By Dr Clive Dalton
Some important basic genetics
Understanding the basic genetics behind what goes on in the cattle yards and paddock has always been difficult for many people to appreciate.
The underlying mathematics seem to be the bit that makes it hard, and when you talk to old students, their teachers didn’t help either!
You may want to skip this next bit, but try and understand it if you genuinely want to leave a better herd for your next generation.
The normal distribution
- Realise that you cannot have all top animals. Your herd, no matter how long you’ll work at it will always show the classical “normal distribution” or bell curve shown in the figure below.
This shows that two thirds of animals in your herd will be around the average, one sixth will be at the bottom and sadly only one sixth will be “top” animals.
- So to make progress you need to move the average upwards, and to do this you’ll need to breed only from the top ones.
- Once you have improved the mean by moving it to the right, you’ll still have the same number of animals above and below the new average in your herd.
- Genetic improvement by selection and culling is slow and it’s a lot easier and faster to improve a herd’s actual performance by better feeding and management – or improving the environment.
- Key point: The main thing about genetic gain is that it’s permanent. So understanding the difference between Genetics and the Environment in your herd, and how they interact is very important.
Genetics (G) and the environment (E)
- Environment (E) has a massive effect on the animal’s genetics, as many an animal’s “genetic potential” (G) has been masked by poor rearing which is environment.
- Unfortunately we can never know exactly how much of an animal’s performance is G and how much E.
- Many people in the past have paid big money for a bull and have mistakenly bought E when they thought they were buying G.
- Their error was not apparent until five years later when the bull’s genes had been spread through at least three generations of offspring and the animals bred from them!
- You don’t get breeders going around advertising this fact – they try to forget about it as it’s very disappointing.
Phenotype and genotype
- When you look at an animal you see its “phenotype” (P).
- Its phenotype doesn’t always reflect how good it will perform, or how good it will pass on its genes or “genetic potential” to its offspring.
- An animal’s genetic potential is called its “genotype” (G).
- So phenotype may not always be an accurate indication of genotype because of the environment – the point made above about buying the good looking bull which was a genetic failure.
- So this is put into a neat formula that says an animal’s Phenotype is made up from its Genotype plus the Environment or P=G+E. It’s worth remembering.
- We also talk about how strongly traits are inherited or passed on from parent to offspring. Some traits are regularly passed on to the next generation – i.e. are highly heritable or have high heritability. We use a scale of 0-100. You can think of them as a percentage.
- Other traits are not regularly passed on to future generations so have low heritability.
- Traits concerned with growth, efficiency of weight gain and carcass traits are high (40-70), whereas traits for fertility and survival are low (0-15).
- It all depends on how strong the effects of the environment are – if environmental effects are large the result is low heritability.
- So don’t be surprised if you find poor-looking animals that produce good offspring, and some good looking prize winners being a disappointment in the offspring they produce. This happens because the good-lookers had mediocre genetics but had been well reared, well fed and prepared for sale by a skilled stockperson.
- Key point: The basic rule is to buy on genotype and not just phenotype, and to do this you need to understand performance records.
Heritability estimates for cattle
- No calves born 0-15
- No calves weaned 0-10
- Calving interval 0-15
- Mature cow weight 50-70
- Feedlot gain 45-60
- Pasture gain 30
- Efficiency of gain 40
- Birth weight 20-60
- Weaning weight 20-55
- 18 month pasture weight 30-55
- Final weight 50-60
- Carcass grade 35-45
- Rib eye area 70
- Tenderness 60
- Fat thickness 45
- Milk yield 25-40
- Udder size & shape 20-40
- Peak milk flow 35-86
- SNF 53-83
- In the old days you didn’t worry about records, especially for traits like weight and fertility, and breed organisations did not record these in their stud books.
- Breeders made decisions on which animals to keep and which to cull on the basis of “eye appraisal” or what they could see on the outside, together with detailed knowledge of the ancestors in the pedigrees.
