What impact does selection on Productive life have within a herd?

Since this is a low heritable trait, I doubt if I should select for productive life.

The heritability of PL (productive life) is 8%, which means it takes a lot of culling information before we get high reliable indexes. But, due to the fact that on farm (phenotypic) differences are very high, we are able to show considerable genetic differences between bulls for this trait. When we look at what indexes mean at a herd level, we should expect that 1 month difference in PL between 2 bulls will be the effect between their daughters at a herd level too.
In a within herd analysis, in one of Alta's Advantage herds, it was shown that the impact of a difference of 6.6 months of PL in the index of sires between two groups of animals (138 and 207 first lactation animals), resulted in cull rate at the end of the first lactation of 36% and 14% for the low versus high PL groups; a difference of 22%.
It shows that if you want to improve productive life of your cows, selecting for PL will have an effect.

has anyone done any work on the compound effect a herd would gain after using high PL bulls for several generations?

Hello Steven,

Productive Life is an additive trait so multiple generation selection, such as what you asked about, is actually what has been happening in the Holstein population, leading to genetic progress and making the breed better for PL over time.

To better illustrate this concept, and to demonstrate selection for PL ‘in action,’ check out the example below. This analysis was done using 8 years’ worth of data from 150 Advantage herds averaging 1000 cows. ‘High PL’ denotes cows sired by bulls in the top 10% for PL, and likewise, ‘Low PL’ denotes those sired by the bottom 10% for PL.

http://www.altagenetics.com/upload/files/What_does_PL_mean.pdf

The ‘High PL’ daughters are clearly remaining in the herd longer than the ‘Low PL.’ This difference is especially evident around the 2nd lactation mark when 20% more High PL cows remain in the herd.

The Canadian Dairy Network did a similar analysis with Canadian data looking at Herd Life; Canada’s longevity evaluation. More info can be found at the link below

http://www.altagenetics.com/upload/files/GeneticVariationforDFandHL-May_2006.pdf

Individual herds will function much the same where each generation will be better than the last with selection. However, with more selection intensity or a shorter generation interval, a herd could make more progress than the breed as a whole.

What is the actual formula for Productive Life. I can not seem to find one besides the generic meaning of the formula. Basically, I am looking for the actual formula that gets the PL number on the bulls proof. I would assume the formula is public? Thanks!

The Productive Life calculation combines a lot of information. It is direct culling info, which comes in as soon as animals get culled, and indirect information, which concern the predictor traits like udder score, production, SCS etc. As well environmental factors are looked at etc. It means there is not a simple formula through which we can make the calculation. The USDA calculates the indexes and the link to their website, which gives more info on calculating Pl is http://aipl.arsusda.gov/reference.htm
When you get to this site then look for longevity and click on it.