As the weight and growth rate of beef cattle increases Calving Ease becomes more and more important for the well being of dams and calves and for the subsequent production ability of the cow. Cows who experience more problems at calving will more likely lose the calf, produce less milk and get bred back later. Together with the increased cost for labor to assist a cow these effects of calving problems all contribute to the economic importance of Calving Ease.

Birth weight has long been used as an indicator for Calving Ease. But every observant beef producer knows at the same weight there is still a lot of variation in Calving Ease. Therefore the American Hereford Association (AHA) has encouraged it’s performance recording members to record Calving Ease on a 1 to 5 scale.

Table 1: Calving Ease Score

Note that it is active recording, a blank in the column will not be interpreted as unassisted. Easy pull is defined as one person without mechanical help, while a hard pull requires two persons or one with mechanical help.

Scientists at the Animal Genetics and Breeding Unit (AGBU) of the University of New England in Armidale, New South Wales, Australia have developed analytical software which combines in one analyses the information collected on Calving Ease, Birth Weight and Gestation Length (from AI) to estimate genetic merit for Calving Ease.

What affects Calving Ease?

Calving Ease is affected by:
a) the calf; through sex, size, weight and shape and
b) the cow; through pelvic size, age, parity, fatness and hormonal levels.

Both cow and calf "performance" is influenced by genes but the performance is lowly heritable The goal of the genetic analysis is to separate these different effects using the pedigree of cow and calf. We want to identify sires, which produce calves that are born easy and sires, which have daughters with no problems giving birth to their first calves.

Calving ease is, as described above, a trait recorded in few categories or classes and the majority of observations are scored into the first class "Unassisted" (Figure 1), even so there are considerable differences between calvings which are recorded as unassisted. Due to the inability of our observation and recording systems we can't distinguish between cows that have two hour labor or five hour labor.

Figure 1: Observed and underlying distribution of calving difficulty, a threshold model

 

To calculate EPDs for calving ease we need calving problems. Breeders which are quicker with assistance are an advantage for the evaluation system as it makes it easier to differentiate between animals. Groups of cows with all calves being born unassisted are of no value for a genetic evaluation of Calving Ease if this is the only information available. However birth weight and gestation length do influence Calving Ease. We know that if either of them increases the underlying calving problem will increase too (Table 2). It will be more difficult for a cow to give birth to a calf weighing 95 than to one weighing 85 even though, both might be born unassisted.

To estimate the breeding values a threshold model is used that translates the records from the observed scale 1-3 to a normal distribution on an underlying scale (Fig 1). Scores 3 and 4 are currently combined and abnormal presentations are not included.

The assumption is made that the sum of the different effects on calving ease (genetic and non genetic) is normally distributed on this underlying scale. When the sum becomes larger than threshold T1 or T2 we observe Calving Ease as an easy pull or hard pull, respectively. A population has overall thresholds or average thresholds. The effect of management (breeders readiness to assist), age of cow and sex of calf all have a major influence on the frequency of calving difficulty and therefore modify the values of the threshold s for a particular group of animals. In young cows calving problems are more frequent and the thresholds in Figure 1 move to the left. Female calves are born on average easier than male calves, their thresholds are further to the right. Analyzing data from three breeds found that the differences between the sexes and the cow age groups are very consistent across the three breeds (Table 2) although the overall calving difficulty differ between the breeds. Very similar observations are made for birth weight and gestation length. Male calves have a longer gestation and are heavier at birth (Table 2).

