There are many, many factors that can contribute to a cow experiencing a slow or difficult calving, also known as dystocia. On both beef and dairy farms, any assistance at birth is undesirable and a difficult calving often reduces calf viability, causes maternal injury or requires assistance.
This post is part 1 of 2, for part 2 click here.
Some of the proximal causes of dystocia include:
feto-maternal disproportion (calf too big for the pelvis)
abnormal fetal position (e.g. breech birth)
uterine inertia (uterine muscle's not able to contract and expel the calf).
vulval or cervical stenosis (failure of the vulva or cervix to soften and stretch enough to allow the calf to pass through)
There are 3 layers of causes of dystocia - proximal, intermediate and ultimate.
The ultimate causes of dystocia result in intermediate causes which in turn result in proximal causes.
An example below: feto-maternal disproportion (proximal) is a result of increased calf birthweight (intermediate) which can be a result of increased gestation length (ultimate).
Intermediate and ultimate causes include; gestation length, birthweight, birth canal undersize, metabolic disorders at calving, calf gender, number of fetuses, breed of the sire and dam, parity (number of calvings), prior history of dystocia, or nutrition.
This post is going to focus on feto-maternal disproportion as related to gestation length, birth weight and dam size. These can be further broken down into genetics (Part 1 - this post) and nutrition (Part 2 - found here).
Let us know if you would like a summary on any of the other causes listed above and we will add it to our list of future topics 👍
What is feto-maternal disproportion?
Feto-maternal disproportion is a form of dystocia that is a result of the calf being too big in relation to the size of the cow's pelvis.
This can mean that the calf is too heavy/big for the size of the cow or that the cow is too small for the size of the calf (i.e. undergrown or a younger cow).
Ways to reduce feto-maternal disproportion would include decreasing the size/weight of the calf and/or increasing the size of the cow. In other words, a smaller key and a bigger hole! Interestingly, heavy heifers tend to produce heavy calves, which counteracts the expected reduction in dystocia from the cow having a larger pelvis. This then suggests that a focus on reducing birthweight may have greater benefits to reducing dystocia than increasing cow size.
Feto-maternal disproportion is the main type of dystocia in first-calving heifers, although it also occurs in cows that are older.
The chance of a cow experiencing a difficult calving has been shown to increase as the birthweight of the calf increases (e.g. Holsteins in the USA, Australian Angus heifers)
However, it is probably more important to consider the birthweight of the calf in relation to the size of the dam rather than birthweight alone (birthweight as % of cow weight).
For a given birthweight of a calf, small cows are more likely to experience dystocia than large cows, and for a given size of cow, a large calf is more likely to result in dystocia than a small calf.
Genetics of Calving Difficulty
We have had a good read of the studies around calving difficulty and genetics, and a large amount were focused on beef cattle rather than dairy cattle. We have found a few really good studies on dairy cattle, unfortunately, not NZ based ones.
Birthweight and the length of gestation are determined by the genetics (genotype) of the calf (direct genetic effect), genotype of the dam (maternal genetic effects) and environmental effects. Although the majority of all the genetic effects are determined by the the genetics of the calf.
The maternal effects contribute to the likelihood of a cow experiencing dystocia when giving birth. The maternal ability of the cow to nurture the fetus influences the calf's birthweight, and the dam’s genetic potential for her own growth influences the size of her pelvic area.
For both beef and dairy cattle, the relationship between the direct and maternal genetic effects is a negative one. This means that if bulls are selected for reduced calving difficulty then it is likely that their daughters will experience calving difficulty when they calve.
So if they have the genetics to not experience a difficult calving when they are born they may have the genetics that increase their chance of having a difficult calving when they calve themselves. As we mentioned earlier, we couldn't find NZ data on dairy cattle for this section so it may not be the same for NZ-bred dairy cattle.
The use of a selection index that considered both direct and maternal genetic effects is recommended by most studies on calving difficulty in beef and dairy cattle. More on selection and breeding values is further on in this post.
Heritability of Calving Difficulty
Calving difficulty as a trait of the calf has been reported as having a low direct heritability of 0.03 to 0.43. For the NZ genetic evaluation of calving difficulty the heritability of calving difficulty in calves born to heifers (first-calver) is 0.045 and in calves born to cows (2nd and later calver) is 0.03.
Heritability of Birth Weight and Gestation Length
The heritability of both birth weight and gestation length were similar at around 0.4–0.5 indicating that the calf's genetics had a large influence on birth weight and gestation length.
Birthweight increased with length of gestation by 0.15–0.38 kg/day for different breed groups, and as a result, calves with longer gestation periods tended to have an increased incidence of dystocia. Interestingly, the trend of longer gestation periods being related to increased dystocia also existed when birthweight was constant.
There was a close genetic correlation between birthweight and dystocia score, ranging from 0.90 to 0.98. This high genetic correlation means that selection for decreased dystocia score would be associated with decreased birthweight in the selected animals.
Heritability of Mature Weight
Weight of dairy cattle has been reported to be a moderately heritable trait; between 0.16 and 0.61. Overseas studies have reported positive genetic relationships between calf birth weight and mature cow weight, so selection on small mature weight would likely result in a decrease in birth weights as well.
What does all this genetics mean?
A heritability of 0 (or 0%) indicates that all of the variation in calving difficulty was caused by environmental factors (e.g. nutrition, the weather etc.), whereas, a heritability of 1 (or 100%) indicates that all of the variation in calving difficulty was due to genetics.
In the case of calving difficulty here a heritability of 0.045 or 4.5% means that the majority of calving difficulty is caused by environmental factors and a small amount due to genetics.
