Genetic Improvement and Crossbreeding in Meat Goats Lessons in Animal Breeding for Goats Bred and Raised for Meat
Will R. Getz - Fort Valley State University
Backcrossing can be defined as the mating of a crossbred back to a purebred of one of the parental breeds or lines. It is most often used to import a specific allele to a herd or breed that lacks the allele. A classic example might be using the Boer to create a red head and white body on other breeds and types of meat goats; to import the color pattern allele into populations where it did not exist before. It is likely that other alleles influence the extent of the red color on the head and neck. Always going back to the breed with the red head until the frequency of that allele is high enough to close the herd. Subsequently the males and females mate with one another according to some plan. Culling against the original red color continues through all phases.
Grading up or “topcrossing”, is a mating system that involves repeated backcrosses but does not include any attempt to select for a specific allele. It is a system designed to accomplish one or the other of the following breeding objectives:
- Convert a herd from one breed to another by mating successive generations of does descended from the first breed to bucks of the second breed.
- Create a purebred population by mating successive generations of non-purebred does to purebred bucks.
An example of upgrading might involve a foundation group of local brush mixed-breed goats mated to successive unrelated Kiko bucks, whereby:
- First cross of Kiko x Brush goat = (½ Brush + ½ Kiko); x Kiko = (1/4 Brush + ¾ Kiko); x Kiko = (1/8 Brush + 7/8 Kiko); x Kiko = 1/16 Brush + 15/16 Kiko, and so on.
This will create a purebred population from a nonpurebred base, although technically the herd will never reach 100 percent.
Converting a population from one breed to another will use the same approach. The first mating will create a halfbred, followed by consecutive backcrosses to the breed of choice.
A risk associated with upgrading is that the genetics in the original foundation breed may be lost or put at such a low frequency that if needed in later generations may be difficult to recover, e.g., finding good adapted local stock after many have been upgraded to Kiko or Boer.
A mating system is a set of rules for making mating decisions. Hence, there is almost no limit to the number of possible mating systems. Instead we prefer to speak of mating strategies. Some are based on animal performance or expectation of performance, while others are based on pedigree relationship.
There are two components for those strategies based on animal performance. The first is in an individual context, while the second is a population context, i.e., strategies for crossing breeds. First, we consider strategies for making individual matings — specific bucks to specific does.
This is a planned mating system in which mates are chosen at random, and should not be taken lightly just because of its simplicity. All conceivable matings are equally likely. Single sire herds represent a form of random mating --- all the does will be mated to the same sire. Often however, random mating involves multiple-sire herds. Random mating may be either a lazy way to breed animals, or a deliberate, carefully chosen technique. Random mating is easy. It has nothing to do with selection. Random mating occurs within the context of breeding animals that have been selected under the existing criteria. There is no special skill required once the selection decisions have been made. Random mating should not imply an absence of selection. Random mating is underrated by many breeders. Given the randomness of inheritance in general, the breeder exercises only a modest amount of control over the outcome of matings at best.
Planned assortive mating
This mating system involves mating of either similar individuals through positive assertive mating, or mating of dissimilar individual through negative assertive mating. Similarity is in the context of performance or set of traits. As with random mating, assortive mating occur after selection decisions have been made. All the animals involved have been selected on merit. Assortive mating is more difficult than reandom mating. It requires performance records, genetic predictions, or some other mating criterion. Animals must be ranked, which is not necessarily a simple matter when multiple traits are considered.
Examples of positive assortive mating include the mating of males with high EPDs to females with EPDs, or phenotypically mating the biggest bucks to the biggest does, and likewise the smallest to the smallest. It tends to create more genetic and phenotypic variation in the offspring generation as compared to random mating. Increased phenotypic variation is generally seen as a drawback of this system. It tends to drive the population toward extremes, and away from the center. Increased genetic variation can be beneficial from a selection standpoint. There is a direct relationship between genetic variation and rate of genetic change. Most breeders use this strategy however for genetic change. They are attempting to produce more extreme individuals which may be highly sought after by other breeders. If they are males they can have a large impact on the next generation. The key is to know if the extreme is useful to the breed as a whole or only to some producers who want and need to change their herd rapidly.
