April 2020 genetic type changes; Base change and Type Merit Formula update

The April 2020 proof run will see the introduction of a new genetic base for Holstein and Friesian type traits, as well as an update to the overall Holstein Type Merit Formula. 

Resetting breed average PTA

Every five years, the average for type trait PTA is updated to account for genetic change in the population. The 2020 Holstein base is calculated using the genetic merit of 100% pure Holstein bulls born between 2004 and 2013. Using stature as an example, bulls born between 2004 and 2013 have an average standardised PTA 0.39 points higher than the previous base (bulls born between 1999 to 2008). The base change for stature PTA will therefore be -0.39. The full breakdown of changes in each Holstein type trait are shown in Table 1.

 

Table 1: Change in Holstein average type trait values between the 2015 and 2020 bases

Trait	Base change
Mammary	-0.78
Legs and feet	-0.45
Stature	-0.39
Chest Width	-0.06
Body depth	-0.02
Angularity	-0.14
Rump angle	-0.02
Rump width	-0.32
Rear leg side	0.05
Foot angle	-0.14
For udder attachment	-0.65
Rear udder height	-0.66
Udder support	-0.44
Udder depth	-0.63
Front teat placement	-0.36
Teat length	0.09
Rear teat placement	-0.30
Teat position side	-0.02
Temperament	-0.26
Ease of milking	-0.32
Locomotion	-0.50
Condition score	-0.03

 

Although the changes are mostly negative, this is a good thing (in many traits) as it indicates genetic improvement since the last Base change. Overall mammary for example shows the highest improvement of 0.78 PTA points and noticeable gains have also taken place in locomotion and legs & feet. In traits where an optimum is desirable, such as rump angle and rear leg side, as expected little change has taken place. Genetic teat length has shown a minor decrease of 0.09 points, which although non desirable is not of a large enough scale to be of concern. Breeders should however still pay careful attention to individual bull PTA for this trait if they do not wish to shorten teats in their herd. The increase in genetic stature is therefore the one area of concern. 

Friesians are initially evaluated together with Holsteins, and then scaled according to their own breed base. The average change in Friesian type traits will be -0.59.

Alongside the change in the average of the population for each trait, the standard deviations (which scale the PTA to the approximately -3 to +3 range that is published) will also change in since the last base change. This will also affect the new PTA published in April.

Interpreting PTA

It is worth considering that a PTA score of 0 for a given trait is not necessarily a bad thing – that simply means the animal has breed average genetic merit for that trait. A bull with a PTA of 0 will not weaken that trait in the average UK herd. 

Those breeders that have genetic type trait information available for their females (e.g. from genomic results) can also take these into consideration when choosing sires. For example, a female with a chest width PTA of -1 can be mated to a sire with a PTA of 0, with the resulting calf expected to be better than the dam. So, in this case even a 0 PTA bull will be an improver, if more chest width is desired.

That said, the biggest improvements will be achieved by using sires with positive PTA of 2 points or more, so breeders looking for an improvement in a particular trait should focus on those bulls.  

Overall Type Merit update

After reviewing the latest analysis on the effect of selection for type traits on lifespan, from April 2020 the overall Holstein Type Merit formula will be adjusted to increase the weighting for overall legs & feet and implement a negative weighting on stature. The weightings of the other traits in the formula will remain unchanged. Taking into account these changes, in conjunction with the Base change, the average Holstein Type Merit value will decrease by -0.55 in April 2020 as compared to December 2019. The Friesian Type Merit Formula remains unchanged and the average drop in Friesian TM values will be -0.59, as a result of the base change.

For full details of the latest type trait PTA please visit the animal data section of the Holstein UK website. In the member services section of the website you will also find an easy way of searching, short listing and creating your own bull list with the ‘BullSelector’ tool.