New Zealand’s seasonal dairy farming system entails a condensed calving pattern with cows required to conceive within approximately 12 weeks of the planned start of calving. This has resulted in strong selection for fertility through culling of non-pregnant cows and relatively strong emphasis on fertility in Breeding Worth (BW), the national breeding objective that drives sire selection.
Fertility breeding values
Fertility breeding values (BVs) are comparative measures expressed as a percentage of daughters that re-calve within the first 6 weeks of calving. The fertility BVs can be used to compare sires of all ages, breeds and crosses in New Zealand.
Fertility BVs for bulls and cows are relative to a genetic base, being the average of cows born in the base year. So cows with fertility BVs of 0% have the same genetic merit for fertility as the base cows born in 2005.
In comparing cows with fertility BVs of +5% (high genetic merit for fertility) with cows with fertility BVs of -5% (low genetic merit for fertility), you can expect 10 more high-merit cows per hundred to re-calve in the first 6 weeks of the herd’s calving period.
Bulls transmit half their fertility BV to their daughters. The other half comes from the dam.
Predictor traits used to measure an animal’s fertility breeding value include
- Whether the cow was mated within 21 days of the planned start of mating
- Calved in the first 42 days of the planned start of calving
- Milk volume in a cows’ first lactation
- Body condition score in a cows’ first lactation at 60 days in milk
See Breeding values for more information.
Sire selection and fertility genetics
Some artificial breeding (AB) sires produce daughters that are genetically more fertile than others. This means that the genetic make-up of your herd for fertility may be a little better or worse than the average herd.
You can check your herd's fertility genetics by requesting your herd’s average fertility breeding value (BV) from your herd improvement organisation.
Breeding Worth (BW) takes reproductive performance into account, ensuring there will be ongoing genetic progress long term in cow fertility.
Farmers who are struggling with reproductive performance could find it valuable to specifically target bulls that rank well for fertility. This will effectively increase the rate of gain in fertility genetics within their herd, and this should have a large impact on their herd’s overall reproductive performance
Genetic fertility research
An ‘animal model’ research herd with extreme diversity in genetic fertility has been established and these heifers calved in 2017. Their progress has been monitored and the research team are looking for answers to the following fertility questions.
- How accurate is the current fertility breeding value?
- What new traits can we measure to help us better predict fertility?
- What underlying physiology is driving differences in fertility?
Crossbreeding adds hybrid vigour
If you are breeding crossbred cows they will have additional hybrid vigour for fertility. New Zealand Animal Evaluation data measures the hybrid enhancement of fertility beyond the effects of breeding values.
The hybrid vigour advantage for a first cross animal is that the 6-week in-calf rate is around 3.4% higher than you would expect from mating parents of the same single breed.
The hybrid vigour advantage for subsequent crosses is that the 6-week in-calf rate is around 2% higher than you would expect from mating parents of the same single breed.
Crossbred AB sires will retain a 1.7% hybrid vigour advantage.
In New Zealand, the average gestation length of dairy cattle is 282 days, but there is genetic variation around this.
Dairy sires gestation lengths can be found in the Animal Evaluation section.
Interest in short gestation breeding options has increased and some farmers include short gestation length sires in their mating programme, at the tail end of AB or as an AB restart near the end of mating.
You should be aware of the gestation length BV of the sires you are using (both beef and dairy), as it will affect he calving date of your cows.
See Calving pattern.
Selecting bulls to minimise calving difficulty
Subsequent reproductive performance is worse after an assisted calving.
Some sires, especially Holstein-Friesian, cause higher rates of assisted calving when mated to yearling heifers. This is a direct genetic effect of the bull.
This sire effect is reflected in a sire’s calving difficulty BV.
A sire's calving difficulty BV predicts the percentage of assisted calvings expected when he is mated to yearling heifers. It can also be used to identify bulls that are expected to increase the rate of calving assistance for cows carrying the bull’s calves.
When artificially inseminating yearling heifers, use sires proven under New Zealand conditions with a low calving difficulty BV.
Inbreeding tends to have a negative effect on a cow's profitability through lower fertility, lower production and a higher incidence of genetic disease. Avoid inbreeding by not mating sires with closely related cows. Your AB company has programmes to help you avoid inbreeding.