Crossbreeding has been used to great effect in other industries to maximise desirable production traits. The Isa Brown hen was originally the result of crossing Rhode Island Reds and Rhode Island Whites, although it now contains a range of other breed genetics – the exact combination being carefully guarded by the company that created it (much like KFC with their eleven secret herbs and spices). The end result is a modern-day hen that combines the best traits of multiple breeds: large egg size, early maturity, high production, and a lesser likelihood of becoming broody.
This idea of crossbreeding to infuse a variety of desirable traits into a single animal, while reducing the impact of negative ones, is known as complementarity. Could this be applied to dairy breeding?
Well, for more and more farmers, it seems like the answer is a resounding ‘YES!’ Certain breeds in dairy are known for their characteristic advantages: Holsteins tend to have great production traits, Jerseys are known for high butterfat levels, and Red breeds have excellent functional traits (like health and longevity). By combining these (or other breeds) together, the theory is that you produce a cow with great production, good components and easy-care management.
This is significant mostly if you’re seeking to improve a variety of traits within your herd. If the main thing you’d like to focus on in your breeding program is production, for example, then cross-breeding is not likely to accelerate you towards this goal – you’d be better off selecting a single breed with the highest production traits.
However, farmers are increasingly seeing that many traits contribute to profitability. This was shown most strongly in the recent ADHIS National Breeding Objective review, where certain farmers spoke loudly and clearly about their desire for increased emphasis on non-production traits such as longevity, health and fertility. While these can be compensated to a degree with good management, some farmers are seeking easy-care cows for their use in larger herds, and for reduced time and stress spent dealing with higher-maintenance or ‘problem’ cows.
The other theoretical benefit of crossbreeding is the near-mythical concept of hybrid vigour. This phrase often gets thrown about – but what does it actually mean?
Let’s start with the scientific definition: ‘the increased performance of crossbred animals compared with the average of their pure-bred parent populations.’ This is thought to be due to an increase in the number of heterozygous loci (Aa) compared to homozygous loci (aa) in individual genomes, as purebreed populations tend to have more homozygosity due to selection.
What does this mean in plain English? Well, the graph above represents the result of hybrid vigour in a single trait. If Parent B is a sire from high-producing breed B, and Parent A represents a dam from a lower-producing breed A (that presumably has other desirable traits, such as fertility), you’d expect their calf to perform at the average of her parents. So, if Breed B produces an average 100 litres of milk, and Breed A produces 50 litres, you’d expect their offspring to produce 75 litres. This is known as the ‘parental average’.
However, this is where the magic of crossbreeding kicks in. The crossbred calf will actually exhibit the parental average performance plus an additional bonus. This is known as hybrid vigour or heterosis. In this scenario, the calf would actually produce 83 litres – a 10% bonus due to heterosis. In real terms, the calf’s performance in this trait may still not equal or exceed its Breed B sire – however, it will have other traits (e.g. fertility, longevity, etc) inherited from its Breed A dam with their own applied degrees of bonus gain.
This brings me to my next point, which is that the amount of extra gain is not the same for all traits. Some traits benefit more from heterosis than others – with longevity and functional traits thought to derive the greatest benefit (up to 30% additional bonus), and milk production traits still benefiting but to a lesser degree (up to 10%). (Some traits may even have negative heterosis – a Danish study found that this was the case for calving ease and stillbirths, although subsequent studies have not been able to replicate this). These exact percentages are not set in stone, as a variety of results have been produced across multiple international studies (perhaps because of the different breeds and lines used). However, the degree of heterosis is thought to be inversely related to the heritability of the trait – hence why it is generally lower for production, but higher for traits such as fertility.
So, just to recap:
- Complementarity is the idea of mixing breeds that complement each other so that the strong points of one breed offset the weaker characteristics of another. It’s about infusing desirable traits into an animal, while reducing the impact of negative traits.
- Hybrid vigour (or ‘heterosis’) is the bonus in performance obtained when joining two different breeds. The degree to which a trait benefits from this depends on its type, with longevity and functional traits benefiting more than production.
Many farmers have a good understanding of the practical benefits of crossbreeding in improving the functional, fertility and health traits of their herd. If this sounds like you, then I’d love to have a chat with you – what system do you use, and what effects have you seen? How did you implement it, and what would you do differently?
The scientific basis of crossbreeding, however, is still undergoing further research, as the exact mechanisms behind complementarity and hybrid vigour are being painstakingly ironed out. So, in the next few weeks, we’ll be talking to a researcher about her efforts to compare popular crossbreeding strategies – and how crossbreeding farmers can help.
In our next post we’ll also cover some practical considerations (as well as disadvantages) of crossbreeding in further detail. As with any management strategy, it’s up to you to decide whether it suits the type of farm you’d like to run – we just provide the facts. There are some great resources out there on the subject too, so I’ll make sure to cover that as well.
In the meantime, are there any questions you’d like to get answered about crossbreeding? Did you have an opinion about its suitability for different farms? Write to me at firstname.lastname@example.org or even better still – leave a message in the comments below.