In the case of negative assortive mating, the smallest are mated to the largest and biggest to the smallest. Negative assortive mating tends to decrease variation. The system tends to more intermediate types and reduces the number of extreme offspring. It is not a good strategy if the goat breeder needs to make rapid change in one direction or another. If the goal is to create phenotypic uniformity about some intermediate optimum this strategy can be very helpful. Negative assortive mating is best used to produce intermediates. Some of this type can be considered corrective matings. These are matings designed to correct in their progeny, faults of one or both parents.
In many cases each of these mating systems will be used simultaneously where by individual breeding animals might benefit from the various advantages. Simplicity and having breeding objectives are key elements. The next two mating strategies are based on pedigree relationship rather than performance criteria.
It might be thought that few breeders use inbreeding in any constructive way, but as it is defined here anyone working within a breed is doing inbreeding. Inbreeding can be defined most simply as mating of relatives. Goats of the same breed are more closely related to each other than to goats of other breeds.
The chief effect of inbreeding is an increase in homozygosity — an increase in the number of homozygous loci in inbred animals, and an increase in the frequency of homozygous genotypes in an inbred population, e.g. herd or breed. An extreme outcome of inbreeding is to fix traits, wherein the frequency of one allele goes to 1 and the frequency of the other allele becomes zero. A consequence of the increase in homozygosity is greater prepotency in the inbred population.
Individuals are said to be prepotent if the performance of their offspring is especially like their own and (or) is especially uniform. Why? Because inbred individuals have fewer heterozygous loci than do non-inbreds, they cannot produce as many different kinds of gametes (sperm and egg combinations). The result is fewer different kinds of zygotes and therefore less genetic variation and more uniformity among the offspring.
A second consequence of inbreeding is the expression of deleterious, “bad” recessive alleles (genes) with major effects. It is this aspect of inbreeding that gives inbreeding a bad reputation. Producers associate inbreeding with genetic defects. While it is true that defects caused by recessive alleles often show up in inbred population, inbreeding does not create deleterious recessive alleles; there were there to begin with. When these defects come to the surface and opportunity exists for cleansing the population, unless, of course, they exist at such a high frequency the population is overwhelmed. The dwarfism problem in a couple of beef cattle breeds in the 1950’s might have been less devastating has there been more inbreeding going on, thus allowing the undesirable allele to show up earlier.
Inbreeding by itself simply increases homozygosity, and it does so without regard to whether the newly formed homozygous combinations contain dominant genes or recessive genes. It therefore increases the likelihood of deleterious recessive alleles becoming homozygous and expressing themselves. When combined with rigorous selection, inbreeding can be used to detect and greatly reduce the faulty recessive alleles from a population. Be aware however, this can be tricky business especially if there is a lot of genetic trash that might surface and greatly reduce productivity and survivability.
Inbreeding depression is a phenomenon which is the reverse of hybrid vigor, that is, a decrease in the performance of inbreds, most noticeably in traits like fertility and survivability. In simply-inherited traits such as dwarfism in cattle and spider syndrome in sheep the effects of inbreeding can become very visible. For polygenic traits (those involving many pairs of genes and many loci) the effects are less graphic or obvious. The individual effects of these genes are small but when taken together, can significantly decrease performance.
