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116
figure for calculating the number of calvings of home-reared heifers. To the number of calvings must be added the importation of cows and heifers from Ireland and elsewhere, the sum representing the total number of cows and heifers drafted into herds in Great Britain for the first time during the year in question. Figures for the imports of cows and heifers are given only for calendar years, and have to be adapted to the year ending on the 4th June. No account can be taken of the external trade in cows and heifers of Northern Ireland.
The wastage of cows and heifers from breeding herds in Great Britain. 1928-29 to 1930-31.
not in milk beginning during the
at end of
Estimated number of cows and belfers imported during the year.
Net increase (+) or decrease (-)
in the size
of herds
during the year.
Wastage of
cows and heifers in
milk and
Wastage
25
cows in calf per cent. of herd but not in milk during the year.
(4) + (5)
-
at
beginning
of year.
Number of
cows and heifers in
Estimated
Year ending
milk and
Number of heifers
number of
cows in
in calf at
4th June.
calf but
calvings of home-reared heifers
of year.
year.
year.
(1)
(2)
(3)
(4)
(5)
(6)
1928
2,775,190
---
1929
2,750,190 407,515 560,500
40,900
25,000
626,400 22.6
1930
1931
2,719,825 416,102 572,300 2,762,735 408,171 559,900
44,600
- 30,400
647,300 23-5
46,500
+ 42,900
562,600 20.7
(8). (7)
(8)
...
The average wastage for the three years is 612,100, or 22.3 per cent. of the average herd at the beginning of the year.
The principal objection that may be brought against this calculation is that it assumes a greater degree of accuracy in the returns of farmers to the agricultural census than can be reasonably anticipated. The variation shown between different years is itself an indication of this. It is not, however, obvious in which direction, if in either, there is likely to be a permanent bias.
A further objection is that the above figures include beef cattle as well as dairy cattle. Dr. Wright's article gives reason for supposing that cows in beef herds have a rather longer life than those in dairy herds.
In view, however, of the fact that the numbers of beef cattle are small in comparison with dairy cattle, this is not a point of major importance.
The three lines of investigation discussed above point with some unanimity to an average annual wastage among dairy cattle in the neighbourhood of 23 per cent. None of them, however, is so free from the possibility of error that implicit reliance can be placed on this result. It may, however, be asserted with some confidence that an annual wastage of less than 20 per cent. or of more than 25 per cent. is very improbable.
Annex A to appendix 2.
Extracts from an article by Dr. Norman Wright entitled "Wastage in
dairy cows.
A FAIRLY reliable indication of wastage can, however, be obtained from the study of the mortality rates of cattle at different ages, since such figures will provide an accurate estimate of the average age of dairy cows throughout the country and, therefore, of the extent of premature losses
* First read as a paper at the meeting of the British Association for the Advancement of Science at York in 1932, and subsequently published in January 1933 (Scottish Journal of Agriculture, 16, no. 1).
117
among stock,
Actual mortality statistics are, of course, unobtainable, but approximate values can be fairly accurately calculated from a study of the age distribution among large and representative groups of cows. calculations have been made by Buchanan Smith for random groups of six Such breeds of cattle. Buchanan Smith tabulated the ages of the parents of about 2,600 pedigree calves born during a single year, and calculated the average age of the parents. For cows the average age at calving worked out for all breeds at 54 years, and for dairy breeds 5 years.
In such stock it
is possible that there will be an abnormally high weeding out of unsatisfactory animals, particularly on account of poor milking capacity. It has already been pointed out that disposals under the latter heading cannot properly be classified as wastage, and Buchanan Smith's average values may therefore be too low. It appeared that a better estimate would be obtained by taking a selected group of cattle in which voluntary disposals would probably be reduced to a minimum. Such a group is available in the class I dairy cows in the annual reports of the Scottish Milk Records Association. The minimum yield which must be attained by an animal in this group corresponds to 800 gallons of milk at 3-5 per cent, butter-fat, and, since such a yield can be classed as highly satisfactory for Ayrshire cows, it is reasonable to assume that a high mortality rate at a premature age would, in such a selected group, hardly be due to voluntary disposal by the owner. Yet, the mortality rate, as judged by age-distributing calculations, is abnormally high during the earlier productive years. For instance, it was found that, in 1931, the average calving age of over 7,000 animals* whose ages were recorded, worked out at 5-6 years, and that only 25 per cent. calved at ages over 6-5 years.
Annex B to appendix 2.
The relation between average herd age and wastage as a percentage of the herd. On the assumptions that-
(1) the number of cattle entering the herd in the course of a year is
unity;
(2) the rate of entrance is uniform;
(3) all cattle (irrespective of age) are exposed to a risk of death such that the ratio of survivors of a group at the end of a year to the members of the group at the beginning of the year is A:1;
(4) all cows die on attaining (n + m) years of age;
(5) the age of all cows on entering the herd is m years;
(6) the numbers in the herd are constant;
then the total number of cows in the herd is-
-
H (say),
where x is the time which an animal has spent in the herd. The rate of
100 wastage is therefore per cent., and the average age of the herd is-
L (say).
*
H
m +
- {( { '"' x A * dx) / {( { "' 4* dx)=:
n H - A11 - 1
Log. A
(A
- 1) / Logę A.
11
A
1.
and Lm+nA"
As the percentage of heifers whose ages were recorded very considerably exceeded the percentage of cows, an appropriate adjustment was made in the figures used as a basis for these calculations.
124
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