- Today, things are now very different and studs can provide extensive performance records so you can buy on genetic potential and not just looks. But it’s up to you to ask for the records and find out what they all mean.
- The stud breeder is the best person to explain these to you – and then you can judge how serious he/she takes them in their breeding policy.
- A stud stock agent will also help you understand the paper work and how to use it along with a physical inspection of the animal.
- Key point: If you are not happy with the answers you receive - keep on asking questions. You are allowed to show your ignorance – they are not!
- When trying to make a decision about different traits on an animal, look at the EBVs or Estimated Breeding Values.
- They are mathematical “predictions” of the performance of the animal’s genetic potential when it becomes a parent. Note these can only be a prediction and not a guarantee!
- So you’ll see an EBV for a wide range of traits from birth weight to 400-day weight, and indeed the large number of them can be confusing when you are deciding which ones you’ll need for your particular herd. Here again, ask the breeder or stock agent for help.
- For example is you were buying a bull, you would look for a negative (below herd average) EBV for birth weight and a negative EBV for gestation length and a positive (above herd average) EBV for calving ease.
- This would aim to improve your herd by having cows that had shorter gestations producing small calves at birth and no calving problems.
- If you want to work out the EBV of an offspring, then add the sire and dam’s EBV together and divide by two. So for 400 day weight if the sire’s EBV is +40 and the dam’s +20, the offspring will be +40kg heavier at 400 days than offspring from parents with EBVs of zero.
- Then there are all the EBVs for weights at different ages such as 200, 400 and 600 days of age. Can you see how it would be easy to get confused?
- To make things easier, and improve general understanding of all these figures, some of the important EBVs have been put together into “Indexes” to predict the value in dollars what the animal will earn through its offspring.
- Remember that these are not real dollars – the dollars are only a handy value to use as a prediction. So use performance records when you buy stock and emphasise them when you are selling.
- Note that the physical features of an animal you want to buy are still important, and breeders are very concerned about what they call “structural soundness”. These are traits like feet, legs, reproductive organs, correct walking and correct jaws, and a defect in any of these could not be compensated for by top records on paper.
- If you were just farming commercially and not showing cattle, then you need not be concerned about traits like colour oddities on the belly or the tail twitch.
- Key Point: Remember another very important principle of genetics which is that the more traits you select for the slower the rate of gain in any one of them. So keep your objectives to a minimum and make sure they are money makers.
The three bits of genetic gain
You can forget everything else about genetics as long as you understand this next bit. By this time in most animal breeding lectures students are sound asleep when they should leap up and say – “Ah is that all there is too it?” “Well why did you bore us to death with all that other guff?”
So to make some real genetic gain in the herd in your human lifetime you have three things to get right:
- Heritability – i.e. how strongly or weakly inherited the trait is. Growth rate is high and fertility and survival are low. Go for high heritability.
- Selection Differential. This is amount by which the animals selected as sires and those selected as dams are above the average of the herd. You calculate this for sires and dams separately and then add them together an divide the sum by two. Go for a high selection differential.
- Generation Interval. This is the average age of the parents when the offspring are born and is about 5 years in cattle. There’s a limit to this as you can’t breed from animals before puberty though we can now get eggs out of new-born calves. Keep generation interval short.
Genetic gain/year = (Heritability x Selection Differential) / Generation Interval
Think about the maths in this formula.
When you multiply things together on the top line, the result gets bigger. When you divide by something on the bottom line the result gets less. So to really make rapid progress in your lifetime, you need to:
- Select for clearly-defined traits that are strongly inherited – and restrict their number to no more than three so go for high heritability.
- Select parents as far above the average of the herd as possible so go for a high selection differential.
- Breed from young animals so keep the generation interval short.
How breeders get it wrong!
- Breeders regularly get it wrong and end up being very frustrated at having made no genetic progress.
- They select for far too many confused traits that are weakly inherited, they select only average animals as parents when they are short on numbers of top stock, and they only keep offspring from their older animals.
- This formula is the core of animal breeding so think about it a lot.
This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.