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"Population Dynamics of the Dexter Breed of Cattle"

by G.B Young

This article was published in The Journal of Agricultural Science, Volume 43, 1953, pages 369-374.

G.B. Young was listed as from the Animal Breeding Research Organisation in Edinburgh.

CONTENTS

Part 1 - Introduction; The Characteristics of the Population Recorded in the English Herd Book (1900-1948)

Part 2 - The Effect of the Lethal on Replacements

Part 3 - The Effect of the Lethal on Selection and Inbreeding in Dexters

Part 4 - Significance of Dexter Breed History for Other Breeds; Summary; References

INTRODUCTION

The Dexter breed of cattle has attracted interest because of the lethal 'bulldog'. While the pathology of the condition has been studied in some detail, the effect of the gene on breed structure has been little investigated. Regular loss of more than half the heifer calves because they are either deformed or of Kerry type (Young 1951) must, however, have had its repercussions on breed structure, and it has been the main object of the analysis reported in this paper to discover them.

The chief effect of these losses will centre round replacements and breed numbers, but other factors such as herd duration and selection will probably also have been affected. A general statistical study of the breed has therefore been undertaken.

THE CHARACTERISTICS OF THE POPULATION RECORDED IN THE ENGLISH HERD BOOK (1900-1948)
 

Herd number and size
 

The herd book contains a list of all the female animals in members' herds each year. The numbers may, therefore, be counted directly, and they are shown in the third column of Table 1 (below). The general picture is one of expansion from 237 females of all ages in 1900 to 1119 in 1925, with a subsequent contraction to 317 in 1940, after which numbers increase again.

Table 1: Herd Characteristics of the Dexter Population

Table 1 also shows some herd characteristics of the breed, and compares their fluctuations with those of the total females in the population. Table 2 (below) compares these features with those of Ayrshires and Red Polls, the comparisons being made at an 'early' and a 'late' period in the three breeds' histories.

Table 2: Herd Characteristics of Dexters Compared with Ayrshires and Red Polls

The numbers of herds in Dexters has, in the selected years, never exceeded seventy, and for those years the average number has been about thirty-four.

Dexters are characteristically kept in small herds. Assuming that about half the herd consists of young stock, the mean pedigree Dexter herd contains only six cows. This may be compared with the corresponding figure of about twenty for pedigree Red Poll herds which are representative of pedigree herds in general. Pedigree Ayrshire herds are considerably larger as a rule than those of other breeds and average about thirty cows.

Moreover, as would be expected from the overall average herd size, the percentage of herds with more than forty females of till ages, and the percentage of the total females in these herds, is considerably smaller than in Ayrshires or Red Polls.

In contrasting the 'early' periods with the 'late' periods, in Red Polls and in Ayrshires there has been a steady increase in average herd size, in the frequency of herds with more than forty females, and in the percentage of animals contained in those herds. In Dexters, all of these characteristics have remained more or less stable.

There is a permanent core of larger herds which always hold about 22% of the breed. This comparative constancy of herd size, in spite of considerable fluctuations in total numbers and number of herds, is probably to be expected in a 'smallholder's' cow. Increase in numbers would be associated chiefly with more breeders, rather than with expansion of both numbers of breeders and herd size, as occurred in Ayrshires and Red Polls.

More generally, these observations on herd size in Dexters suggest that the breed must suffer more than usual from difficulties arising from small herd size (Donald & El Itriby, 1946). In small herds, the small numbers of the progeny reduce opportunities for selection. Adequate progeny testing is well nigh impossible, and random gene fluctuations play a large part.

Duration of herds
 

One measure of duration of herds is the average length of life of all herds started in a certain period of time. This has been calculated for Dexter herds begun in the period 1925-30. The figure obtained is shown as average expectation of life in the first line of Table 3, and compared with similar data for Ayrshires and Shorthorns (Donald & El Itriby, 1946).

Table 3: Duration of Dexter Herds Compared with Ayrshires and Shorthorns

The expectation of a life of 4.7 years is very short, but over the relevant period the breed was slowly contracting. As can be seen from Table 1, the number of herds was being reduced. A very short herd life was, therefore, to be expected.

Another measure of duration is provided by the average age of herds existing at any given time, that is, the average length of time since they were established. Since it is not possible to estimate the expectation of life of herds recently started, this second method has been adopted to describe the herds functioning in 1947 which followed a period of moderate expansion starting in 1940. As Table 3 shows, average age in 1947 was greater than average expectation of life in 1925-30 - which is the result to be anticipated from conditions favourable to breed expansion and herd life.

In both calculations, the very short average duration of the great majority of Dexter herds stands out clearly. Whether this short herd duration is associated with the presence of the 'bulldog' calf is impossible to state, but it is certain that it affects Dexter herds as much as those of other breeds. The majority of herds do not last sufficiently long for their owners to do much constructive breeding.

The geographical distribution of the breed

The geographical distribution of the breed has shown little change over the last 50 years. Dexter cattle are found moat commonly in the South of England with an outlying, but persistent scattering of herds in Wales and the North of England, and occasional herds elsewhere. If from 1900 onwards, at 5-year intervals, the total females are summed by counties, Sussex, Essex, and Gloucester each account for about 10% of the females, and Kent, Yorkshire, Wiltshire, Norfolk and Suffolk for about another 30%. The remainder is very thinly scattered.

On to Part Two
 

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