19
12
20
An
to maintain an average economic yield in his milking herd. In part, however, they are indirect losses attributable to disease. interesting example of the importance of the latter was given to us by one experienced witness, who had made an analysis of the milk yields of the cows in a large herd, dividing them up into groups according as they were or were not infected with mastitis or contagious abortion (not in an active form). The average yields in the different groups (corrected to allow for the various extraneous factors which normally affect productivity*) were as follows:-
TABLE 2.
Average milk yield in healthy and infected cows.
Disease condition.
No. of
COWS.
882
Corrected yield of milk in pounds
per annum.
Group (1)
Mastitis free, abortus free
39
9,718+243
Group (2)
Group (3)
Mastitis infected, abortus free Mastitis free, abortus infected...
56
8,116 +274
12
7,702 ± 630
Group (4)
Mastitis infected, abortus in-
fected
19
7,416 + 350
It may therefore be safely inferred that many of the animals sold out of herds nominally on account of their low milk yield, are in fact the victims of disease.
17. The next group in table 1 refers explicitly to disease; to tuberculosis, Johne's disease, udder disease (principally mastitis), and to miscellaneous diseases. Almost last in the list comes old age. The small number of disposals recorded under this head is not due to a commendable preference on the part of farmers making returns to giving a definite rather than an indefinite reason for each disposal; but simply to the fact that very few cows survive so long that their death can be attributed to old age. Such evidence as we have suggests that the rate at which cattle are disposed of is practically constant at all ages up to perhaps eleven or twelve years, and is so high that only few cows reach these latter. Disposals on account of reaction to the tuberculin test are practically confined to tested herds. Though an unimportant cause in the aggregate, it is in these herds a serious drain, being responsible for as much as one sixth of the total losses. Animals sold on account of reaction to the tuberculin test are frequently again bought into dairy herds. Though the statistical inquiries which we have been discussing in the preceding paragraphs do not enable us to assess the absolute importance of each disease, they show how serious a part disease
* The most important of these factors are age and date of last calving.
18
plays in the economic management of the country's herds in bringing about the disposal of cattle. Of all disposals, 58 per cent. are directly attributable to disease; of the remainder, some are indirectly attributable to disease and many are only transferred to other herds, and, though not lost to the dairy industry, do nothing to raise the general standard.
18. If the statistical proof is deficient there is unanimity among experienced veterinarians that four diseases are pre-eminently responsible for the farmers' losses, namely, contagious abortion, bovine tuberculosis, mastitis, and Johne's disease. Of these four diseases it is agreed that on the whole contagious abortion is the most, and Johne's disease the least, serious. Our report is mainly concerned with the steps which can be taken to lessen the incidence of these four diseases.
(iii) Influences affecting the spread of disease.
19. The influences affecting the spread of disease among dairy cattle may be divided into those which predispose the cow to fall a victim to, or resist, an infection if encountered, and those which If we increase or diminish the chances of encountering infection. make small reference to consideration of the first class it is not because they are unimportant. A cow, producing perhaps a thousand gallons of milk in the course of a year, in addition to supporting a growing foetus, is making unnaturally severe demands upon her powers of assimilation. In such circumstances quite minor deficien- cies in her diet may have serious results. Several witnesses have argued that correct feeding and environment are necessary to the prevention of disease. Government action in this field is necessarily confined to the encouragement of research and to the dissemination of the results of research among farmers. It should be fully realised that owing to the heavy strain to which they are subject, dairy cows are liable to fall easy victims to disease, and that the high incidence of disease among them is closely related to this fact. Moreover, the modern tendency to require a constant increase in yield is likely to increase the incidence of disease. We are further impressed with the importance of the proper feeding of calves and young heifers in order that their constitutions may not be impaired and their vitality lowered.
20. The chances of a cow encountering infection depend, in the first place, on housing conditions. Milking herds spend prolonged periods congregated in buildings. If these are of such construction as to harbour infection, they may be a powerful factor in the perpetuation of disease. The absence of proper premises for isolation of diseased animals or for calving, overcrowding, unsatisfactory ventilation and a water supply which is liable to become contaminated are all dangerous. Buildings to be hygienic require impervious