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Context: Nitrate salts can be utilised by the rumen bacteria as a nitrogen source. Nitrate salts can induce a methaemoglobinaemia in cattle if consumed in sufficient quantities. Methaemoglobinaemia is the principal factor that leads to the onset of clinical signs for nitrate toxicity in cattle. A methaemoglobin concentration ≥20% is considered unsafe for cattle. There are, however, limited studies on the longer-term effects of nitrate supplementation on methaemoglobin formation in Bos indicus steers consuming forage that is reflective of northern Australia's poor quality, native pasture in the dry season. Aims: We hypothesised that the Australian government's recommended daily dose of nitrate salts given to Bos indicus cattle would not cause a methaemoglobinaemia in the blood >20% throughout a 70 day treatment period. Methods: A 70 day study was conducted to determine the methaemoglobin, carboxyhaemoglobin, total haemoglobin, growth rate and forage intakes of cattle supplemented with a non-protein-nitrogen treatment containing nitrate (6.48 g NO3/kg dry matter intake (DMI) or no nitrate and consuming a chaffed Flinders grass hay (Iseilema spp.), a C4 species. The dose rate of nitrate was selected to match the Australian government guidelines. Ten 3-year-old fistulated Bos indicus steers (mean liveweight ± s.d., 400.7 ± 26.2 kg) were randomly allocated into two groups (n = 5). Blood samples were collected at 0, 2, 4 and 6 h after treatment with nitrate or no nitrate on days 10, 30, 50 and 70 to measure haemoglobin fractions in the blood. Key Results: Nitrate treatment caused the mean methaemoglobin (P < 0.001), peak methaemoglobin (P < 0.001) and carboxyhaemoglobin (P = 0.008) concentration to be greater in the blood of steers compared with steers given no nitrate. Nitrate treatment had no general effect on the total haemoglobin, DMI or bodyweight of steers. Conclusions: Bos indicus steers treated with 6.48 g NO3/kg DMI develop a methaemoglobinaemia that does not exceed 20% of total haemoglobin for 70 days. This data supports the Australian government's recommended feeding rate of nitrate to sedentary Bos indicus steers. Implications: The Australian government's method for feeding nitrate to cattle is safe under the conditions of this study. Within the conditions of the study, Bos indicus steers treated with nitrate develop a methaemoglobinaemia that does not exceed 20% of total haemoglobin for 70 days. Although the data in the current study supports the Australian government's recommended dose rate as safe for cattle the data represents Bos indicus steers in a sedentary state. Further research is required to validate the dose rate of nitrate for cattle in an active state. [ABSTRACT FROM AUTHOR]

Heifers that have an earlier age at puberty often have greater lifetime productivity. Age at puberty is moderately heritable so selection should effectively reduce the number of days to puberty, and improve heifer productivity and profitability as a result. However, recording age at puberty is intensive, requiring repeat ovarian scanning to determine age at first corpus luteum (AGECL). Genomic selection has been proposed as a strategy to select for earlier age at puberty; however, large reference populations of cows with AGECL records and genotypes would be required to generate accurate GEBV for this trait. Reproductive maturity score (RMS) is a proxy trait for age at puberty for implementation in northern Australia beef herds, where large scale recording of AGECL is not feasible. RMS assigns a score of 0 to 5 from a single ovarian scan to describe ovarian maturity at ~600 d. Here we use multivariate genomic prediction to evaluate the value of a large RMS data set to improve accuracy of GEBV for age at puberty (AGECL). There were 882 Brahman and 990 Tropical Composite heifers with AGECL phenotypes, and an independent set of 974 Brahman, 1,798 Santa Gertrudis, and 910 Droughtmaster heifers with RMS phenotypes. All animals had 728,785 real or imputed SNP genotypes. The correlation of AGECL and RMS (h 2 = 0.23) was estimated as −0.83 using the genomic information. This result also demonstrates that using genomic information it is possible to estimate genetic correlations between traits collected on different animals in different herds, with minimal or unknown pedigree linkage between them. Inclusion of heifers with RMS in the multi-trait model improved the accuracy of genomic evaluations for AGECL. Accuracy of RMS GEBV generally did not improve by adding heifers with AGECL phenotypes into the reference population. These results suggest that RMS and AGECL may be used together in a multi-trait prediction model to increase the accuracy of prediction for age at puberty in tropically adapted beef cattle. [ABSTRACT FROM AUTHOR]

