Inbreeding is generated by the mating of related individuals. The objective of inbreeding is to concentrate the desirable genes of a family such that they are consistently transmitted to offspring. Unfortunately, inbreeding will concentrate some of the undesirable genes as well. This may result in the occurrence of lethal genetic traits. In general, the net result of inbreeding is known as "inbreeding depression". Research has shown that each percentage increase in inbreeding in dairy cattle results in a 22 kilogram decrease in milk production, and may have greater impact on reproduction and calf viability.

Not all inbreeding is bad. Inbreeding has been a useful tool in plant breeding and in the poultry industry where the occurrence of undesirable genes is quickly overcome by short generation intervals and vast numbers of offspring from single matings. Dairy cattle do not have these advantages, and therefore inbreeding could have more long lasting effects. Mild forms of inbreeding have been used in dairy cattle. For example, some of the Roybrook sires in the Holsteins breed have been inbred. When these sires were bred widely to unrelated cows in the population, the offspring performed fairly well. In general, inbreeding should be done with caution.

One factor that determines the rate of increase in inbreeding in a population is the number of breeding bulls and cows in the population. As population size decreases, the rate of inbreeding tends to increase, and therefore one might suspect that inbreeding rates in Ayrshire, Jersey, and Guernsey might be greater than in Holsteins. While the number of breeding cows in a population is very large, the number of breeding bulls has been reduced substantially by artificial insemination (AI). While there are approximately one million dairy cows in the Holstein breed, there are only 300 young sires progeny tested per year, from which 30 may be kept for more extensive use. Over 83% of registered Holsteins are the product of AI breeding. In addition, the AI studs each have sons from the same proven sires. There appears to be a narrowing of the genetic base in that the pedigrees of today's young bulls can be traced back to relatively few sires. Perhaps there should be limits on the number of breedings that each AI sire is allowed to have, and limits on the number of sons in total from each proven sire?

How bad is the inbreeding rate today? A study by Filippo Miglior and Ted Burnside at the University of Guelph addressed this question for the Holstein and Jersey breeds. The data consisted of approximately 195,000 Jersey cows and bulls born since 1955, and 1.1 million Holstein cows and bulls born since 1982. Another factor that influences the amount of inbreeding is the point from which you start to do computations. The pedigrees in the Jersey breed went back to 1950 while those in the Holstein breed were traced back to 1970. Thus, if the Holstein pedigrees could have gone back to 1950, then the amount of inbreeding may be greater than that found in the study.

Looking only at the bulls, 32% of the 2,300 Jersey bulls and 5% of the 4,300 Holstein bulls were inbred. The average inbreeding percentages of all bulls were 0.15% in Holsteins and 1.2% in Jerseys. The researchers did not look at the inbreeding percentages of the most recent group of AI bulls in each breed. The average inbreeding percentages in the cow population were 0.3% in Holsteins and 1.2% in Jerseys, and the percentages of inbred cows were 10% in Holsteins and 36% in Jerseys. In looking at the average percentage inbreeding by year of birth in the cows, Miglior and Burnside found a decrease in inbreeding percentage in the Jersey breed, and a slight upward trend in Holsteins. They concluded that inbreeding was not a major problem at this time.

Recently, multiple ovulation and embryo transfer (MOET) schemes have been initiated in the Ayrshire and Holstein breeds in Canada. The objective is to select the very best cows and to generate cow families through embryo transfer. Some of the embryos have been split to produce identical twins, and in theory many identical individuals could be produced from one embryo. By reducing the number of breeding females through MOET schemes, there is a further potential for increasing the inbreeding rate in a population. Perhaps MOET should not be used in small populations? When does inbreeding become a serious problem? Researchers will likely continue to monitor inbreeding rates very closely in the future. If you are concerned about the level of inbreeding in your herd, then write to Dr. E.B. Burnside to obtain a summary of inbreeding percentages for your current animals, with recommendations for future breedings.