THE ROLE OF CLASSIFYING IN THE GENOMIC ERA
In the last few years dairy cattle breeding has been heavily focused on the potential of predicting animal performance using DNA
technology. In contrast to genomics, type classification has taken place in the UK for more than 50 years. Darren Todd asks what role classification plays in the modern world and in particular what can it tell us about the key breeding goal of lifetime yield?
Text by Darren Todd, Geneticist, Holstein-UK Breed R&D Department Data Analysis and charts by Henry Richardson, Centre for Dairy Information
So what does classifying tell us about dairy cows, in comparison with information from genetic values for type merit? Classifying describes the physical cow, sometimes referred to as the phenotype. This phenotype is a reflection of both the cow’s DNA and the environment in which the she has grown up. Genetic or breeding values only reflect the cows DNA makeup.
When a heifer only has pedigree information available, typically about 35% of her genetic merit for type is known. This is referred to as reliability. As type traits are quite heritable, the phenotype of a heifer is a reasonable reflection of her genetic merit. Classification, therefore, adds about another 10 percentage points of reliability on top of this.
Of course, genomic testing can provide even more insight, with reliabilities close to 70% achievable for young animals. We should remember that genomic calculations rely on a huge bank of classification data to enable the DNA code to be translated into type merit values. Moreover, as the Holstein population evolves, type data used in genomic calculations needs to be updated with fresh classification results, to maintain the high level of reliability of genomic tests.
Consider also that while a heifer may have a good DNA profile for type traits, she may have encountered disease, sub-optimal nutrition
Figure 1 Average lifetime yield per overall classification category, of Holstein heifers classified between 2000 and 2002 inclusive. Data analysis by CDI
or injury during her life on the farm. When these factors have adversely affected her physical development, classifying will also enable this to be quantified and recorded.
Classifying, therefore, gives both information about a heifer’s functionality and her genetic merit for type traits. Importantly classification data underpins genomic type calculations.
Classifying and lifetime yield Holstein UK classifiers score tens of thousands of heifers a year, providing a large dataset with which to examine the relationship with lifetime yield (LTY). The following series of figures, produced from data collated by CDI, display the LTY (until September 2013) per heifer classification category, of Holstein heifers classified over a three year period, from 2000 and 2002. Figure 1 shows the LTY of heifers per overall classification category. Those in the higher classification categories went on to produce considerably more milk than heifers with lower scores. For example, heifers scored VG overall typically produced 15,000kg (e.g. £ 3600 worth of milk @ 24 ppl) more in their lifetime than those scored fair. Not surprisingly, this trend is also found for the two major components of overall classification, mammary composite score and feet and legs composite.
The classification of individual traits uses a one to nine scale, where extreme scores are not necessarily considered as ideal, depending on the trait. When plotted against LTY, some traits display intermediate scores as
60 THE JOURNAL AUGUST 2015
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