Table 2: Sex and age of cow effects on Calving Ease, Birth Weight and Gestation Length

		Calving Ease SD	Birth Weight kg	Gestation Length days
Phenotypic SD		1	4.6	5.2
Sex:	F	+0.28	-1.1	-1
	M	-0.28	+1.1	+1
Dam age:	2	-0.70	-2.8	-2
	3	+0.11	-1.6	-1
	5	+0.67	0	0

From the normal distribution in Figure 2 we can see that the difference eg. between male and female calves of 0.56 SD will result in a bigger difference in calving problems in two year old cows between the two sexes compared with the same difference in mature cows. The mature cow threshold for unassisted birth is about 1.4 SD to the right of the threshold of two year old cows. Any deviation from the two thresholds eg. by difference in sex, will have a far greater effect on the left threshold for two year old cows as is demonstrated in Fig 2 by the area, under the curve.

EPDs on underlying scale

Direct and maternal Estimated Progeny Differences (EPDs) are also calculated on the underlying scale in units of standard deviations. Calving Ease is a trait with a low heritability and the variation of these EPDs is considerably smaller than eg. the differences between the sexes or the age effects. As sires in natural matings tend to not have too many progeny their EPDs will be further regressed to the mean. The extreme EPDs on the underlying scale will therefore only vary between +0.4 and -0.4.

Figure 2: Female calf effect in young (Y) and old (O) cows for Calving Ease

 

EPDs on the observed scale

The EPDs on the underlying scale do not mean much to most producers and breeders. Therefore the EPDs are expressed on an observed scale as plus or minus % additional animals born unassisted. The threshold used for the transformation is the one for two year old cows, which gives the largest differences between sires.

One EPD is for the direct effect, the second one is for the maternal effect. The direct effect is important for a sire of calves, as it indicates how many more or less % calves from this bull are expected to require assistance at birth out of two year old cows. A sire with an EPD of +8% for Calving Ease is therefore expected to have 8% more calves being born unassisted out of two year old cows than a sire with an EPD of 0%.

The second EPD called Calving Ease – Maternal or Daughter's Calving Ease is an indicator for the daughters of a sire to give birth unassisted as a two year old. It consists of the maternal component and half the direct component. Again it is an EPD. A difference of 8% between two bulls means we can expect that 8% more daughters of the sire with the higher EPD will calve unassisted at two years of age.

These EPDs are based on a breed average threshold for two year olds. This threshold can vary from year to year and from herd to herd and with it the expected difference between progeny performance either as calf or as daughter. Regardless of all that the progeny of a bull with the higher EPDs are always expected to have less problems being born or giving birth.

Starting with the Fall 2002 EPD analysis, the AHA & CHA will begin reporting these two new Calving Ease EPDs. Remember, CE EPDs take into consideration the animal’s birth weight information so that essentially means that you can ignore the BW EPD and focus on CE EPDs to make genetic progress. In fact, selecting on both CE and BW EPDs is strongly discouraged since doing so will have the effect of maximizing selection pressure on BW due to the fact that it is a component of the CE EPD  In the future, it is possible that we may cease to continue reporting of BW EPDs in favor of CE EPDs since they are a more direct measure of the trait of concern. That does not imply that birth weights are not important and should not be continue to be collected, it simply means that reporting both BW and CE EPDs makes it possible for breeders to select on both traits and in doing so, they will actually over emphasize selection on birth weight since it is a component of the CE EPDs.

You will find that some high BW animals are high for CE-Direct, CE-Maternal or both which means that animal tends to have larger calves at birth but still experiences little or no problem in calving. Some sires may have an elevated BW EPD, high CE-Direct and low CE-Maternal EPDs. That means those sires direct calves are born unassisted more often but there could be potential problems at calving for daughters of that sire. Others may be just the opposite. It is an interesting study and these new traits have as much economic impact on the commercial cattleman as any other identified to date.

These new calving Ease EPDs will were first available via the AHA's internet search tools with the Fall 2002 EPDs.  The Calving Ease EPDs have been incorporated in all printed reports, including the printed sire summaries, herd reports, etc.  

Remember, selecting on both CE and BW EPDs is strongly discouraged since doing so will have the effect of maximizing selection pressure on BW due to the fact that it is a component of the CE EPDs.  This may over emphasize the effect of BW in a breeders selection decisions