MMN January 2025 – Should You be Using Genomic Testing?

10 January 2025

Genomics uses a young animal’s DNA to estimate their genetic potential and aims to provide dairy farmers with the information and the confidence to make informed breeding decisions with their youngstock. Genomic testing has been increasing in popularity over the last decade, with AHDB now estimating that over 100,000 dairy heifers are tested annually, a figure which is growing by around 15% per year.

How does genomic testing work?

The farmer’s part in genomic testing is relatively simple. A sample of the animal’s hair or tissue is collected and sent off to a laboratory via the farmer’s chosen genetics company. DNA is extracted from the sample and an electronic version of the DNA (known as a SNP-Chip) will then be sent off to geneticists at Edinburgh Genetic Evaluation Services (EGENES). EGENES develops this into a SNP-Key which translates the DNA sequences into a genomic index, expressed as a Predicted Transmitting Ability (PTA). PTAs predict the extent to which a given trait will be passed on to an animal’s offspring. They do not predict the offspring’s actual performance, which will vary widely depending on their environment and management.

Table 1 - Predicted Transmitting Ability categories and their associated traits.

Selection indexes

Selection indexes are used by farmers and semen companies as a quick and easy way to compare bulls. Selection indexes represent the additional monetary profit in pounds sterling that a bull is expected to transmit to his average daughter over her lifetime, compared with a bull with an index of zero.

£PLI (Profitable Lifetime Index) – this is a within breed genetic index, which means that different breeds are not comparable. The indexes are expressed as a financial value, indicating the additional profit which the daughter of the £PLI bull is expected to earn over her lifetime versus that of a daughter sired by a bull with a £PLI of zero.

£SCI (Spring Calving Index) – this is an across breed genetic ranking index, which means that different breeds can be compared. This index was developed to breed cows that produce a lower volume of milk but of a higher quality. The Spring Calving Index is also targeted at herds which are spring calving and are on grazing focused systems.

£ACI (Autumn Calving Index) – this index is also an across breed genetic ranking index. The index was developed to serve Autumn block calving herds, and the weighting of the different PTA traits are weighted to account for the costs of feeding for winter milk production and the higher milk price per litre received at the time of year.

Benefits of genomic testing

Genomics can be used to achieve many different aims, depending on what Indexes the business uses and what PTA’s the business is looking to prioritise. According to recent AHDB analysis completed for the 2023 calendar year, herds that test 75–100% of their heifers have an average PLI of £430. In comparison, herds that only test 0–25% of their heifers have an average PLI of just £237. This is a difference in PLI of £193, which can lead to a theoretical difference in profit for a typical 175- cow herd of £19,300. However, analysis of actual business accounts shows that testing 75-100% of your heifers can lead to a difference in profit of over £50,000.

In addition, genomics and the improved genetic gain which can result from this have the ability to meet several other goals of the dairy sector such as reducing carbon emissions and antibiotic use through targeting health, wealth and fitness traits.

Furthermore, management PTA traits such as temperament can be utilised to improve cow behaviour and improve safety for people working with the cattle.