Original
Article About Dexters
"Population Dynamics of the Dexter Breed of Cattle" - Part Four
by G.B Young (1953)
SIGNIFICANCE OF
DEXTER BREED HISTORY FOR OTHER BREEDS
It would appear that the damaging effect of a lethal gene which does not
frequently result in maternal death at parturition could be overemphasized.
This does not mean that any effort should be spared to prevent lethals from
becoming widespread, or once they have, to reduce their frequency. Calf
losses, however, from all causes can even exceed 50% (as in Dexters) without
extinguishing all hope either of breed expansion or even of improvement by
selective breeding.
SUMMARY
1. It is possible for a breed of cattle to persist and to expand slowly,
despite the existence of a genetic defect which prevents 50% of the calves
born being used for breeding. Moreover, it is possible to exert a certain
amount of selection in such a breed. The Dexter breed provides an example.
2. Before 1924, about 64% of entrants to the breed were from inspected
animals. Between 1924 and 1939 the breed contracted. From 1940 to 1947 the
breed expanded.
3. During the period of decline almost every Dexter-type female born was
registered and raised. The wastage rate and age distribution of the cows are
similar in Dexters to those of other breeds. There would appear to have been
comparatively little breeding from Kerry types. The average registered
reproductive rate of Dexter-type females was about 0.8.
4. While it was not possible to examine the breed structure in the period of
expansion, this was probably accomplished by lengthening the average
productive life of the female, and possibly also by breeding from Kerry
types.
5. The possibility of exercising selection in Dexters is shown to exist.
Only about 13% of the females registered are used for producing registered
bulls. Of the total bull crop, only about 7% are used to produce registered
females, but the bulls selected have different numbers of offspring. The use
of bulls selected after progeny testing is hardly feasible in Dexters. The
increased numbers of females necessary in order to produce an adequate
number of heifers, renders this technique almost impossible.
6. There has been little inbreeding in Dexters. This is possibly due to
deliberate policy, as some of the breeders associate inbreeding with the
production of monstrous calves.
7. It would, therefore, appear that the damaging effect of a lethal in a
breed could be overemphasized.
REFERENCES
CREW, F. A. E. (1923). Proc. Roy. Soc. 95,
228.
DONALD, H. P. (1945).
Emp. J. Exp. Agric. 13, 169.
DONALD, H. P. & EL
ITRIBY, A. A. (1945). J. Agric, Sci. 35. 84.
DONALD, H. P. & EL
ITRIBY, A. A. (1946). J. Agric. Sci. 36. 100.
MILK MARKETING BOARD (1947). Annual Report.
PETTIT, G. H. N.
(1940). J. Agric. Sci. 30, 485.
RENDEL, J. M. &
ROBERTSON, A. (1950). Emp. J. Exp.Agric. 18, 49.
RENDEL, 3. M., ROBERTSON, A. & ALIM, K. A. (1951). Emp. J. Exp. Agric. 19,
295.
ROBERTSON, A. & ASKEB,
A. A. (1951). Emp. J. Exp. Agric. 19, 191.
WIENER, G. (1950).
Aspects of Population Dynamics in Pedigree Ayrshire Cattle. Thesis.
University of
Edinburgh.
YOUNG, G. B. (1951).
Vet. Rec. 63, 635.
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