CrossBreeding and selection in Goats
This paper has presented at the 4th National Mohair Conference, Canberra, November 1991, and was prepared from the recording by L. Hygate and B. McGregor, Agriculture Victoria. The contents of the paper are relevant for all farm animals, meat, fibre and dairy.
Cross-breeding and selection in all livestock species play a role in increasing production and hence determining profit in the enterprise. The genetic theories behind cross-breeding principles and selection practices are explained in this paper so that producers can use these tools to improve the production of their goats.
Inheritance of quantitative traits
Additive gene action
To explain the inheritance of quantitative traits (such as fleece weight or fibre diameter), we can use the example of a single gene which affects something. This could be fibre diameter, for instance. For each type of gene you inherit, one allele, or example of it, comes from the dam (mother) and the one comes from the sire (father), so that you have two copies of these genes. Let's say gene A affects fibre diameter. In this case, we have gene A, which comes in two forms A and a (table 1). If you have two copies of a, it does not increase fibre diameter at all. If you have one A, it is worth one micron. It you have two A's, it is worth two microns. This is a very simple model of how genes influence a quantitative trait like fibre diameter.
In this case it is an additive gene action because two of these A genes have twice as much effect as one.
Table 1. Additive gene action
What happens in a crossing scheme?
In a crossing scheme, we mate AA to aa (table 2). On the line below this mating, the effect on fibre diameter is indicated. If these were two different breeds, then the cross between them would be a first cross (or F1) from this mating the animal will get an A from parent 1, and an a from parent 2, so the offspring will be Aa, which has a value of "1". If we mate one F1 to another F1, there will be a range of gene types (or genotypes) in the offspring. Some offspring will get an A from both parents, others will get an a from both parents and half will get both an A and an a (table 2). The first thing to notice with the F2 generation is there will be an increase in variance. That is, there will be an increase in the range of fibre diameter observed between the parent generation and the F2 generation. When we look at the average for each generation, the parents' average will be "1", all of the first F1 generation is "1" and the average of these F2 will be "1". Thus, in this case, the crossing does not change the average at all, but it increases the variation in the F2 generation.
Table 2. Progeny and merit using additive gene action
Dominant gene action
Table 3 shows the effect of dominant gene action. In this model, the aa genotype is still "0", and AA is "2" but A is dominant over a, so Aa performs just like AA, that is "2". This is simply dominance applied to a quantitative trait like fibre diameter. What happens with the genes in the crosses is the same result as before but because of the dominance, Aa's perform like AA's. We mate the F1 to an F1 and we get an F2 generation. This has the same genotypes as before, but now the Aa's have the same merit as the AA's. Thus, the averages will change. The average of the parents is "1", but the average of the F1 first cross is "2" and the average of the F2 second cross is "1 1/2". This demonstrates the other general phenomenon that we see with cross breeding: crosses perform better than the average of the parents and half of that superiority is lost in the next generation.
It stays the same from there afterwards, that is, if we mated two F2's together on average, it will remain at this 1.5 level.
Table 3. Progeny and merit using dominant gene action
Accordingly, the two phenomenon you will see with cross breeding are an increase in the average and an increase in the variance of the progeny. There is more variability in the F2 generation than in the F1 generation. In general, it is the traits that are related to fitness which show this hybrid vigour (or heterosis) as it is called - an increase in the average of the first cross. Traits such as fertility, growth rate, and probably fleece weight show this hybrid vigour, whereas typically, the quality traits, such as fibre diameter, just show additive gene action. That is, the average of the cross is just half way in between the average of the parents. It is important to realise that some traits will show some hybrid vigour and some of them will be just half way between the parents.
Traits caused by more than one gene
The above examples are slightly misleading, because it might make you think that there is just one gene affecting the trait - for example, fibre diameter. In fact, most of the traits that we are interested in are affected by a whole series of genes and by environmental factors. The phenotype (or performance - what you observe) of an individual animal is simply the sum of the environmental influences which are experienced by that animal and the genes it carries. (table 4).
table 4. The phenotype of an animal
Environmental factors include nutrition, age of mother, rearing status and birth date. For example, if we are making selection decisions post weaning and select animals based on their performance at that time, it is likely the selection group will all be early born, single kids. However, some of the later born twins, although they do not look as good as their counterparts may have progeny which perform as well or better. Often, we do not know what these environmental factors are. We think that because animals in one mob are treated the same as the rest, they should all be the same, however, they all experience the environment differently. That affects the way they perform and there are a number of genes that affect traits like fleece weight and fibre diameter. The outcome of these many factors which affect a trait like fleece weight is that instead of having strict categories like high and low or 0, 1 and 2, we end up with more or less a continuous range of fleece weights. The best way to demonstrate the distribution of fleece weight is with a bell-shaped curve (Figure 1). This shows that the most common fleece weight is in the middle, around the average, but there are a few animals well-above the average, and a few well-below the average. This is most commonly observed for most traits.
