Since its inception, BREEDPLAN has provided birthweight EBVs for those breeds where there is interest in improving calving performance. Recognising that there are aspects other than birthweight involved, some breeds now also provide calving ease EBVs . This Agnote explains these EBVs, describes the information needed to generate them, and briefly explores the genetics of calving ease.
Calving ease is affected by many environmental factors and several genetic ones. It is of course the genetic factors which BREEDPLAN can assist to improve. They include:
Many large studies have consistently shown birthweight to be by far the most important, though calf shape and the maternal factors also need to be considered for best genetic improvement.
It should be noted that overemphasis on any form of selection can have unwanted consequences. Calving ease factors are no exception. For example, selection on light birthweight alone will lead to lower growth rates later in life and increased chances of female progeny that have poorer calving ability. Similarly, selection on large pelvic area alone will increase birthweight and reduce the beneficial effects on calving ease. It is therefore important to understand the correlations involved, many of which are included in the EBV calculations.
The BREEDPLAN calving ease EBVs are therefore currently calculated from three sets of records , which can be reliably taken and which exert most influence on calving ease:
Seedstock breeders supply the following scores with their calf registration details:
|1||Unassisted (either through observing the calving or noting cows with calves after no apparent difficulty)|
|2||Easy pull (one person without mechanical assistance), or calf unassisted but cow and/or calf show signs of difficult birth|
|3||Hard pull (two people, or one person with mechanical assistance)|
Like all BREEDPLAN records, comparisons are initially only made from calving ease scores in like-treated groups. They are then combined with available birthweight and gestation length information, and the pedigree links and correlations are also used in the calculation. The EBVs developed from this information allow predictions for:
Calving ease (DIR) EBVs are estimates of genetic differences in the ability of bulls’ calves from 2-year-old heifers to be born unassisted. The EBVs are reported as differences in the percentage of unassisted calvings. Higher, more positive, calving ease (DIR) EBVs are more favourable ; for example, a bull with an EBV of
Calving ease (DTRS) EBVs are estimates of genetic differences between animals in the ability of their 2-year-old daughters to calve without assistance. The EBVs are also reported as differences in the percentage of unassisted calvings. Higher, more positive calving ease (DTRS) EBVs are more favourable ; for example, a bull with an EBV of
The following exercise shows the format in which calving ease EBVs are presented, and demonstrates some principles of their use. People interested in intensive selection for calving ease would be advised to consider using an index calculated by BreedObject (see BreedObject - software to balance EBVs (Agnote DAI-149)).
Exercise: Advise each of the following three bull buyers on their selection of a bull from bulls A to D listed in the table below.
|Birthweight EBVs||400-day weight EBVs||Calving ease EBVs|
|Buyer 1:|| Bull B. Positive calving ease direct EBV with moderate accuracy and with the highest 400-day weight EBV.|
Note: Bull A has a similar calving ease EBV, but it has a lower accuracy. Lower 400-day weight EBV.
|Buyer 2:||Bull C. Positive calving ease daughters EBV, with acceptable 400-day weight EBV.|
|Buyer 3:||Bull D. Positive calving ease direct EBV, with the highest accuracy, as calving ease is critical. For the retained females, calving ease daughters EBV is average.|
Graser, H & Goddard, M Understanding Calving Ease , AGBU Tech Note 1/95, available on the Animal Genetics and Breeding Unit website.