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Inbreeding in Canadian Dairy BreedsBrian Van Doormaal, CDN General Manager With a greater intensity of selection by Canadian dairy producers and A.I. sire selection personnel, rates of genetic progress have steadily increased during the past decade. Although genetic progress is the key to breed improvement, changes in levels of inbreeding should also be monitored. Inbreeding results when two related animals are mated together. The progeny of this mating is inbred and is likely to have more genes in common compared to the offspring of two unrelated animals. The main consequence of producing an inbred animal is an overall decrease of its average performance, referred to as inbreeding depression. Research done using Canadian Holsteins has shown that for every 1% of inbreeding, the average cow will produce 25 kg milk, 0.9 kg fat and 0.8 kg protein less during first lactation in conjunction with a poorer reproductive ability. The same research found that this average response could vary from family to family; some with no negative effect of inbreeding and some with an effect two or three times the average magnitude. Based on all official pedigree information at Canadian Dairy Network, which dates back several decades, the average level of inbreeding within each year of birth was calculated for each dairy breed in Canada. Figure 1 shows the inbreeding trends for the Holstein, Ayrshire, Jersey and Guernsey breeds for animals born from 1968 to 1998. Statistics pertaining to this graph are presented in Table 1. Although the Ayrshire breed currently has the highest average level of inbreeding at 5.6%, the rate of increase has dropped from decade to decade. The Holstein, Jersey and Guernsey breeds all currently have an inbreeding level just over 4%. These breeds had relatively little accumulation of inbreeding until the past decade. For Holsteins in particular, the average level of inbreeding has been increasing by an average of one-quarter of one percent per year. Although this has been mainly due to the superiority of Hanoverhill Starbuck bloodlines in Canada, the same rate of increase in inbreeding exists in the United States and the current level of inbreeding in Holsteins is similar in both countries at about 4.5%. Table 1: Current Inbreeding Level and Change in Average Inbreeding for Each Dairy Breed in Canada
Controlling the accumulation of inbreeding in a population is mainly done at two levels. A.I. centres must maintain a balance between genetic superiority and diversity when selecting young sires. Diversifying young sire pedigrees without sacrificing high genetic potential is the most powerful tool for controlling inbreeding. The second way to control inbreeding is at the farm level. At the time when each cow is mated an awareness of how inbred the resulting calf will be can ensure that high levels are avoided. Figure 2 lists five simple matings that should be avoided and their associated level of inbreeding. Although there are other more complex matings which can lead to significant levels of inbreeding, these five matings represent the major cases to avoid. Given the current level of inbreeding in all major breeds in Canada, it is suggested that matings which result in more than 6% inbreeding should be avoided. The majority of the A.I. centres in Canada offer a computerized sire recommendation service which will assist producers in controlling inbreeding. Obviously, producers should also maintain diversification in the young sires and proven sires they use in their herd. This approach will allow the flexibility that is required to ensure a minimum usage of all elite proven sires, without substantially increasing levels of inbreeding in the herd. Figure 2: Simple Matings to Avoid for Controlling Inbreeding
Paternal Grandsire
Normally, the notion of inbreeding makes producers panic, most likely due to a lack of understanding. Inbreeding levels are not automatically transmitted from generation to generation. It only exists in the progeny of parents which are related to each other by a common ancestor. The first step to controlling inbreeding is monitoring it after which time A.I. personnel and producers can make their selection decisions accordingly. | ||||||||||||||||||||||||||||||||||||||||||||
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