Figure 1. The Bell shaped curve
Dominance and CrossingTake another case where we have some dominance occurring and therefore, some hybrid vigour. If the last example were for fibre diameter, this one might be for fleece weight. Because there is some hybrid vigour due to some dominance, the first cross performs closer to parent P2 than it does to parent P1. The spread is the same, but the mean increases. The second cross drops back compared to the first cross (remember we went from 2 to 1 1/2 when there was a single gene). So we move back half way towards the parent average and we also have an increase in spread. The first cross is above parent average, no more variable than the parents, and the second cross is still above parent average, but not as much, and it has a larger spread. That is what you would expect to observe for things that are related to natural fitness like fertility or ability to grow or survive and fleece weight falls into that pattern.
Selection of animals
As mentioned previously, the final performance of an animal is the sum of the effect of the genes and the effect of the environment. All animals get half their genes from each parent. The genes passed onto each offspring are randomly selected. When you are selecting animals for breeding, the aim is to select the ones which will pass on good genes to their offspring. These environmental factors that make an animal good or bad are really a nuisance when you are trying to figure out the merit an animal will pass on to its offspring - that is, what its breeding value is. What you can observe is its performance and that is a guide to its breeding value but an imperfect guide because of environmental factors which cause animals to perform better or worse that you might have expected. If you imagine Figure 8 represents a population of goats, the average of that generation is in the middle, (M1) and we select animals from the shaded area as parents for the second generation. We select the best goats to breed the next generation and what we find is that the average of this generation is not as good as the average of the selected parents. The reason for this is that the selected parents were good partly because they had better genes but partly because they had a better environment. If you think about it, if merit or whatever trait you are talking about is partly due to genes, partly due to environment, and you go through and pick out the best ones, you are tending to select animals that are both good for genes and good for environment. However, they will only pass onto their offspring, the superiority of their genes. They will not pass on their superiority in the environment. Thus, the offspring, on average, will not be as good as the selected parents. Although they may not look as good, however, genetically they are as good. They will be better than the average of the previous generation so you will make progress, but not as much as you thought you would. The proportion of the difference which you observe in the offspring is called the heritability.
From the second generation, we select the best and breed from them. Again, their offspring are better than the last generation average, but they are not as good as their selected parents. This explains a commonly observed phenomenon where people say, "you know old buck 123 was terrific - but he didn't leave any progeny as good as him, so we kept using him for 20 years or whatever, because he was so good himself." The obvious explanation for that is that the reason he looked so good was partly environmental. He wasn't passing that on to his offspring. If he had been genetically that good, he would have produced offspring of that calibre and some of them would have been better than he was unless, of course, you had been mating him to does that are not nearly as good. Quite often, breeders over-estimate the genetic merit of their favourite animals because they only look at their performance. They do not realise that the fact they are not passing it on is just straightforward evidence that they are genetically not as good as they thought they were.
Estimated Breeding Values (EBVs)
Gains from selection are reasonably slow because you keep slipping back part of the way that you through you were going to improve. The aim in a breeding program is to try and make those gains from selection move as fast as you can. The first way of doing this is: to increase the accuracy of selection.
By this, I mean, how well you estimate the breeding value or the genetic merit of the animals as opposed to just their own performance. If we go back to our equation, P=G+E, what we are trying to estimate is the G part of the P. What information is there? The first thing is the animal's performance. In trying to estimate breeding value, you want to correct for its environmental factors as much as you can. There are basically two ways that you can do that. You should only compare animals which have been raised at the same paddock, in the same time, in the same mob. There is no point in comparing the performance of a goat from one property with a goat on another property because the environmental affects are big enough to swamp any genetic effect there might be. Your accuracy of guessing the breeding value when you are looking at animals from different environments is just too poor. Thus, you should only compare animals which come from the same environment, meaning they have essentially grown up in the same mob. The other thing you can do to some extent is to correct for some environmental factors, if they consistently occur. For example, if you know that mature does have faster growing kids than maiden does, then you can make allowances for this when comparing, for example, the weaning weights of kids from maiden does with those from mature does.
The other sort of information that you can obtain in trying to predict the animal's breeding value is: the performance of its relatives
because all of its relatives share genes with this animal and provide you with some information.
