10 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



The effects of inbreeding and crossbreeding on fertility are discussed 

 later. For the present it will suffice to say that inbreeding is very 

 likely to lead to a reduction of fertility, both as regards regularity of 

 breeding and number at a birth. That this is an inevitable result is 

 not, however, indicated by experiment. Among a number of inbred 

 lines, some will be found which appear to suffer little or no loss of 

 fertility. Experiments indicate that the fertility of a herd which has 

 declined through inbreeding can usually be restored by an outcross 

 even with another inbred line of reduced fertility, if the latter is not 

 closely related. 



Among factors other than heredity, a warm climate, a reasonable 

 amount of exercise, and a condition neither fat nor thin are conducive 

 to fertility in both males and females. Insufficient exercise is con- 

 sidered to be an especially common cause of failure of stallions. 

 Undernourishment reduces the activity of the reproductive function 

 in both males and females. Excessive fatness is, however, as great 

 a cause of failure to breed in both sexes, in some cases owing to 

 mechanical closure of ducts, in other's to fatty degeneration of the 

 sex glands, leading to permanent sterility. Sterility from this cause 

 is a recognized danger in fitting breeding stock for the show ring. 



A gaining condition at the time of conception, following a somewhat 

 thin condition, is considered most conducive to fertility. This prin- 

 ciple is much used in Great Britain in the so-called practice of flushing 

 ewes. Ewes which have been maintained on pasture are fed liberally 

 for about 3 weeks before breeding, using, especially, succulent feeds. 

 Similarly, fresh, green pasture is recommended for cows which have 

 failed to breed. On the other hand, certain feeds, among which 

 sugar and molasses may be mentioned, are considered likely to lead 

 to sterility. 



There are a considerable number of pathological causes of infer- 

 tility. By far the most important is the financial loss which it occa- 

 sions is contagious abortion of cattle, a germ disease. Other causes 

 of abortion are also sources of much loss. A small percentage of ani- 

 mals are congenitally sterile. The freemartin heifer is an example 

 which is discussed later. Permanent sterility which is not congenital 

 may result from fatty degeneration, as already mentioned, or from 

 tumors. There are, finally, a number of conditions causing sterility, 

 some of which can be overcome by the use of artificial insemination. 

 The subject is too large to be more than touched on here. 



HYBRIDS. 



While wide crosses within the same species tend to increase fer- 

 tility, crosses between different species are apt to result in offspring 

 which are either wholly sterile or of reduced fertility in the few cases 

 in which such crosses can be made at all. The rather common stories 



