What’s new in the latest genetics bull proof run?

Five-yearly base change

Genetic indexes are expressed as a comparison to the breed average, which is set at zero. However, assuming the national herd makes genetic progress, the average creeps up over time. So, every five years, the national average is recalculated in the UK and reset to zero. This means all other genetic index figures change in their difference from breed average – although the order in the rankings remains the same.

This so-called base change has been carried out with the April 2020 index run, for which the average has been reset to reflect the average genetic merit of cows born in 2015. This has pulled down almost all genetic index figures for all dairy breeds, with the magnitude of the change detailed on our website. For the Holstein, the overall decline in £PLI is £142, meaning each bull’s £PLI declines by this amount purely due to the base change. All of this ensures genetic index figures don’t keep getting higher and higher, moving so far away from the original average it becomes hard to see where a bull stands in relation to the UK’s national herd.

Genomic evaluation system upgrade

When an animal’s genotype (its genetic make-up) is assessed it is done by tracking its DNA through a number of markers, or so-called SNPs. These genetic markers are then analysed by the AHDB and its partners at SRUC-EGENES who are able to interpret them as Predicted Transmitting Abilities (PTAs). The number of markers under scrutiny had, until this proof run, been 43,000. However, in April 2020, the density was upgraded to 80,000 markers which will now be routinely used. This enhanced marker density enables AHDB Dairy to track the genes involved in better performance more closely and to improve our genomic predictions. This fine-tuning has caused some minor reshuffling in the ranking of bulls.

Digital Dermatitis Index

Digital dermatitis has been collected as part of the NBDC’s dairy breed societies’ classification process for many years and has been used as part of the Lameness Advantage calculation since 2018. However, many producers are keen to know which bulls specifically transmit better resistance to digital dermatitis to their daughters, so from this proof run onwards, it will be made available to producers as a standalone index. The Digital Dermatitis Index (DD) is expressed on a scale of about -2% to +2%, with positive figures being desirable. Daughters of a bull with a +2% DD are expected to have 2% fewer cases of digital dermatitis than daughters of a bull whose DD is zero.