Calving ease EBVs

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Introduction

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.

Genetic factors in 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:

  • calf — weight and shape
  • cow — pelvic area and calving ‘will’.

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.

Information used by BREEDPLAN

  • Scores for the ease of each calving, that is, calving ease scores, are recorded by breeders. They are the most important information required to generate accurate calving ease EBVs. Calving ease scores are, in effect, measuring factors such as pelvic area and calving ‘will’.
  • Birthweight is directly genetically influenced by the sire’s and the dam’s ‘growth genes’. This is highly heritable (around 40%). It is called birthweight direct in genetic evaluation programs. As outlined above, it is a major influence on calving ease, so must be part of any calving ease prediction. Cows also influence birthweight through the maternal environment they provide in the uterus. This is called birthweight maternal. It is not as heritable as ‘birthweight direct’, but it does have an effect and is also used in the analysis.
  • Gestation length is another factor that is genetically influenced. Calves carried for shorter terms are generally born a little lighter and therefore more easily. Gestation length can of course only be easily measured in artificial insemination (AI) programs, but it does provide useful information when available.
  • Male and female pelvic areas can be measured. They are quite well correlated with each other and have some relationships to calving ease. The results obtained by incorporating these measures in genetic analyses have, however, been variable. They are therefore currently not used by BREEDPLAN, which instead prefers the calving ease scores.

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:

  • calving ease score
  • birthweight
  • gestation length

Recording calving ease scores

Seedstock breeders supply the following scores with their calf registration details:

Table 1. How seedstock breeds should record calving ease scores
ScoreDefinition
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)
4 Veterinary assistance
5 Malpresentation

Notes

  • If the breeder does not include a calving ease score, it is recorded as ‘Not observed’ and the information is not used in the analysis.
  • If, however, the calving herd is inspected regularly and a healthy live calf is seen, this should be recorded as ‘Unassisted’ and not ‘Not observed’.
  • Scores and weights for dead calves should be recorded if possible.

Developing the calving ease EBVs

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:

  • the direct effect that bulls have on the ease with which their calves are born, that is, calving ease direct or calving ease (DIR) ;
  • the calving ease of a bull’s daughters, that is, calving ease daughters or calving ease (DTRS) .

Interpreting calving ease EBVs

Calving ease direct (DIR)

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 +5.0% would be expected, on average, to produce 3% fewer ‘difficult’ calvings from 2-year-old heifers than would a bull with an EBV of –1.0% (6% difference between the sires, then halved because they contribute half the genetics).

Calving ease daughters (DTRS)

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 +4.0% would be expected, on average, to produce 2-year-old daughters that have 3% fewer calving problems than the daughters of a bull with an EBV of –2.0%.

Notes

  • Because of all the environmental influences on calving, the calving ease scores have a relatively low heritability. In turn, the calving ease EBVs have quite a low accuracy until large amounts of data are collected. They are usually only reported on sires.
  • Like all traits, the magnitude of the differences expressed from matings of stock with different EBVs will depend on the environment.

Bull buying exercise

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.

  • Buyer 1 seeks a terminal sire to join with crossbred heifers for yearling production. Calving ease is of moderate importance.
  • Buyer 2 is straightbreeding for yearling production and wishes to improve the calving ease of the females.
  • Buyer 3 is straightbreeding and seeks a sire to join with heifers. Calving ease is of considerable concern to this breeder of grass-finished yearlings.
Table 2. Calving ease EBVs
Sire Birthweight EBVs 400-day weight EBVs Calving ease EBVs
kgAcc.kgAcc.DirectDaughters
A +0.2 65% +6 60% +10 35% –6 30%
B +0.5 79% +25 75% +9 67% –9 51%
C +1.3 83% +21 80% +1 58% +5 60%
D +0.7 95% +18 93% +8 85% 0 75%
Breed average +1   +16   0   0  

Answers

Table 3. Answers to bull buying exercise
BuyerBull
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.

Further reading

Graser, H & Goddard, M Understanding Calving Ease , AGBU Tech Note 1/95, available on the Animal Genetics and Breeding Unit website.

BREEDPLAN EBVs are calculated using software developed by the Animal Genetics and Breeding Unit (AGBU), a joint Institute of NSW Department of Primary Industries and the University of New England. AGBU receives a majority of its BREEDPLAN research and development funding from Meat & Livestock Australia (MLA).