Calving PTAs

These PTAs (predicted transmitting abilities) cover calving ease and related aspects, such as direct and maternal calving ease and gestation length. Find out what they are and how they are calculated.

Animals that calve easily and milk calmly help reduce labour time.

Calving Ease (CE) indexes are expressed on a scale of -3% to +3%, with a breed average of zero. Positive figures predict easier-than-average calvings and negative figures predict more difficult calvings.

There are two CE indexes that together give a complete picture of a bull’s ‘calving performance’:

• Direct Calving Ease (dCE%) – this predicts the ease with which a calf by that sire will be born
• Maternal Calving Ease (mCE%) – this predicts the ease with which a daughter of that sire will give birth

You should avoid negative dCE% when breeding maiden heifers, as these are predicted to give a higher proportion of difficult calvings. However, dCE% shouldn’t be ignored in older cow matings, where it’s wise to avoid bulls which are likely to produce very difficult calvings.

Long-term selection for dCE% without any regard to mCE% could set up problems for the future, so you should also consider maternal calving ease.

It is recommended to select primarily for Profitable Lifetime Index (£PLI) and use the Calving Ease indexes – as well as other fitness traits – as secondary criteria. Both dCE and mCE are incorporated into the £PLI, £SCI and £ACI.

The sex of a calf and lactation number affect gestation length so are used when calculating Gestation Length PTAs. The use of sexed semen does not affect the prediction ability of this model.

The heritability of gestation length is 43%, indicating that genetics plays a large role in reducing gestation length.

Gestation length is expressed in days and ranges from +7 days (longer) to -7 days (shorter). For every one point change in a bull’s Gestation Length PTA, a change of one day of gestation length for the bull’s calf is predicted. 

For example, a bull with a -5 gestation length is expected to have his calves born five days earlier compared with a 0 PTA bull.

Gestation Length is published as a stand-alone trait.

If you would like to order a hard copy of the InCalf for farmers farming all-year-round, or the Calf management guide, please contact publications@ahdb.org.uk or call 0247 799 0069.