Genetic indexes for selection

Our three economic breeding indexes – £PLI, £SCI and £ACI – help you make quick decisions about choosing bulls most suited to your calving system.

Our selection indexes – £PLI (Profitable Lifetime Index), £SCI (Spring Calving Index) and £ACI (Autumn Calving Index) – can save you time analysing the many individual traits that are available to dairy farmers.

The indexes allow you to easily rank animals that transmit the most profitable combination of genetic traits to suit your farming system.

They show the additional profit a bull is expected to pass on to his daughters compared with a bull with an index of zero in a year-round, spring block- and autumn block-calving system respectively.

These indexes are reviewed every year by a forum that includes farmers, breed societies, vets and other breeding industry stakeholders. The forum fine-tunes the indexes as the need arises, looking at the genetic progress dairy breeds are making and the long-term market outlook for inputs and output.

The Profitable Lifetime Index (£PLI) is a within-breed genetic ranking index, meaning different breeds are not directly comparable, expressed as a financial value, and indicates the additional profit that a daughter of a high £PLI bull is expected to earn over her lifetime, compared with a daughter sired by an average bull with a £PLI of zero.

The Spring Calving Index (£SCI) is an across-breed genetic ranking index developed to breed cows that produce lower volumes of milk of a higher quality. It’s specifically for spring block-calving herds, which rely heavily on grazed grass and calve in a tight (12 weeks or less) block.

The Autumn Calving Index (£ACI) is suitable for autumn block-calving herds and reflects the costs of feeding for winter milk production and the higher milk price per litre received at that time of year.

The systems that the three different rankings are designed for are:

• £PLI – Profitable Lifetime Index for all-year-round calving herds
• £ACI – Autumn Calving Index for autumn block-calving herds
• £SCI – Spring Calving Index for spring block-calving herds

The £PLI should be used to compare bulls within a breed, but both the £ACI and £SCI can be used to compare genetic qualities of animals from different breeds directly with each other.

Since each of the three indexes is designed for different systems, each has a different relative performance to suit the different needs. The graph below illustrates how the £PLI, £ACI and £SCI emphasise different traits.

*Trait reversed for presentation purposes

As the national herd makes genetic progress, the average trait for each animal also goes up. So, every year, the national average for every trait is recalculated and reset to zero. This ensures the base animals used are as current as possible, making bull selection more meaningful, which, in turn, will make genetic progress in your herd easier.

If we didn’t recalculate the average (or change the base), almost every animal would eventually be better than the ‘average’ determined many years before.

The important point to remember is to compare like with like and only compare PTAs (predicted transmitting abilities) from the same proof run. Be particularly cautious of published bull catalogues, which are generally out of date within four months of publication.