PLI: what’s behind it and what lies ahead

Published 25 March 10

Developing a national breeding index for the UK dairy industry has been a cornerstone of the DairyCo breeding+ research strategy for several years. It has always been considered important that such an index would represent the economic benefits a dairy cow could bring to a business, while also addressing the need for better health, fitness and welfare standards to which both farmers and consumers aspire.

To this end, funding has been directed to both SAC and Edinburgh University, who have analysed millions of cow records* and assessed the relative importance of different traits under a variety of possible economic conditions.

The outcome of this work and the economic assumptions that were made were then considered by an industry group, on which breed societies, milk recording organisations, the AI industry, the milk processing industry and many practising dairy farmers were all represented. And the final result of all the discussions was the development of an index in which each trait is included in line with our best estimate of its ongoing economic importance. That index is the current Profitable Lifetime Index (PLI).

This PLI is a revision of previous formulae for the index, but, for the first time, represents the financial improvement an animal is, on average, predicted to pass on to its offspring over its lifetime, rather than for a single lactation.

As with the previous PLI, it still has strong emphasis on production - retaining 45 percent of its emphasis on the milk, fat and protein traits - but now includes a 55 percent emphasis on a variety of fitness traits. The particular traits included are those which have been proven to be most strongly associated with the cow's lifetime profitability, full details of which are given in the pie chart.

Since the launch of the current index in 2007, we at DairyCo breeding+ have been encouraged and reassured by how positively it has been received throughout the dairy industry. Many leading breeders and commercially successful dairy farmers have strongly endorsed the index and we have been able to vindicate their confidence with some encouraging results, including genetic improvement across the national herd in many of PLI's component traits.

Similarly, when we analyse the actual performance on the farm of animals with different indexes, we can clearly see that the genetic prediction is accurately pinpointing the best animals.

This has been seen in some recent research undertaken by DairyCo breeding+ which shows that daughters of bulls with high Lifespan indexes (positive scores) live considerably longer than daughters of bulls with low Lifespan indexes (negative scores). The same relationship between the genetic predictions and daughter performance has also proven to be the case when other components of PLI are looked at individually.

Naturally, we don't expect every single dairy farmer in the UK to religiously select their AI sires for PLI, although we would highly recommend that it was the first selection goal. We know there will be farmers with specific problems to correct, who may wish to use PLI as their first screening tool and then focus on a secondary trait which is important to their particular situation.

But we can confidently say that if the country as a whole followed the PLI breeding route, the national cow population as a whole would steadily begin to reduce the recent decline in dairy cow fertility and make improvements in each of the other traits in the index. And since genetic improvement is cumulative, significant improvements can be built up over the generations.

* Records analysed in the process of creating PLI are obtained from the milk records organisations (NMR, CIS and UDF), and the dairy cattle breed societies.

Key facts about PLI

  • The current formula was introduced in August 2007
  • 45% of the index is based on milk production (milk, fat and protein)
  • 55% of the index is based on health, welfare and fitness traits
  • £ value represents improved potential profitability over a daughter's lifetime
  • Top bulls have PLIs of over £200
  • Daughters of top bulls will earn around £200 more than average daughters
  • This is worth £30,000 for a 150 cow herd
  • The improvements build up over the generations

How to use PLI

Use the highest PLI bull you can without compromising on any of the other traits you wish to address. Try to select from the top 50 percent available, with PLIs of £70 or more.

How to check a bull's PLI

PLIs are always available on the DairyCo breeding+ website at www.dairyco.org.uk. Here, it is also possible to re-rank the top PLI lists to highlight the best bulls for individual traits such as production, fertility, lifespan or cell counts.