NUTRITION IN RELATION TO REPRODUCTION         201

performances in milk production. He points out that the animals are usually
stall-fed for long periods on diets exceedingly rich in concentrates, and that actual
symptoms of deficiency (bone disease) occur. The output of mineral matter in
high milk production is enormous, and unless special provision is made to meet
the requirements, depletion of the skeleton is bound to occur. Stockklausner (15)
and Frei (26) both regard this as one of the causes contributing to produce
sterility.

                 MALNUTRITION IN RELATION TO STERILITY.

Under-nutrition.—Fertility depends, in the first place, on the simple quantita-
tive adequacy of the diet. Famine and war sterility or diminished fertility,
is known in all classes of farm animals and in human beings. Following the
Great War, sterility was common in cattle, goats and sheep in Germany and
Hungary (14). In goats sterility increased from the normal, about 5 per cent.
to 20 or 25 per cent., and in sheep from 10 to 40 per cent. Under-feeding is well
known as a cause of sterility in range herds and in sheep on poor pastures. Indeed,
it has been suggested that the natural habitats of sheep have been determined
by the limits within which climatic conditions allow a sufficiency of natural
pasture (27). On poor grazings the lamb crop may be as low as 25.50 per cent.
(28).

Experiments by Loeb (29) indicate how this sterility may be produced.
In guinea-pigs, underfed on a normal ration, degeneration and atresia of the
maturing ovarian follicles occurred, with failure of ovulation and complete
sterility.

Over-feeding.—On the other hand, over-feeding is also known to be detrimental
to optimal breeding performance. In cattle ' show condition ' and in sheep ' high
condition ', as produced for flock competitions, are associated with low fertility or
failure to breed (30, 27). In brood sows, over-fatness appears to be a common
cause of sterility. According to some veterinarians, cystic changes in the ovary
occur most frequently in fat animals.

Flushing.—In contrast to the effect of continued high condition, a stimulating
effect of ovulation is produced by flushing (31). It appears that the nature of the
feed used to produce the flush is relatively unimportant. In animals that are fit,
but not over-fed, any sudden increase in feed will stimulate ovarian activity,
promote the occurrence of œstrus and increase the number of ova shed. In a
series of observations (32) on a total of 1,033 flushed ewes, the number of lambs per
100 ewes was 154, as compared with 140 in 4,054, ewes not flushed. In another series
(33), flushed on extra good pasture, the lambs per 100 ewes 164, as compared with
143 from control ewes.

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