Genomic evaluation is the process of estimating the genetic merit of an animal based on the animal’s DNA information. Genomic evaluation can be carried out for a number of dairy breeds including: Holstein, Friesian, Guernsey, Ayrshire and Jersey.
The first step in genomic evaluation is to build up a reference population for a breed. A reference population contains the DNA information of sires with a high reliability daughter-based proof. Any animal with a high reliability proof can be used, whether they have poor or excellent genetics. This wide spectrum of genetics allows geneticists to identify markers (SNPs) in the animal’s DNA that are related to poor or favourable traits shown in the animal’s genetic proof. These associations are then used to create a so-called ‘SNP-key’, which can then be used to evaluate young animals without daughter or production information based only on the DNA present in a tissue or hair sample.
Reliability of genomic proofs
Genomic proofs are produced by the Agricultural and Horticultural Development Board (AHDB) and are identical in layout to production and daughter-proven proofs, but are flagged a green ‘G’ symbol to indicate that the proof uses DNA in addition to parent average information to estimate the animal’s genetic potential. Genomic proofs have a higher reliability than traditional proof testing pedigree indexes, as illustrated in Table 1. Genomic tested young bulls retain this gain in reliability over traditionally tested bulls, up until the second crop daughter information is available.
Table 1. Reliability of genomic and traditional testing sire genetic proofs over time
|Sire testing stage||Age of sire||Traditional||Genomic|
|Second crop daughter proof||6||99%||99%|
It is only when second crop daughter production information is available that the gap between traditionally proven sire proofs and genomic proofs is fully closed. Although genomic young bulls have a higher reliability than their traditional counterparts, caution should be taken when using them across a herd. The reliability indicates how likely the bull’s proof will change as more information contributes towards his proof. Therefore, a genomic proof with reliability of 65–70 per cent could still drop or improve when daughter information becomes available. Using a team of around five or six genomic young bulls across your herd is advised to ensure that if one bull changes significantly, his genetics will not contribute to a large proportion of your replacement heifers.
Genomic evaluation within the CAFRE dairy herd
All female Holstein cattle have been tested within the CAFRE dairy herd. Heifer calves are tested once all the heifer calves have been born usually by around February each year. The herd genomic data for individual replacement heifers and cows is used to decide which cows and heifers are used for breeding replacements, or if there is a surplus of heifers, which animals to sell. In addition to the genomic data, consideration is given to timing of calving and thus breeding as a key objective is to have the replacement heifers born early in the calving season to have a uniform batch of heifer calves to rear as a batch.