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"Population Dynamics of the Dexter Breed of Cattle" - Part Three

by G.B Young (1953)

THE EFFECT OF THE LETHAL ON SELECTION AND INBREEDING IN DEXTERS

Selection of dams of bulls
 

In a breed free from a lethal and subject to mass selection, most progress is made by selection of dams of bulls. In Dexters, selection of dams of bulls may not be as effective as in other breeds, since the cows selected for breeding will only produce one suitable Dexter-type bull in four calves. This means fewer bulls from each good dam, and more dams will be required to produce the requisite number of bulls, thus lowering the selection differential. Between 1921 and 1943, 13% registered females produced registered bulls. Corresponding figures for other breeds cannot be given, but generally it will be found in dairy cattle that the number of bull registrations is about 20% of the number of female registrations. If the total cows in the breed is about 2.5 times the number of female registrations, then the percentage of cows producing registered bulls each year is about 8%. It would appear, therefore, that the fraction of cows producing registered bulls may be greater by about a half in Dexters than in other breeds. This effect may, however, also be associated with the small herd size in Dexters.

Table 8: Differential distribution of registered male and female offspring of bulls (1924-1943)

Despite the greater number of dams of bulls required, there is still room for considerable selection. By studying the birth lists and registrations between 1924 and 1943, it was found that of the total male births, 16% only are registered, and only 7% produced registered male offspring, and 11% registered female offspring. The average age of the dams of these bulls was 5.2 years, so selection could be based on the dams' production.

Selection of sires of bulls and cows

Between 1926 and 1945 the average age of sires at the birth of their sons was 3.8 years, and only 13.5% were over 4.25 years. The corresponding figures for the birth of daughters were 3.5 years and 12%. In Dexters there are difficulties additional to those normally encountered in attempting progeny testing in small herds. If only Dexter-type female calves are retained, twice the usual number of dams would be needed. This increase from the twelve or fifteen generally considered desirable to twenty-four or thirty, means that few herds are large enough to test even one bull each year.
It has already been mentioned that 7% only of all registered males are selected as sires of bulls, and 11% as sires of cows. It remains to show the distribution of male and female offspring from these bulls.

3% of sires, leaving more than six bull calves each, account for 19% of the offspring. At the other end of the scale 76% of the sires, leaving one or two offspring, account for only 46% of the male progeny. The average number was 2.3. The 10% of sires leaving more than ten female offspring account for about 30% of the offspring, whereas the 38% who have one or two female offspring account for only 12% of the calves. The average number was 4.6. These figures indicate that, as in other breeds, the influence of a small percentage of sires may be out of all proportion to their numbers.

Selection of dams to breed dams

The contribution which this can make to genetic advance is relatively not great, yet Rendel, Robertson & Alim (1951) estimate that in dairy breeds in Britain it constitutes about 25-50% of the progress actually achieved. In Dexters, since the culling level and wastage rate is about the same as in other breeds, a similar contribution to the rate of progress would be expected.

The general conclusion emerges from this discussion that despite the presence of the lethal gene, selection can still be exerted in the breed, mainly through selection of bulls on their dams' production. Even when the lethal gene removes 50% of the bull crop for breeding, the percentage required is so small that there is still room for considerable selection.

The degree of inbreeding in Dexters

The methods used were based on the technique described by Robertson & Asker (1951). The year analysed was 1947, and the pedigrees of the eighty-six heifer registrations were taken back for six generations. The results are shown in Table 9.

Table 9

The estimated total inbreeding is only 2.4%, which suggests that inbreeding is avoided, possibly because, in the breeder's mind, inbreeding is associated with the production of monstrous calves. The effective population size calculated from the inbreeding is equivalent to twenty-five males. The comparatively large resultant expected long-term inbreeding indicates that strain differences are not important.

The low degree of inbreeding implies that there have been no animals with a predominant influence on the breed. Indeed, only one animal, Ratcliff's Negro Manager, born in 1930, has a direct relationship to the breed of more than 2%, the actual figure being 5.2%. Despite this, there is however one important herd. 40% of the lines trace back through the Grinstead herd, founded about 1912. Its influence is at least partially due to its large size and long duration.

On to Part Four
 

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