There are two kinds of decisions that meat goat breeders must make. They first must make decisions such as which individuals become parents, how many offspring they may produce, and how long they remain in the breeding population. Meat goat breeders then must also decide which bucks to breed to which does. That is a mating decision.
Mating decisions fall into two general categories, inbreeding and outbreeding. Inbreeding is most often practiced by purebred breeders who are producing the parent stock for commercial meat producers. Even linebreeding is a mild form of inbreeding. The other category is outbreeding, which includes the often discussed practice of crossbreeding.
Crossbreeding can be used for upgrading, i.e. moving from breed to another, or “upgrading” from common stock to more superior stock. This is accomplished by backcrossing.
Crossbreeding can be used in a perpetual system to produce market stock and replacement females at the same time. There are some significant advantages seen in using this mating system for meat production. The advantages have been documented in other livestock, and there is a growing body of evidence that suggests the advantages apply for goats raised for meat too. A number of practical systems are available.
Two major factors make systematic crossbreeding effective: a.Combining the attributes of two or more breeds (There is a practical limit to the number of breeds that can contribute positive aspects.) and b. Taking advantage of hybrid vigor (heterosis or the condition where the offspring show more vigor or growth than the average of the parents), which in a sense provides a free boost for some traits.
Crossbreeding also can be used to produce the foundation for new breeds, synthetics, or composites. The following website provides many useful details of the mating systems available to meat goat breeders and producers: (http://www2.luresext.edu/goats/training/appendixI.html).
Mating systems should be evaluated on several criteria. These include: •Merit and availability (physical or financial) of the breeds to be used. •Expected level of heterosis. •Complementarity of the breeds available. •Replacement stock considerations. •Simplicity.
Some specific crossbreeding examples are provided below.
Rotational crossbreeding systems
Rotational crossbreeding systems involve rotating sire breeds across the female population. Such systems produce replacement females internally, yet manage to maintain acceptable levels of the original heterosis. Either purebred sires or crossbred sires can be used. You can use the breeds of sires simultaneously by placing them in separate physical locations; or you can use the breeds sequentially over time. Any number of breeds can be involved but generally the system involves only 2 to 4 different breeds. Although adding more breeds will maintain heterosis at a higher level, it may be a challenge to identify more than four breeds that compliment one another.
Two-breed rotational system
A two-breed rotational system, also known as a crisscross system, represents the simplist system available. Suppose you have chosen to use the Kiko and the Spanish breeds to use in the rotation. The first step is to make the initial cross. Then backcross to one of those original breeds. The female offspring from that backcross will then be mated to the other breed; then back to the first breed. By following this system you will maintain about 67 percent of the original 100 percent level of heterosis. The actual result in terms of measurable differences and units will differ from trait to trait. This system is illustrated here using a rotation over time.
Two-breed Rotational System
Generation 1 – Kiko x Spanish = (.5 Kiko + .5 Spanish) Generation 2 – (.5 Kiko + .5 Spanish) x Kiko = (0.75 Kiko + .25 Spanish) Generation 3 – (.75 Kiko + .25 Spanish) x Spanish = (.375 Kiko + .625 Spanish) Generation 4 – (.375 Kiko + .625 Spanish) x Kiko = (.6875 Kiko + .3125 Spanish)
After about the 3rd generation, the relative percentages in breed composition will be about 2/3 for the most-recently-used breed, and 1/3 for the breed next in rotation. These same crosses can occur simultaneously if the system is applied on a spatial basis where different herds of goats are in different locations.
Three-breed rotational system
Three-breed rotational crossbreeding increases the level of complementarity, and the level of hybrid vigor maintained after the first crosses. In this system, approximately 86 percent of the original level of heterosis will be maintained on average over time. In this next illustration, the Boer has been included as the third breed. Three-breed Rotational System Generation 1 – Kiko x Spanish = (.5 Kiko + .5 Spanish) Generation 2 – (.5 Kiko + .5 Spanish) x Boer = (.5 Boer + .25 Kiko + .25 Spanish) Generation 3 – (.5 Boer + .25 Kiko + .25 Spanish) x Spanish = (.25 Boer + .125 Kiko + .625 Spanish) Generation 4 – (.25 Boer + .125 Kiko + .625 Spanish) x Kiko = (.125 Boer + .5625 Kiko + 3125 Spanish) Generation 5 – (.125 Boer + .5625 Kiko + 3125 Spanish) x Boer = (.5625 Boer + .28125 Kiko + .15625 Spanish) Generation 6 – (.5625 Boer + .28125 Kiko + .15625 Spanish) x Spanish = … and, so on.
After about the 5th generation, the relative percentages in breed composition for the offspring will be about 57% for the currently used sire breed, about 28% for the next most recently used breed, and 15% for the breed next in line for mating. The complementary effects will increase slightly and the level of retained heterosis will increase as compared to the two-breed rotational scheme. Crossbred replacement does will be produced internally from the mating plan.
Terminal sire systems
Terminal sire systems are systems applicable where breeds are unquestionably identified as maternal-breeds, which excel in maternal traits like conception rate, number born, milk yield, and that intangible term, mothering ability; or paternal-breeds, which excel in traits like growth rate and carcass yield.
Within the meat goat breeds, there are individual animals that tend to be more balanced in terms of maternal and paternal features for all breeds. However, data from Teneesee State University indate that the Boer may excel in growth and carass trais while the Spanish and Kiko may excel over the Boer in maternal characteristics. One more extreme example is that, if they were used, the Nubian breed would have to be considered a maternal breed. It would be more difficult to identify a similar breed on the paternal side because of the balance of traits. Terminally sired females are not kept as replacement, but are sold as meat animals because there will be other breeds which will do better on the maternal side. While these terminal systems produce ample amounts of hybrid vigor, their most important attribute is breed complementarity. There are two approaches to terminal systems; static terminal systems and rotational/terminal systems.
