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The results in question are as follows:-
Cambridge Reading Combined inquiry. inquiry. result.
1928-29.
Number of cattle in herds Number of entrants Percentage of entrants
13,465
13,465
3,670
1
3,670
27.3
27.3
1929-30. Number of cattle in herds
12,984*
9,478
22,462*
Number of entrants Percentage of entrants
3,479
2,612
6,091
26.8
27.6
27-1
1930-31. Number of cattle in herds
11,367
12.348
23,715
Number of entrants Percentage of entrants
3,482
4,122
30-6
33.4
7,604
32.1
1928-31. Number of cattle in herds
Number of entrants Percentage of entrants
37,816*
21,826
10,631
6,734
59,642* 17,365
28.1
30.9
29-1
1929-30 and 1930-31.
Number of cattle in herds
24,351*
21,826
Number of entrants
6,961
Percentage of entrants
28.6
6,734
30.9
46,177* 13,695
29-7
*
partly estimated.
The counties covered are, by the Cambridge inquiry, Bedfordshire, Cambridgeshire, Hertfordshire, Lincolnshire and Norfolk; by the Reading inquiry, Hampshire, Berkshire and Dorsetshire in 1929-30, together with a few herds in Buckinghamshire, Oxfordshire and Northamptonshire in 1930-31.
It appears from these figures that approximately 30 per cent. of an average herd is disposed of every year. But, in order to arrive at the true wastage in dairy herds, it is necessary first to deduct those animals which, though sold from one herd, are bought into another, and are consequently not lost to the dairy industry. In both investigations there is an analysis of the state in which animals sold from each cause are disposed of.
It may be assumed that animals, sold in milk or in calf, for the trade or on account of low milk yield or on account of failure to pass the tuberculin test, are bought for dairy herds. Animals parted with from one of these causes, but which were sold when dry but not in calf, or when fattened, or which died or were sent to kennels, and all animals parted with from other causes in whatever condition they were disposed of, may be assumed to have passed finally out of dairy herds.
On these assumptions, 27-6 per cent. of animals disposed of were re-absorbed into dairy herds. If it is further assumed that of those low yielders sold in milk only those sold within two months of calving were retained in dairy herds, this figure is reduced to 25-3.
If this latter figure is taken, and 29.7 per cent. (the average for the two years 1929-30 and 1930-31) is taken for the percentage disposed of, the wastage, properly so called, in dairy herds is approximately 22 per cent., a figure which is consistent with an average milking life of 4 years.
(ii) Investigations into the age distribution of large samples of cows.
An account of an attempt to determine the working life of milk cattle from the age distribution of selected groups has been published by Dr. Norman Wright.* Extracts from his article are attached as annex A.
* 1933, Scottish Journal of Agriculture, 16, no. 1.
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Dr. Wright states (in litt.) that he has now come to the conclusion that the method adopted in that paper for estimating the average productive life of animals from their average age cannot be upheld. We have therefore constructed an alternative method of estimating, with which Dr. Wright is in agreement. In a table in annex B, the average ages of herds calculated upon various assumptions are given. The losses from the herd are there assumed to be of two kinds: (i) a percentage annual loss to which all cows at whatever age are equally subject, and (ii) a loss by superannuation, all cows being assumed to die as soon as they reach a certain age. The average age given in Dr. Wright's paper is 5-6 years. This is the age at last calving. But the average age of a herd at any moment is greater than this, by an amount which, if there are twelve months between calvings, may be estimated at 5 months. Allowing for this, the average age of the herd is about 6-05 years. From a rough interpolation from the table in annex B it appears that, if cows are superannuated after 9 years of milking life (or at nearly twelve years of age), an average age of 6-05 years is compatible with a total wastage of 23 per cent. The Cambridge inquiry gives the average life in the herd of homebred cows disposed of on account of old age as seven years.
The Cambridge and the Reading investigations showed that estimates of the average milking life of cows, based upon the direct returns of farmers, were lower than was to be expected from the wastage which more accurate statistics indicated as having taken place. This deficiency is to be accounted for in part by failure to return the ages of the older cows, for which records are less likely to be available; in part, perhaps, by a tendency to understate the ages of cows returned. Both these difficulties are likely to be encountered in an attempt to estimate the average age of a random sample of cows. They would, however, be of less importance in the case of pedigree cows.
In the case of samples which are not random, a further difficulty arises. The cow population of Scotland as a whole is stable. But it does not follow that every section of that population is stable. If in any instance this is not so, the average age of that section (the average life being supposed given) will be affected. Increasing populations or populations from which emigration is taking place are younger, other things being equal, than decreasing populations or populations into which immigration is taking place. Herds of pedigree cattle may be either increasing or subject to emigration. Average ages based upon samples drawn from them may consequently be misleading, if applied to cattle in general,
In fact,
(iii) An estimate based upon the annual agricultural census. It is possible to base an estimate of the number of heifers drafted into herds annually, upon the number of heifers recorded as being in calf on the 4th June every year. If calving were carried on regularly all through the year, the latter number should be 77 per cent. of the former. however, there are considerable seasonal variations in the rate of calving. In the year 1930, in addition to the annual census upon the 4th June, a partial census was made in England and Wales upon the 11th January. From this it appears that the ratio of heifers in calf in June to those in calf in January is as seven to eight. Making an allowance for this, it seems that 72 per cent. of the heifers calving between the 4th June one year and the next should be recorded as in calf on the former date. A slight adjust- ment in the figure for calvings of home-reared heifers has to be made for Irish heifers included in the numbers recorded in the census. From figures published in the 20th number of the Ministry of Agriculture and Fisheries series of economic reports (page 147), it appears that about one-quarter of the cows and heifers imported from Ireland are heifers. If it is assumed that the latter are in Great Britain for six months before calving, one-eighth of the number of cows and heifers imported from Ireland must be deducted from the number of heifers in calf in Great Britain before using the latter
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