FERTILITY IN SHEEP.

Manawatu Standard, Volume LVI, Issue 112, 13 April 1936, Page 5

FERTILITY IN SHEEP.

CAUSES OF VARIATION. HEREDITY IS THE BASIS. Investigations into fertility in sheep have shown that the causes of difference in fertility are fundamentally determined by the genesis or the natural mode of reI production, but this is also governed by I influence of environment. Flockowners, are well aware how essential it is that ewes should be in a rising condition when I bred from, and a little extra food is Well | repaid if it ensures this desired state. I Experiments carried out in Scotland showed that 48 crossbred Blackfaced and Cheviot flocks on the hillsides, which had I no extra food at breeding time, produced 93 lambs per 100 ewes. But 19 similar flocks taken on the pastures three weeks before the breeding season gave 136 lambs to the 100 ewes. Twins are generally born early in the lambing season, and young ewes are less likely to produce twins than old ones, for the young animal is not so certain a breeder as the older one. Fertility in sheep is also influenced by the ram, and, if anything is wrong with the "ram, it' increases the number of barren ewes. Ram lambs are generally considered uncertain breeders, and older rams are more certain to get a good quantity of lambs. The remits of experiments have shown that the average of barren ewes was 2 percent, and anything beyond that was not normal. The fact that heredity is the fundamental basis of fertility is proved by the breeding differences which occur in flocks of the same breed kept under similar conditions, and that it has been proved possible to build up strains of different levels of fertility by selective breeding. Apart from the primary hereditary causes, fertility is influenced by those factors which affect barrenness and abortion, since high fertility depends on a high: percentage of multiple births, associated with low percentage of barrenness and, abortion. INFLUENCE OF MANAGEMENT. \ I The management and food supply of j the flock influence barrenness and abor-j tion, except in those cases where abortion is due to definite pathological causes. I At tupping time such factors as the re- j moval of obstacles to service and the pro- ' portion of ewes per ram need attention, I as these may affect the proportion of barrenness, while, during pregnancy, abortion may be prevented in some measure by, management, ensuring that the ewes are! maintained in a steady condition at first and that all predisposing causes (such as • "sheep-sick" land, leading to debility)! arc, as far as possible, eliminated. A high,! or fat, condition, such as is common] among Show sheep, is antagonisCic to high fertility, for either physical or patholo-. gical reasons. | It has been shown .that five-year-old| ewe* give the highest yield of lambs, but' this does not indicate that this is the best age at which to obtain lambs; such 1 , age must be determined largely by economic considerations in particular dis- ■ tricts. However, the data available for certain of the breeds show that the. fertility of yearling ewes is less than that of older ones, the. incidence of barrenness and abortion being greater. From investigations involving large numbers it has I 'been shown that the sex ratio in sheep may be considered as about 97 males per 100 females, but in small numbers there can be much seasonal variation.