The natural abundance of nitrogen isotopes (δ15N) in animal tissues is related to the ability of cattle to lose less nitrogen in the urine and has been used to estimate nitrogen use efficiency and feed efficiency. Carbon isotope enrichment (δ13C) has also been used to estimate feed conversion ratio and grazing behavior in beef cattle. There are significant variations between the individual beef cattle in the same environment for δ15N and δ13C of the tail hair. Thus, the objective was to determine the heritability of δ15N and δ13C in the tail hair of tropically adapted beef cattle to further validate their use in genetic breeding programs. A total of 497 Bos indicus steers (269 Brahman and 228 Droughtmaster) were used, and the steers had reliable pedigree information for three generations as they were part of the beef cattle genetic evaluation system in Australia. Two single, mixed breed, contemporary groups of steers are used including the steers weaned in 2019 (n = 254) and steers weaned in 2020 (n = 243). Samples of tail switch hair representing hair segments growing during the dry season were collected and analyzed for δ15N and δ13C. Fixed effects fitted in the linear regression model were age, breed, year (as contemporary groups), and average daily gain during the different seasons. The heritabilities of δ15N and δ13C in the tail hair were estimated at 0.43 ± 0.14, and 0.41 ± 0.15, respectively, which indicates both are moderately heritable traits. The genetic correlation between δ15N and δ13C was negative at -0.78 ± 0.16, and the phenotypic one was -0.40 ± 0.04. The negative correlation can be explained by the contrasting isotope fractionation in the urine of cattle, as urine has less δ15N and more δ13C than the diet. The findings of this study have shown for the first time that both δ15N and δ13C in the tail hair of cattle are heritable traits. These results support the feasibility of utilizing tail hair isotopes as a promising approach for identifying cattle with greater nitrogen use efficiency, which is a key trait in low-protein diets. Moreover, genetic selection for nitrogen-efficient cattle can reduce the environmental impact of beef production. [ABSTRACT FROM AUTHOR]

The meta-analysis methodology was applied in this study to evaluate factors that affect the carcass quality components in beef cattle (rib eye area - AOL and fat thickness - EGC, measured between the 12th and the 13th ribs) and to compare them in different genetic groups. Data from 215 measures of AOL and 209 measures of EGC, from 65,174 animals, extracted from 36 papers published between 1985 and 2006 were used. The Restricted Maximum Likelihood Method and an animal model that considered AOL and EGC as functions of the fixed effects of country from where the data come from; animal category; feed system; measure methodology; Bos indicus, british and continental Bos taurus gene contribution; age and weight at measure, were used. The observed averages for AOL and e EGC were 63.5cm² and 4.2mm, respectively. The animals were in average 552.22 days old at the measure of the characteristics and had 468.47kg of live weight. The use of meta-analysis permitted to get generalized conclusions about AOL and EGC: animals rose in the EUA presented values significantly superior for AOL and EGC when compared with those rose in Australia and Brazil; castrated and the entire males presented bigger AOL measures (do not differing among them) compared to females, and females presented bigger EGC. The feed system, confined and do not confined, were superior to the greasing system. The Continental Bos taurus genotypes presented bigger AOL values, British Bos taurus presented bigger EGC values, and Bos indicus presented intermediate values. The measures taken in the carcass (pos mortis) and by ultra son (in vivo) were not different. The increase in the characteristics decreased with the age and live weight increase. [ABSTRACT FROM AUTHOR]

Abstract: In response to uncertainty among cattle producers in Australia regarding the need to treat Bos indicus and B. indicus crossbreeds, the scientific literature relating to the productivity effects of Boophilus microplus on cattle of all breeds was reviewed. Estimates of the mean effect of each engorging tick (damage coefficient, d) were made from a simple analysis of the reported data. On average, each engorging female tick is responsible for the loss of 1.37±0.25g bodyweight in B. taurus cattle. The comparable value for B. taurus × B. indicus cattle is 1.18±0.21g/engorging tick. These values were not statistically significantly different, indicating that if a threshold approach to tick control were taken, then the threshold number of standard ticks would be the same regardless of cattle genotype. No studies provided useable estimates of the effect of tick infestation on pure B. indicus cattle. An economic threshold for treatment, below which acaricide application is not beneficial, can be predicted, using known values for the cost of acaricide application and the price of beef. However, the application of a threshold approach to control has not been embraced by government advisers and runs contrary to the accepted principals of strategic control programs. [Copyright &y& Elsevier]