Linebreeding can be defined as the mating of individuals within a particular “line”. A line may be considered a group of related individuals within a breed. Lines usually have a famous ancestor in common among all individuals. For example Kaptein, Eggsfile, or Tabu among Boer lines, and Goldmine, Lightnin, or Goliath among Kiko lines. While inbreeding generally has an undeserved bad reputation, this mild form of inbreeding called linebreeding does not. It is a mating system designed to maintain a substantial degree of relationship to a highly regarded ancestor or group of ancestors, without resulting in high levels of inbreeding. Due to the absence of very close matings, linebreeding is a slow form of inbreeding. This allows time for selection to off-set some of the adverse effects of inbreeding, such as inbreeding depression. If poor performing individuals from a linebred population are systematically weeded out through selection, inbreeding depression may not be as apparent as it would otherwise.
The two most important genetic reasons for inbreeding are:
- To increase uniformity (prepotency).
- To create an opportunity for hybrid vigor within breeds.
Meat goat breeders who work within a single breed are in fact using inbreeding, but we call it purebreeding or straight breeding and the resulting offspring are called purebreds. When you cross those inbred lines (breeds), you can obtain substantial levels of hybrid vigor (heterosis).
You can rightly conclude that outbreeding is the opposite of inbreeding. It is the mating of unrelated individuals. Now because no animals within a population (breed) are completely unrelated, a more correct definition is the mating of individuals more distantly related than the average of the population. The most common outbreeding mating strategies are linecrossing and crossbreeding.
Crossbreeding involves the mating of bucks of one breed or breed combination, to does of another breed or breed combination, whereas linecrossing refers to the mating of bucks of one line or line combination to does of another line or line combination. Just as the primary effect of inbreeding is an increase in homozygosity, the primary effect of outbreeding is an increase in heterozygosity. The following illustration demonstrates this increase in heterozygosity.
Suppose you have two linebred parental genotypes described as AABbccDDEeff and AabbCCddEEFf. Note there are six loci (pairs of genes) involved. These parents are relatively homozygous, each being heterozygous at only two loci. Each of these parents can produce four different “kinds” of gametes (sperm or eggs). These are illustrated in the top and lefthand side of the following table:
Example No. 2
|Example No. 2|
The body of the table contains all possible genotypes that can be produced when the various gametes are combined as the result of all possible mating combinations of these parents. Note that it the order of alleles, e.g. Cc versus cC, does not matter in terms of describing the genotype. Just looking at the number of heterozygous loci in the resulting offspring it is apparent that the relative inbred parents created relatively heterozygous offspring when crossed. The average number of heterozygous loci has increased from two to four. Some offspring (above, in italics) are heterozygous as only two loci while some are heterozygous at as many as six loci, as in AaBbCcDdEeFf.
It is important to understand that outbreeding does not eliminate deleterious recessive alleles. It actually perpetuates them by masking their expression, which makes natural and artifical selection against them ineffective. By increasing heterozygosity, outbreeding tends to keep most deleterious recessives in a form in which they are not expressed.
The counterpart to inbreeding depression is called hybrid vigor or heterosis, which can be defined as an increase in the performance of “hybrids” over that of purebreds, most noticeably in traits related to fertility and survivability. A hybrid, in the livestock species, is an individual that is a combination of species, breeds within species, or lines within breeds. Both inbreeding depression and hybrid vigor are affected not only by the relative numbers of homozygous and heterozygous loci influencing a trait, but also by the degree of dominance exhibited at each locus. There are times when no dominance or partial dominance or other types of gene action prevail. Hybrid vigor may still occur, but the degree of vigor may be different.
Hybrid vigor is not inherited. It results from the particular gene combinations in each succeeding generation. During meiosis (a type of cell division in the sex cells) those combinations are broken up and then recombined in new combinations --- some of which will be similar to the original combinations. A critical presumption in understanding either hybrid vigor or inbreeding depression, is that recessive alleles are generally unfavorable or at least less favorable than dominant alleles.
In reality, most recessive alleles are less favorable than their dominant counterparts, and the reason for this probably is connected with evolutionary forces over the millennia. Because of the type of genetic combinations involved there tends to be an inverse relationship between heritability and hybrid vigor. That is, those traits that have a moderate to high heritability tend to be associated with low to modest levels of heterosis, and those traits that are lowly heritable exhibit substantial amounts of heterosis. Traits which have a low heritability are usually improved dramatically through outbreeding.
