◽  
Richard G. Holroyd ◽  
...  
Keyword(s):  
Body Condition ◽  
Bos Taurus ◽  
Body Condition Score ◽  
Subcutaneous Fat ◽  
Muscle Area ◽  
Eye Muscle ◽  
Validation Population ◽  
Accuracy Of Measurements ◽  
Condition Score ◽  
Female Cattle

Hip height, body condition, subcutaneous fat, eye muscle area, percentage Bos taurus, fetal age and diet digestibility data were collected at 17 372 assessments on 2181 Brahman and tropical composite (average 28% Brahman) female cattle aged between 0.5 and 7.5 years of age at five sites across Queensland. The study validated the subtraction of previously published estimates of gravid uterine weight to correct liveweight to the non-pregnant status. Hip height and liveweight were linearly related (Brahman: P < 0.001, R2 = 58%; tropical composite P < 0.001, R2 = 67%). Liveweight varied by 12–14% per body condition score (5-point scale) as cows differed from moderate condition (P < 0.01). Parallel effects were also found due to subcutaneous rump fat depth and eye muscle area, which were highly correlated with each other and body condition score (r = 0.7–0.8). Liveweight differed from average by 1.65–1.66% per mm of rump fat depth and 0.71–0.76% per cm2 of eye muscle area (P < 0.01). Estimated dry matter digestibility of pasture consumed had no consistent effect in predicting liveweight and was therefore excluded from final models. A method developed to estimate full liveweight of post-weaning age female beef cattle from the other measures taken predicted liveweight to within 10 and 23% of that recorded for 65 and 95% of cases, respectively. For a 95% chance of predicted group average liveweight (body condition score used) being within 5, 4, 3, 2 and 1% of actual group average liveweight required 23, 36, 62, 137 and 521 females, respectively, if precision and accuracy of measurements matches that used in the research. Non-pregnant Bos taurus female cattle were calculated to be 10–40% heavier than Brahmans at the same hip height and body condition, indicating a substantial conformational difference. The liveweight prediction method was applied to a validation population of 83 unrelated groups of cattle weighed in extensive commercial situations on 119 days over 18 months (20 917 assessments). Liveweight prediction in the validation population exceeded average recorded liveweight for weigh groups by an average of 19 kg (~6%) demonstrating the difficulty of achieving accurate and precise animal measurements under extensive commercial grazing conditions.

Genetic relationships between steer performance and female reproduction and possible impacts on whole herd productivity in two tropical beef genotypes

2014 ◽  
Vol 54 (1) ◽  
pp. 85 ◽  
Author(s):  
M. L. Wolcott ◽  
D. J. Johnston ◽  
S. A. Barwick ◽  
N. J. Corbet ◽  
H. M. Burrow
Keyword(s):  
Beef Cattle ◽  
Meat Quality ◽  
Reproductive Performance ◽  
Feed Intake ◽  
Genetic Relationships ◽  
Carcass Composition ◽  
Female Reproduction ◽  
Muscle Area ◽  
Eye Muscle ◽  
Reproduction Traits

Steer growth and carcass composition, and female reproductive performance have been identified as key aspects of productivity by breeders of tropically adapted beef cattle in Australia. Research has also demonstrated that traits describing meat quality and feed intake and efficiency are of economic importance to Australia’s beef industry. The present study aimed to determine genetic relationships of traits describing steer growth, feed intake and efficiency, carcass composition and meat quality with female reproductive performance in two genotypes of tropically adapted beef cattle. Female reproduction traits describing outcomes of first (Mating 1) and second (Mating 2) annual matings and lifetime reproduction (averaged over 6 matings) were analysed for 1020 Brahman (BRAH) and 1117 Tropical Composite (TCOMP) females. Steer traits were available for 1007 BRAH and 1210 TCOMP half-sibs of the females evaluated for reproductive performance, and measurements of liveweight and body composition for 1025 BRAH and 1520 TCOMP bull progeny of the same females were included in the analysis. Results demonstrated that selection to increase steer carcass weight and eye muscle area and decrease carcass fat depth would have no significant unfavourable impact on female reproductive performance for both genotypes. Measures of liveweight, eye muscle area and P8 fat depth in young BRAH bulls, however, were only moderately correlated with steer carcass equivalents (rg = 0.28 to 0.55) and results showed that selection on the basis of bull measurements alone may negatively affect female lifetime annual calving rate (rg = –0.44 to –0.75) if both were not included in a multi-trait genetic evaluation and considered when making selection decisions. More favourable (lower) net feed intake in BRAH steers was genetically associated with lower Mating 1 weaning rate (rg = 0.76) and higher days to calving (rg = –0.50), although this did not significantly affect lifetime annual calving or weaning rate (rg = 0.10 and 0.29, respectively). For TCOMP, higher steer carcass P8 fat depth was unfavourably genetically associated with female Mating 2 weaning rate (rg = –0.76), although these relationships were not as strong for weaning rate at Mating 1 or when averaged over the animals lifetime (rg = 0.43 and –0.13, respectively). Lower (more favourable) shear force (a measure of tenderness) also displayed a significant genetic association with higher (less favourable) Mating 1 days to calving in TCOMP and, while standard errors were high, tended to be unfavourably associated with other measures of female reproduction evaluated for the present study. Steer growth, carcass composition, meat quality and residual feed intake and female reproduction could be improved simultaneously if measurements describing both are included in a multi-trait genetic evaluation. Results of the present study also showed that expanding female reproduction traits to include descriptors of first and second mating outcomes, as well as lifetime reproductive performance, would allow a fuller account to be taken of genetic relationships of male traits with female reproduction.

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

2014 ◽  
Vol 54 (1) ◽  
pp. 37 ◽  
Author(s):  
M. L. Wolcott ◽  
D. J. Johnston ◽  
S. A. Barwick ◽  
N. J. Corbet ◽  
P. J. Williams
Keyword(s):  
Body Composition ◽  
Body Condition ◽  
Body Condition Score ◽  
Genetic Correlations ◽  
Muscle Area ◽  
Eye Muscle ◽  
Weaning Weight ◽  
Mating Period ◽  
Lactating Cows ◽  
Condition Score

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.

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