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

by G.B Young (1953)

THE EFFECT OF THE LETHAL ON REPLACEMENTS

Replacements and breed structure

In studies on lethals, little attention has been devoted to their effects on replacements. Robertson & Asker (1951) have, however, recently emphasized the considerable reserve of fecundity possessed by dairy breeds of cattle. It would seem reasonable to suppose, therefore, that even if a lethal causes considerable calf losses, a breed may still be able to maintain its numbers and even expand. To investigate this problem, the Dexter breed would appear at first sight to be particularly suitable since the breed has existed for over 50 years.

It is obvious that, if Kerry-type females are not bred from, each Dexter-type female of breeding age must produce an average of four calves in order to produce a Dexter-type daughter to replace her. This calculation, however, makes no allowance for normal mortality. Robertson & Asker (1951) estimate from New Zealand figures that almost 10% of females die before calving or are sterile. If a 10% mortality occurs among Dexter-type heifer calves, only about 90% of such calves will produce offspring. Each breeding female will have to produce, therefore, 4.4 calves to maintain the breed numbers. This is a not unreasonable expectation of life for cows. In a recent summary, Reudel & Robertson (1950) conclude that the average productive life is approximately four lactations.

But an average reproductive life of 4 or 4.5 pregnancies is not the maximum that can be obtained. The Friesian average at the present time is about 5.4 lactations (Robertson & Asker, 1951). If a similar average be applied to Dexters, each female would produce, with normal calf mortality, 1 1/4 replacements. This would mean that the herd could double itself approximately every three generations, i.e. in about 15 years. This may be compared with Robertson & Asker's estimate that with present reproductive rates, the Friesian breed could double itself every 5 1/2 years.

Population dynamics

Before 1922, about 64% of the females were admitted to the herd book by inspection. After then, entry was made with much more difficulty, and the annual number of animals admitted via the grading-up register was only about 2% of the total. The main period of interest, from the point of view of breed numbers, therefore, is from 1922 to the present time.

To obtain a true measure of expansion or contraction for these years in such a small breed is, however, difficult. If only the annual numbers of registrations are considered, a false impression may be obtained due to postponements in registration. A similar objection applies to the total numbers listed in herds. Probably the best measure is the number of registered females born in any year regardless of the year of registration. The total number of calves born each year could also be used, but, unfortunately, since not all 'bulldogs' are recorded, the birth notifications are not complete. In Table 4 a combination of these methods is used to obtain a picture of the breed population changes.

Table 4: Population Changes in the Dexter Breed: Yearly Average Figures

The maximum number of females registered was in 1925. From then, all four measures showed a decline up to 1940. After that the females registered and calves born steadily increased. The total females also increased rapidly in 1946-8. The reduction in numbers had, therefore, been halted and an expansion begun.

(a) Rearing proportions
Robertson & Asker (1951) have recently pointed out that much of the expansion of the Friesian breed could be attributed to an increase in the rearing proportion from 0.26 to 0.37. By rearing proportion, they meant the proportion of births that produce a heifer that survives and is fertile. Obviously, the higher the rearing proportion, the greater the number of heifers available for replacement and expansion. Since they estimate normal mortality to be about 10%, a rearing proportion of about 37% is getting close to the maximum of 45%.

In Dexters, a direct count of this has been obtained from the birth lists for females born in the years 1921, 1930 and 1935, and subsequently registered. The results show rearing proportions of 0.41, 0.40 and 0.38 of Dexter-type births. It would appear, therefore, that in Dexters, even in a period of decline, an. attempt is made to raise practically all Dexter-type females to maturity. This may be contrasted with the Friesians in their period of comparative constancy of numbers (before 1935) when the rearing proportion was about 28%.

Only in a period of expansion did the Friesian rearing proportion approach that of Dexters in decline. This high Dexter rearing figure indicates that there is not a reserve of females born which could be reared if it was desired to expand the breed. Another explanation must, therefore, be looked for to account for the expansion from 1940 to 1947.

(b) Culling and age of Dexters
Table 5 gives the distribution of numbers of lactations of cows born in 1921, 1930 and 1935, and compares them with the grouped figures of Pettit (1940) and more recently of the M.M.B. [Milk Marketing Board] (1947) for several breeds.

The Dexter figures are similar to those of the other studies. The wastage rate in the cows is, therefore, not much different from that in other breeds. The average age of the cows in terms of parturitions (that is, the average of the number of parturitions which they have had at the time of counting) from 1921 to 1940 is estimated to have been 3.2. The average productive life (which is the average lifetime total of parturitions which any group of cows born at the same time will ultimately have) for the years 1921, 1930 arid 1935 was 3.8 - insufficient to provide replacements for a stationary population. The corresponding figures from the M.M.B. data are 3.1 and 3.9 lactations.

Since the average productive life of Friesians in 1947 was equivalent to 5.4 calvings, it seems that there might be a considerable reserve of fecundity in Dexters to be drawn on by increasing the average productive life of the cows, and this may have accounted for the expanding numbers in the 1940-47 period.
 

(c) The possible registration of Kerry-type females
Since breeding from Kerry-type females is not absolutely banned, it is possible that some breeders do breed from these animals. This was the method adopted by the early Irish breeders to avoid 'bulldog' calves (Crew, 1923). The Herd Book, unfortunately, does not lend itself to an exact determination of the number of Kerry-type females bred from. From the birth lists and registrations of 1924-43, it was found that 68% of the heifers notified at birth were subsequently registered. This would seem to indicate that 18% at least of Kerry-type animals were being registered. However, only about 10% of all heifer calves notified were recorded as having died, and only 1 or 2% were recorded as 'bulldogs'. It is clear, therefore, that not all females born are notified. The percentage of heifer calves subsequently registered will be less than 68% if the minimal allowance of 13-14% of unnotified 'bulldogs' is made. It can be concluded that, at most, only 7% of Kerry-type animals are registered.

The features discussed in the preceding paragraphs indicate that during the period of decline in numbers from 1925 to 1940, the Dexter breeders registered practically every Dexter-type female born, raised as many as possible to maturity, but did not keep the cows longer than any other breeds. They seemed to breed from comparatively few Kerry-type females.

The results of this policy can be soon in the total registered reproductive rate (the average number of registered heifer calves produced by each of a group of heifers themselves counted at registration (Robertson & Asker, 1951)). Since each Dexter cow mated to a Dexter bull produced a Dexter-type heifer calf only once in four births, and since it only has 3.8 births, and also allowing for normal death-rates, it is clear that the total reproductive rate must be below 1.

Table 6 shows 5-year averages of the actual reproductive rates for the female animals born in these years, and indicates how the total number of females born and registered has declined due to the low although fairly constant reproductive rate.

Table 6. Dexter reproductive rates

These reproductive rates and the main features of breed structure which cause them are summarized and contrasted with the corresponding figures for Friesians (Robertson & Asker, 1951) in Table 7.

Table 7. Reproductive rates in Friesians and Dexters

With an average reproductive life of only 3.8 years, it is clear that even with a rearing proportion near the maximum of 0.25, Dexter cows cannot replace themselves and breed numbers must decline. A reversal of this trend could be achieved only by keeping cows longer, or breeding from Kerry-types.

On to Part Three
 

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