There is much yet to be said about hybrid vigor or heterosis, but that will be the subject of another module. The two most important genetic reasons to outbreed are:
- To add hybrid vigor or heterosis.
- To take advantage of breed complementarity.
The most common form of outbreeding in meat goats is crossbreeding. Crossbreeding is most often practiced by “commercial producers’, those who are involved primarily in selling goats for meat. They need to use every tool that can increase production and production efficiency if they raise goats as an enterprise to create revenue for the household. Adding hybrid vigor and complementarity are important tools of the trade. Seedstock producers or breeders focus on supplying the seedstock inputs to commercial crossbreeding programs and are less concerned about heterosis and complementarity. Not all seedstock are purebred. Hybrid seedstock are available even within the meat goat sector. The Genemaster (3/8 Kiko+5/8 Boer) is an example of a composite or hybrid, which is designed to incorporate some level of heterosis in the seedstock.
Crossbreeding is a mating system that uses crossbreeding to maintain a desirable level of heterosis and(or) breed complementarity. Systematic crossbreeding does not lead to mongrels — animals composed of a hodge-podge of breeds. If you are involved in producing meat goats for sale as live animals ready for harvest, it is most likely you will benefit from a systematic crossbreeding program. The major task is to consider the appropriate system.
Crossbreeding systems should be evaluated on several criteria. These include:
- Merit and availability of the breeds to be used.
- Expected level of heterosis.
- Complementarity of the breeds available.
- Replacement stock considerations.
The basic merit of each breed used in a crossbreeding system is of critically importance. If alternative breeds are not superior in some aspect to the existing single breed, it is likely that crossbreeding will not be an appropriate choice in mating systems. You are better off sticking with the single breed. Working within a breed is certainly more simple. Generally the lack of merit is not a problem among breeds of meat goats. Scientific-based characterization of the several breeds of meat goats available has progressed to the extent of other livestock species. More useful information is becoming available annually. It is not unusual for meat goat producers to determine the optimal combination of breeds for their environment, only to learn that some of the breeds are not available near by, or at a reasonable price.
Generating heterosis is one of the most important reasons for crossbreeding. Generally, the more hybrid vigor the better. However, you do not want to include breeds in the cross just to generate that hybrid vigor and not give attention to the breed merit. Maximum heterosis is only obtainable in the first cross of unrelated populations; the F1. To sustain heterosis at that level (100% of what is available) you must avoid backcrossing completly. This is not possible in sustainable systems. Therefore the aim should be to sustain the hybrid vigor at some acceptable level.
Breed complementarity refers to the production of a more desirable offspring by crossing breeds that are genetically different from each other, but have complementary attributes. One breed may be known to add muscle to the carcass; another breed is known to be attentive mothers, another may have inherent tolerance to a prevailing disease, e.g. parasite resistance. Keep in mind that all breeds will contribute to the net merit of the offspring, but some breeds will have a more desirable influence in some environments than in others.
Because the ultimate level of performance is usually with the F1, the first cross between purebreds, commercial producers that are raising goats for meat would like to have nothing but those in the herd. The question then is how do they produce a continuous supply of F1s? Either you produce them yourself, by keeping some purebred stock of each breed or you buy them from someone else. Keeping the purebreds requires resources that could otherwise be used by the crossbred animals which in the end will be more productive meat producers. Buying from someone else runs the risk of introducing disease and the price may be relatively high when their value is known. Some crossbreeding systems generate their own replacements, but in doing so there is a loss in hybrid vigor, breed complementarity, and simplicity.
Crossbreeding systems should be relatively simple. It is important in a crossbreeding system to be in harmony with other aspects of the farm enterprise. If many breeding pastures are required it may interfere with the proper rotation of grazing areas and require additional fencing, which is an expense that may not impact profitability for the better.