For birth weight and gestation length the moderate heritabilities of 40 - 50% mean that differences in birth weight or length of gestation were fairly evenly split between genetics and the environment.
Estimated Breeding Values (EBV)
Beef
For beef bulls, the EBVs provided (from BREEDPLAN) are for "calving ease direct" and is expressed as "the influence of the sire on calving ease in purebred females calving at two years of age."
A higher calving ease direct EBV is the best for beef bulls as it indicates an easier calving.
As we mentioned before, the use of a selection index that considered both direct and maternal genetic effects is recommended in beef and dairy cattle.
For beef bulls, this is provided for by the calving ease direct and daughter EBVs and bulls should be selected that have high EBVs for both of these traits (when calves are kept as replacements). If the calf is being bred purely for beef (i.e. they won't have a calf themselves) then selection only needs to be made using the calving ease direct EBV and not the calving ease daughter EBV.
BREEDPLAN provides EBVs of sires and percentile bands for each breed registered. It is important to note that EBVs cannot be compared between breeds, for example, an EBV of 10 for a Hereford is different to an EBV of 10 for an Angus or a Charolais.
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Dairy
In the NZ Dairy Industry "calving difficulty" is the measure of the genetic component of dystocia. The estimated breeding value (EBV) is expressed as the percentage of assisted calvings expected when a bull is mated to 15-month-old heifers.
For example: A bull that has a negative calving difficulty EBV is expected to cause less calving difficulties in first calving heifers than the average bull that has an EBV of 0%.
This means that when selecting bulls to use, a lower calving difficulty EBV is the best for dairy bulls.
For dairy, EBVs are calculated for all bulls that have data in the national database (DIGAD at DairyNZ). This includes beef, dairy and crossbred bulls. However, only bulls that are enrolled with NZAEL will have their EBVs available on the DairyNZ website.
Different to beef, EBVs generated by the dairy industry are comparable across all breeds, for example, an EBV of 10 for a Jersey is the same as an EBV of 10 for an Angus or a Holstein-Friesian.
Selecting for lower calving difficulty
For both beef and dairy cattle, the incidence of dystocia can be genetically changed by selecting for animals with favourable genotypes (i.e. EBVs).
This can be achieved using EBVs for low birthweight, low calving difficulty (dairy) or high calving ease (beef), and shorter gestation length. By selecting bulls in this way we will identify sires that will generate calves that are born easily each year, whether it be dairy bulls in dairy cows, beef bulls in beef cows or beef bulls in dairy cows.
Why do we see calving difficulty when using bulls with low calving difficulty EBVs?
As we mentioned earlier there are genetic AND environmental impacts on calving difficulty. This means that selecting for 'easy-calving' bulls does not guarantee less dystocia at birth... it just helps to reduce the chance of dystocia occurring (minimises the genetic component).
EBVs can be used to identify bulls that are more likely to result in easy-calving, however, there is a difference between the estimated breeding value (EBV) and the true BV. We can never know the true BV, only an estimate of it, even with high accuracy/reliability of a bull's estimated BV there is always a chance that it will change as more records are added.
The graph below shows a bull with an estimated BV of -2% and a high reliability (i.e. lots of daughters). The blue line shows the distribution of the estimated BVs with the peak being the estimated BV with the highest probability of being the true BV.
If we take a bull with the same calving difficulty estimated BV of -2% but a moderately high reliability (not as many daughters as bull 1 above) we can see that the peak is still -2%, but the spread of potential estimated BVs is wider for bull 2 (purple) than for bull 1 (blue) because we have less data available for bull 2 so it is more likely to change as more data is added.
Both bulls have the same EBV for calving difficulty but because of the higher accuracy for bull 1 (blue) it is very unlikely to change as more data is added.
Another way to look at it is for bull 1 our best estimate of BV is -2% and we are 95% sure that the true BV is between -3.8% and -0.8% (range of 2.4 percentage units). For bull 2, our best estimate is also -2% however, we are 95% sure that the true BV is between -4.4% and +0.4% (range of 4.8 percentage units).
So choosing high reliability bulls may be a safer option when selecting against calving difficulty. But no guarantees!
What can you do to help?
Record the difficulty of ALL CALVINGS in your herd recording software.
This includes bobby calves, calves reared as replacements, calves reared as beef, calves that were stillborn etc.
We are only familiar with the recording system from LIC (MINDA) which gives you the following 3 options for recording calving difficulty:
Reported no assistance
Minor assistance given
Major assistance given
In our view, it is just as important to record option 1 (reported no assistance) as it is to record options 2 or 3. Why you might ask? Because the more information that is recorded about each bull the better the predictions for EBVs are going to be.
What happens if you don't record any calving difficulty?
On the database used to generate the EBVs (DIGAD at DairyNZ) it is recorded as "Not reported" and is treated as missing data in the estimation of EBVs!
A very common statement made by geneticists is that "genotypes are only as good as the phenotypes that have been provided".
What does this mean? This means that if we have a small amount or very skewed data (e.g. only the major assistance recorded) for an animal we are never going to accurately be able to get a good estimate of it's genetics (i.e. our EBVs will not be close to the TBV). We need good quality data as well as a good quantity of data.
Our challenge to you...
For future calving seasons to record all calving difficulties even when there was no assistance given. This will help in the calculations of EBVs in the years to come and help you in managing bull allocation come mating time.
If you have the facilities (and the time and man/woman power!), consider recording the weight of calves as they enter the rearing shed. Maybe one day the dairy industry will have enough data on the weight of calves to establish a birthweight EBV 🤞
We would like to acknowledge Associate Professor Rebecca Hickson and Dr Lucy Coleman for providing references and insights into the preparation of this post.
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