The best way of putting all of this information together is to statistically calculate and estimate the breeding values. An EBV or estimated breeding value is just the best estimate you can make for that animal, given all the pieces of information you have, that is, the pedigree and the performance of all the relatives.
Using EBVs will improve the accuracy with which you select. The first thing to do is try and not be put off by environmental factors. Try and make your comparisons free of environmental factors. That is, use information from relatives and get someone to calculate EBVs for you. The last thing I have is one that only applies when there is cross-breeding occurring. As I stated earlier, the cross breed, particularly those from the first generation performs better than the average of its parents, because of this hybrid vigour. The F1 parent will not pass on that hybrid vigour to its offspring. Thus when comparing animals of different crosses say an F1 with a purebred, it is important to realise that although animals will be equally good, the F1 will not pass on its superiority which results from the hybrid vigour effect to its offspring,.
Maximising progress from selection
To maximise the progress that you will get from a selection program, you need to increase the intensity of selection, decrease generation interval and minimise inbreeding. These will now be discussed in more detail.
Increasing selection intensity
Increasing the selection intensity increases genetic progress. If you select the best buck from 100 contemporaries, it will be better than the average of the best 50 out of 100. If you have only got two males that you have left entire, then it is a one in two selection and, you will not make that much progress. Your really need to have numbers from which you can select the best and you need to do it intensively. It is not good enough to just say, "Well, OK, I've only got three bucks and I need two of them so I'll cull this one". It is better than doing nothing but it is nowhere near as good as having 500 and saying I'll pick the best five. This will present a big difficulty for people with small flocks, however, you need to find some way around this. This can be achieved possibly by collaborating with other breeders.
Decreasing generation interval
In all those graphs shown earlier, which indicated how much progress is made by selection, the progress illustrated was that which you could make in one generation. If one generation takes 10 years, then you only make that progress in 10 years. If one generation takes you two years, then you make that much progress in two years. Clearly, you want the generations to be as short as possible. This must occur within the constraints of the other important factors because you cannot just cull all your does and start again every year. Within reason, generations need to be turned over fairy quickly. It comes back to the example shown before. If you are making progress, then each year's drop should be better than the previous years so that if you cannot find a better buck out of this year's then you are not making progress. You need to think about what you are doing. If you are making progress, then you should be able to find a better buck out of this year's drop than the ones you selected out of the previous year's drops. People typically keep animals, particularly males for too long. There are umpteen stories in other industries (and I cannot believed goats are any different) of people keeping sires for years and years, because they thought they were so good. If he is still the best sire that they have bred at 10 years of age, then it does not say much for what they have achieved in the last 10 years.
All the good things that occur with crossbreeding in terms of improvement go the other way with inbreeding. We know that dog breeders traditionally inbreed their animals. but the people who make a living from their animals such as dairy farmers, never deliberately inbreed their animals. They know that everything that means money decreases with inbreeding; production, fertility, survival and so they always try to minimise inbreeding. There is a conflict here between minimising inbreeding and selecting intensely. If you select intensively, it means that there are relatively few males contributing to the next generation. Therefore, eventually, you are going to run into some level of inbreeding, but put it off for as long as you can. Try to keep enough males or introduce animals into the flock so that you can minimise inbreeding.
An example from the dairy industry illustrates this. The situation with Holsteins or Friesians on the world basis is frightening. There are a small number of Holstein bulls who are the sire to almost all of the next generation throughout the world. If you go to a trade fair in Bangkok, you can buy a son of Bell from America, you can buy a son of Bell from France or any other country. It will still be a son of Bell or Valiant or whoever was the leading sire that year so they selected enormously intensely. It concerned me that they were overdoing it so I did some pen and paper work to see whether they would not be better off to select a few more of these super sires so be able to avoid inbreeding for a little while. It turns out that what they were doing was not too far from the economically optimum thing to do.
If you are worried for example that over the whole of Australia, you are allowing the Angora goat population too become too small, my advice would be to stop worrying. If the whole Holstein population could be bred by half a dozen top bulls then surely we are not breeding Angoras from too few bucks. However, if on an individual flock basis you are trying to run a closed flock and you are breeding all of your flock from two bucks then you may have a problem. Inevitably there will be some maidens that are inbred to those two bucks used a few years earlier and some other problems may ocurr such as cryptorchidism. Consequently, you will need to do some planning to make sure that you have more than a couple of bucks breeding the next generation. The good news is that you do not need to have a huge number. It is best to really concentrate on the superior sires across the industry than to use a lot of mediocre sires.
