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      Genomic evaluation of age at first calving

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      Journal of Dairy Science
      American Dairy Science Association

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          Genomic evaluations with many more genotypes

          Background Genomic evaluations in Holstein dairy cattle have quickly become more reliable over the last two years in many countries as more animals have been genotyped for 50,000 markers. Evaluations can also include animals genotyped with more or fewer markers using new tools such as the 777,000 or 2,900 marker chips recently introduced for cattle. Gains from more markers can be predicted using simulation, whereas strategies to use fewer markers have been compared using subsets of actual genotypes. The overall cost of selection is reduced by genotyping most animals at less than the highest density and imputing their missing genotypes using haplotypes. Algorithms to combine different densities need to be efficient because numbers of genotyped animals and markers may continue to grow quickly. Methods Genotypes for 500,000 markers were simulated for the 33,414 Holsteins that had 50,000 marker genotypes in the North American database. Another 86,465 non-genotyped ancestors were included in the pedigree file, and linkage disequilibrium was generated directly in the base population. Mixed density datasets were created by keeping 50,000 (every tenth) of the markers for most animals. Missing genotypes were imputed using a combination of population haplotyping and pedigree haplotyping. Reliabilities of genomic evaluations using linear and nonlinear methods were compared. Results Differing marker sets for a large population were combined with just a few hours of computation. About 95% of paternal alleles were determined correctly, and > 95% of missing genotypes were called correctly. Reliability of breeding values was already high (84.4%) with 50,000 simulated markers. The gain in reliability from increasing the number of markers to 500,000 was only 1.6%, but more than half of that gain resulted from genotyping just 1,406 young bulls at higher density. Linear genomic evaluations had reliabilities 1.5% lower than the nonlinear evaluations with 50,000 markers and 1.6% lower with 500,000 markers. Conclusions Methods to impute genotypes and compute genomic evaluations were affordable with many more markers. Reliabilities for individual animals can be modified to reflect success of imputation. Breeders can improve reliability at lower cost by combining marker densities to increase both the numbers of markers and animals included in genomic evaluation. Larger gains are expected from increasing the number of animals than the number of markers.
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            The effect of respiratory disease and a preventative antibiotic treatment on growth, survival, age at first calving, and milk production of dairy heifers.

            Bovine respiratory disease complex (BRD) is a common disease in weaned dairy calves that incurs economic and welfare costs. This study was an extension of a randomized clinical trial in which a single injection of tulathromycin (TUL) or oxytetracycline (TET) was administered at first movement to group housing for the prevention of BRD in the 60 d following antimicrobial treatment (BRD60). Calves treated with TUL were 0.5 times [95% confidence interval (CI): 0.4 to 0.7] as likely to be treated for BRD60 as calves treated with TET. The objectives of the current study were to evaluate the long-term effects of BRD and antibiotic treatment on growth of heifers until breeding age, age at first calving, incidence of dystocia, milk production, and mortality before first calving and mortality before 120 d in milk. At entry to the breeding barn (382 d of age), calves that experienced BRD60 weighed 16.0±2.3 kg less than calves that did not. Survival to first calving was recorded for 98% (1,343/1,392) of the heifers on this trial. For TET and TUL heifers with BRD60, 63% (94/150) and 73% (64/88) survived to first lactation, respectively. For TET and TUL calves without BRD60, 84% (436/517) and 84% (494/588) survived to first lactation, respectively. The median age at first calving for heifers with and without BRD60 was 714 (95% CI: 705-723) and 702 (95% CI: 699-705) days, respectively. Heifers with BRD60 were 1.5 (95% CI: 1.1-2.2) times more likely to have a calving ease score ≥2 at their first calving compared with heifers without BRD60. The administration of TUL at movement to group housing may have a role in the prevention of BRD and in mitigating some of the long-term effects of this disease.
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              What affects the costs of raising replacement dairy heifers: a multiple-component analysis.

              A DP model of a dairy replacement herd was developed to analyze the impact of different dairy and replacement herd variables on the cost of rearing replacements for a representative dairy herd of 100 cows. A model was developed with Pennsylvania and US average information as the basis for the parameters. We used age at first calving of 25 mo, calving interval of 13 mo, herd-culling rate of 25%, and preweaned calf death of 10% as the base for comparison. We examined the impact of factors including age at first calving, calving interval, PDR, and the number of replacements required. From the base model, the total cost of rearing sufficient replacements for a 100-cow herd was $32,344. A reduction in culling rate to 20%, holding all other factors fixed, caused the net costs of raising replacements for the dairy herd to fall by $7968 or 24.6%. Increasing the culling rate above 25% led to a deficit in replacements for maintaining constant herd size, assuming a closed herd. The average age at first calving also affected the net costs of raising replacement heifers; reducing the age at first calving by 1 mo lowered the cost of a replacement program by $1400 or 4.3%. Changes in the length of the calving interval or in the PDR had marginal impacts on the net costs of replacement programs when compared with either herd-culling rate or average age at first calving.
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                Author and article information

                Journal
                Journal of Dairy Science
                Journal of Dairy Science
                American Dairy Science Association
                00220302
                August 2017
                August 2017
                : 100
                : 8
                : 6853-6861
                Article
                10.3168/jds.2016-12060
                28624286
                1718de64-69e1-4f87-b5f6-d81872936f6c
                © 2017

                https://www.elsevier.com/tdm/userlicense/1.0/

                http://www.elsevier.com/open-access/userlicense/1.0/

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