Inbreeding and Linebreeding

The concept of inbreeding and linebreeding are often misunderstood by cattle producers. Inbreeding usually has a negative connotation due to the fact that humans are encouraged to avoid inbreeding.

Inbreeding in livestock species is, however, quite common. Inbreeding is the mating of individuals that are related. In the broad sense, all members of a breed are related. As a result, any seed stock producer is practicing some level of inbreeding. Therefore, we generally reserve the term inbreeding for the mating of animals that are more closely related than the average of the breed.

Linebreeding does not have the same negative reputation as inbreeding, but it is a mild form of inbreeding. Linebreeding is the mating of individuals within a particular line. It is a mating system designed to maintain a substantial degree of relationship to a highly regarded ancestor without causing high levels of inbreeding.

Animals that are related have a certain percentage of their genes in common. For example, brothers and sisters will have 50% of their genes in common due to their relation. Inbreeding is a measure of how many genes are identical by descent, which is the opposite of crossbreeding.

The main benefit of inbreeding is increased uniformity in offspring. Since inbreeding increases the number of genes in common, these animals will have more similar performance numbers. Most breeds have used inbreeding to fix such traits as coat color or horn status.

There are some negative aspects of inbreeding as well. As the amount of inbreeding increases, so does the occurrence of genetic defects (see Curly alf Syndrome in this issue). Also evident in inbred populations is an issue called inbreeding depression, which is a decrease in performance of inbred animals – most noticeable in traits like fertility. Inbreeding depression is the opposite of heterosis or hybrid vigor that is accomplished by crossbreeding.

Linebreeding has the advantage of maintaining genes from outstanding individuals that are no longer available for breeding purposes. Also, it helps in recognition, since the breeder can use the name of the ancestor at the center of linebreeding. This is fine if this ancestor is truly outstanding.

Let's look at an example of linebreeding:

In this example, the linebred offspring is 47% related to sire A, but is only moderately inbred. This level of relationship is almost that of full siblings. This type of breeding can be beneficial to purebred breeders who are trying to produce calves that are similar within year and between years. It does take some study to minimize any issues that may be present due to inbreeding. If you have any questions or would like to have a more in-depth discussion on inbreeding and linebreeding, please feel free to contact me.

In the past few years, sex-sorted semen has made its way from the lab and field trials to use in beef cattle production. Shifting the sex ratio for a calf crop has been tried several different ways, but the idea of sorting or selecting semen to favor male or female offspring has been researched since the advent of artificial insemination. The fact that makes this concept possible is that the fertilizing sperm cell determines the gender of the calf. Due to the genetic makeup of the cells, sperm that produce female offspring are referred to as X-bearing sperm while those that produce male offspring are referred to as Y-bearing sperm.

There have been several attempts to develop a method that efficiently separates bovine semen into fractions containing higher concentrations of X- or Y-bearing sperm. These technologies have utilized sex-specific antibodies, centrifugation and flow cytometry. Of these attempts, the only one that has proven to be commercially viable is flow cytometry. This type of sorting was first researched in the 1980s but yielded very low conception rates when the semen was used fresh after processing. Work in the laboratory and field has improved the results, and the first gender-selected calf using frozen semen was produced in 1999. Eventually, sexed semen became commercially available on a large scale in the U.S. in 2004.

The inefficiency of flow cytometry comes from its complexity and slow pace. The principle of this method relies on the fact that X-bearing (female) sperm contain 3.8% more DNA than Y-bearing (male) sperm. Before sorting, the sperm cells are stained with a fluorescent dye and then passed through the flow cytometer as drops of liquid containing a single sperm cell per droplet. Because of the difference in amount of DNA, the X-bearing sperm shine brighter than the Y-bearing sperm when exposed to light. This allows the cytometer's laser and detector to determine the gender of the sperm cell based on the amount of light it emits. A positive or negative charge is then applied to the droplet containing the single sperm cell.

Positively charged drops are deflected one way, negatively charged drops deflected the other and uncharged droplets pass straight through. The uncharged drops may contain multiple sperm, damaged material or cells that were not aligned in the proper direction.

The sperm cells pass through the machine at about 60 miles per hour. This seems fast but, considering that they pass in single file and one ejaculate can contain more than 7 billion sperm, it takes about three to four times longer to process sex-sorted semen than conventionally processed semen. Therefore, this technique yields fewer straws of frozen semen per ejaculate at an increased cost and results in lower conception rates. Current research shows that, in an ideal situation, pregnancy rates will be about 70 to 90% of that for cows or heifers bred with non-sorted semen. This sorting method is not perfect, but it does shift the ratio to about 85 to 90% of the desired sex. As with any other market-driven technology, it is reasonable to consider that sex sorting will evolve to become more efficient and less costly. In the meantime, the benefit from shifting the sex ratio of a calf crop has to be weighed against the increased cost and lower fertility.

In some regards, sex-sorted semen is more applicable to the dairy industry. Milking herds are almost always more interested in producing replacement heifers while bull calves are drastically less valuable. For purebred beef cattle producers, bull sales to commercial cattlemen are often the most significant source of revenue, while heifers are also important for genetic improvement as replacements. For producers who rely on bull sales and require fewer replacement heifers (or purchase bred replacements), breeding with Y-bearing (male) sorted semen could be a practical management decision.

Use of sex-sorted semen should be reserved for herds where reproductive efficiency has been optimized through intense reproductive management. Pregnancy rates to sexed semen will be highest in virgin heifers that are bred 12 hours after the beginning of standing heat, and mass insemination or timed breeding is not an acceptable method. Semen handling is similar to traditionally processed semen, even though it will likely be packaged in a ¼ cc straw as opposed the usual ½ cc straw. Keep in mind that semen handling during storage and just prior to insemination is equally critical for success as proper insemination technique. Most of the research to this point indicates that the number of transferable embryos produced by using sex-sorted semen in super-ovulated cows will be half that of conventionally packaged semen.

With new technologies that have some potential for controversy, it is important to understand the under lying science before making a judgment on its ethical implications. Sex-sorted semen is not genetically engineered or modified. It is a natural product that is simply separated into fractions that contain a higher percentage of sperm cells that will produce either male or female offspring.

Calves from these matings do not have an increased risk for death or abnormalities compared to calves from conventional artificial insemination or natural breeding.

Source: Justin Rhinehart, MSU
Extension Beef Cattle Specialist