Combining selection with crossbreeding
This is what people are interested in doing with the Texan and South African Angora imports and their existing goats. This can also apply to crossbreeding Boer and cashmere goats, Boer and Angora goats or Angora with cashmere goats. Assume we take two parent breeds (P1 and P2) and the F1, the first cross. I would assume the F1 is closer to P2 than P1, so I am also assuming that there is some hybrid vigour. We select the best of those F1's to breed the next generation and we cross it back to this parent 2 so B2 is a 3/4 bred. Because there has been some selection practiced, the mean of the B2 is higher than the average of the P1 and the P2 population. Remember, because the trait is not completely heritable, you are only going to get a proportion of the difference between the selected animals and the average translated into the offspring. This is the pessimistic situation. In the backcross generation, (B2) there will be an
increased variance. The fleece weights for example, will be more spread out. We select again and we inter se mate, in other words, we mate a backcross buck to a backcross doe and the inter se mating, B2 shows the resulting population. As a result of this selection, we are steadily moving forward, because of the combination of the selection and the crossbreeding. Now, if you accuse me of talking commonsense, then I agree with you.
In your crossbred flock, there will be some F1's, some 3/4 breds and some 7/8's. You will want to select from among all of those for the best ones for breeding purposes. The basic things is to select the parents which will give the best offspring. That sounds obvious, but as long as your think about it in that way, you can see what you need to do. The performance of the offspring will be made up of a number of things:
When selecting the parents, you can predict the average progeny performance of the cross, because it will usually be the average of the parents. If the parents are above average for their generation, then they are going to produce above average progeny in the next generation. How much heterosis or hybrid vigour will you get for a particular cross? If the offspring will all be F1's, there will be a lot, if they are from a backcross mating, there will be less, and if the progeny are 7/8th, the amount of hybrid vigour will be low. Thus, it is a matter of when you are comparing animals to try and say, well, what sort of progeny can I expect to get from using all these different sires which originate from different breeds and crosses? That is, how will the progeny from buck A perform relative to those from buck B? This does become complicated, and to get it absolutely right requires someone to do a formal analysis of your records.
Combining different traits to maximise profit
In this paper, I have assumed you are only selecting for one trait, such as fleece weight and fibre diameter, for example. In reality, of course, you want to select for a number of different traits. The question is how to combine all the information on all the different traits in which you may be interested, in order to make the most progress. You can move from a situation of many traits to a situation of one trait if you simply calculate, for all the animals which you have available, the amount of money which they made last year, or over their lifetime or whatever. Essentially, what I am advocating is that you select on the profit which that animal makes for you. This means that you should include in your selection decision, all of the traits that contribute to making money. If this means that you have to select for a number of traits, then so be it. If you find that in your selection decisions, that you are putting a lot of weight on things that have a relatively small effect on dollars, then you are probably doing the wrong thing. One of the most common mistakes that people make in breeding livestock is that they put too much emphasis on traits which do not have that big an effect on dollars.
Figure 2 illustrates this principle using two traits, for example, fleece weight and dollars per kilogram, you receive for the fleece (quality). In this graph, I am comparing two different ways of breeding Method 1 and 2.
Figure 2. Ilustrates the principle of using two traits to select on the profit which an animal makes
In Method 1 we have the two parents, X and Y. We have selected one parent which is very good for fleece weight, but poor for quality and have mated this to another which is good for quality and poor for fleece weight. In my experience, this happens frequently. Breeders assume that if they mate these two animals, they will get progeny which are good in both directions. However, you do not get the best of both of them, their progeny will be half-way in between each other, so that you end up with an animal which is intermediate for both fleece weight and quality.
In Method 2 a slightly different philosophy is used. Neither of the two parents is the best animal for fleece weight or the best for quality. They are more in between animals but they are the best for making money. In this case we are using a Bell shaped curve for financial returns. The same thing happens here, the progeny will be intermediate between the two parents. However, all of the progeny from this mating are more profitable than those from Method 1. Thus, if you have been selecting on profit, the trait you really wish to improve, you would prefer Method 2 animals over the Method 1 animals and have obtained more profitable progeny as the outcome.
I tend to think of this sort of breeding as a cooking mentality of breeding livestock. People imagine that if they could get so much of fleece weight and so much of density, and so much of size and then mix it all together, they would come out with an ideal animal. However, what they end up with is an animal which is about average of both its parents. If you want to maximise dollars, you need to use the parents which have the highest economic merit, that is whose progeny are worth the most money.
© 2000 B.McGregor