Static terminal-sire crossbreeding system
The static terminal-sire crossbreeding system is considered static because the proportional breed composition does not change over time as it does with rotational systems. The system does not provide for replacement females internally. Obtaining those replacement does is the most difficult aspect. A static terminal system that uses purchased does is very simple from a management standpoint. The system produces a lot of hybrid vigor. An example of this system is shown in the next box.
Static Terminal-Sire System
Generation 1 – Spanish x Boer = (.5 Spanish + .5 Boer) Generation 2 – (.5 Spanish + .5 Boer) x Tennessee Meat Goat = (.5 Tennessee Meat Goat + .25 Spanish + .25 Boer) The two-breed males are harvested. The females in Generation 1 are used as breeding females to produce Generation 2 market stock. All animals in the three-breed group will be harvested.
Rotational/terminal systems are designed to solve the replacement problems associated with static terminal systems. They combine a maternal rotation for producing replacement females with terminal sires for producing market offspring. A portion of the goat herd is bred to “maternal sires” to produce the replacement does. The remaining does are bred to terminal sires to produce market offspring. Obviously the males from the maternal sires will be marketed too. The system is illustrated below: Two-breed Spatial Rotational/Terminal System Nubian x Boer = (.5 Nubian + .5 Boer)x Boer = (.75 Boer + .25 Nubian) x Nubian = (.375 Boer + .625 Nubian)x Boer, and so on for replacement does x Kiko = (.50 Kiko + .25 Boer + .25 Nubian) all of which go to market for harvest
This system provides the breed complementarity that would be missing from purely rotational systems, and the crossbred replacement does missing from a purely terminal system. Approximately 25 percent of the two-breed does would stay in the system as replacements.
Rotational/terminal systems provide more hybrid vigor and breed complementarity than comparable rotational systems, but less than comparable static terminal systems. Whenever you combine two crossbreeding systems, you can expect the combination to be more complex than its separate parts. In addition to the requirements of the rotation, an additional pasture is needed to accommodate terminal matings. Using a rotation in time would simplify the rotational component. Using artificial insemination on one of the groups would reduce the number of breeding locations required.
Composites are derived from crossbred foundations. They can be considered new breeds. Although developed initially through various crossbreeding systems, the intent of a composite base is to develop a new breed. The simplest way to use composite animals in commercial breeding is as one breed. Once the breed is closed (usually after about three generations of inter se matings (that is, matings among the crossbreds themselves), there is no longer any crossbreeding. A composite breed can be considered a breed made up of two or more component breeds and designed to benefit from hybrid vigor without crossing with other breeds. Development of new composites should be based on research information regarding the true need for both paternal and maternal characteristics within the new breed. Research should be conducted on the optimal mix of breeds to obtain the desired mix of characteristics— that is, what the foundation breeds should be, and in what the proportion.
Pure composite systems can produce considerable hybrid vigor. When two F1s (first offspring of the cross) are mated to produce F2s (second generation of the cross), half of the F1 heterosis is lost but half still remains in the F2, F3 and subsequent matings. This remaining 50% of the original level of heterosis is retained in what becomes a two-breed composite. A four-breed composite is expected to retain about 75% of the F1 level of heterosis. This is on the assumption that inbreeding within the composite is kept at a minimum. As a point of clarification, F2s are produced only through inter se matings of the F1 (first generation offspring are mated to other first generation offspring), and F3s are produced only through inter se matings among F2 individuals. Inter se matings are mating only among themselves. These terms should not be used simply to designate generations. They convey information about what type of animals that went into them. Livestock people, including meat goat producers, often make this error when speaking of various generations in crossbreeding.
The amount of vigor retained in a composite depends on the number and proportions of component breeds in the composite. This can be proven mathematically. Because the goats within a pure composite system are all of the same biological type, there is little opportunity for breed complementarity between the sire and dam, i.e. the newly-formed breed may have a deficit but it will not be corrected within the composite breed.
Once the composite is formed, the management and operational aspects are similar to any other pure breed — relatively simple. Keeping several breeds around is no longer necessary, and the advantages of retained hybrid vigor can be used even in small herds. On the other hand, developing a new composite breed is not simple. Assembling the composite can be very complex, and should be based on scientific studies done in advance for justification. If it is done right, a predictable four-breed composite population can be created in seven generations.
The usual justification for composites is to fit in a specific environment where suitable alternative breeds do not exist. Whenever the environment poses special challenges, there is an opportunity for an appropriately designed composite breed.
Utilizing studies to determine which breeds should be involved and in what proportions, the following steps are required to create a composite: 1.Select the foundation animals – The foundation animals set the base on merit. 2.Make the initial crosses – Apply selection to replacement females. 3.Make subsequent crosses to reach the desired proportions 4.Mate within the population – Apply selection to replacement females 5.Mate within the population – Apply selection to replacement females 6.Mate within the population – Apply selection to replacement females 7.If the new composite is suitable, begin the controlled release of breeding stock for multiplication.
In developing a useful composite breed, it is critical that those individual animals and families of animals within the foundation that are not performing be rigorously and quickly culled. Nothing can kill the success of a new composite breed better than nonperformance of too many animals in those early generations. New meat goat producers may try a composite one time, but they will not stay with it if too many animals fail to reach expectations. Another key is to begin with a large population. This will help manage the rate of inbreeding. Unfortunately there is no single answer to the question, how large is large enough. Apply selection with extensive culling at each successive generation.