18 September 1998

LIFESPAN PREDICTION IS COMING OF AGE…

Lifespan is the latest trait

to be included in dairy

sire evaluations.

Emma Penny reports

TYPE traits and actual completed lactations are to be included in a new lifespan predicted transmitting ability (PTA) for dairy sires.

These PTAs will also be included in a new index which will shortly replace ITEM.

The interest in lifespan comes as reducing replacement rate becomes an increasing concern in dairy herds. Producers, faced with low cull values from the over 30-month scheme, are keen to increase cow longevity. That is where new information on lifespan PTA could help, says Gordon Swanson of the Animal Data Centre.

"When researchers at Edin-burgh University and SAC started to develop ITEM, they identified type traits – angularity, foot angle, udder depth and teat length – which were associated with animals surviving for longer in the dairy herd.

"But these type traits have only a moderate association with longevity. Because of that, their inclusion in ITEM means there is really only a small difference between PIN and ITEM for most animals.

"With only a small difference between the two figures, ITEM, which is a useful measure, has not really taken off. That led us to start looking for ways of improving accuracy of herd life prediction, which would give a bigger difference between PIN and ITEM," says Mr Swanson.

Assessment of an increased pool of linear type data, coupled with introduction of actual survival based on lactations completed by a cow produces the new lifespan PTA. This can be used by producers to assess whether a bull will increase daughter longevity.

The increased pool of linear type data from the Holstein Friesian Society has helped researchers at SAC and Edinburgh University reassess which traits are associated with longevity.

This re-analysis showed that fore-udder attachment, foot angle, udder depth and teat length had the greatest effect on herd life.

"Angularity – which was previously used in ITEM – was found to have no correlation, while fore-udder attachment had a strong relationship with lifespan," he adds.

The analysis was repeated by the ADC with linear trait data from Ayrshires, and the same four traits were found to be highly correlated with longevity, says Mr Swanson.

"We will repeat the analysis with linear traits from other breeds as soon as we have enough information. But we are fairly confident that the same four traits reflect longevity in all breeds, meaning we produce lifespan PTAs for all breeds."

Now all type classified animals will be awarded a longevity score, based on fore-udder attachment, foot angle, udder depth and teat length. This data is available during the first lactation, meaning a prediction of how long a cow will survive in the herd can be obtained early in her life.

The number of lactations a cow completes are analysed to calculate a lifespan score. Cows are awarded an incremental score the longer they remain in the herd, so cows with good longevity will score most; those lasting one lactation will score one, while those achieving five lactations and still living will be awarded a lifespan score of about eight.

Combining type trait information, the longevity score, lactation information and the lifespan score, means a lifespan PTA can be calculated, says Mr Swanson. This joint evaluation of scores improves accuracy of the final PTA.

The evaluation is carried out using Best Linear Unbiased Prediction scores, which, as well as including data from daughters, can also take information from relatives into account to help calculate the PTA of any individual. Using BLUP also removes any factors, such as management, which could unduly influence scores.

"Initially, as linear type information is obtained during the first lactation, lifespan PTA is based predominantly on this information. But increasing lactation information will substantially increase lifespan PTA accuracy," says Mr Swanson.

Minimum reliability for ADC publication is 30%, which means that about 18 daughters with linear type classification will be required before a bull can have a published lifespan PTA. "Higher reliability can only be obtained once completed lactation number is included. This means that current low reliability should be viewed with caution, they may change greatly as more information becomes available."

Bulls with best lifespan scores are likely to transmit an additional half lactation the their progeny, while the poorest ones might reduce herd life by half a lactation. In practice, this indicates the ability of daughters to withstand involuntary culling.

Mr Swanson advises treating lifespan as a secondary selection trait, with selection still being concentrated on ITEM and PIN. &#42

LIFESPAN PTA

Linear trait assessment.

Completed lactations.

Combined as lifespan PTA.

Currently low accuracy.