INBREEDING IN RELATION TO EGG PRODUCTION 



By F. A. Hays, Research Professor of Poultry Husbandry 



Introduction 



Inbreeding has been practiced for more than a century and a half by breeders 

 of domestic animals as a method of establishing uniformity in herds and flocks. 

 In the formation of new breeds the general pohcy has been to cross together 

 several existing breeds or strains each carrying distinctive characteristics that 

 were desired. The plan of matings for the polyhybrids thus obtained has gener- 

 ally been to select and inbreed the few animals showing desirable qualities until 

 a degree of uniformity has been attained. Inbreeding was a necessity because 

 few animals exhibited the characteristics desired and because it enabled breeders 

 to "fix" the desired quaUties. There is ample evidence in breed history to indi- 

 cate that inbreeding has been advantageous in founding breeds where the original 

 stock came from very diverse sources. 



At present, evidence is not conclusive that inbreeding within a herd or flock 

 of an established breed, and hence from a restricted foundation, is advantageous. 

 This second type of inbreeding is comparable to most experimental inbreeding as 

 carried on in recent years. 



The very exhaustive analysis made by Wright (1922b, 1922c) of the results 

 of inbreeding guinea pigs for 13 years using brother-sister matings led him to 

 make two very significant deductions: 1. Compared with control stock derived 

 from the same source as the inbreds and raised under identical conditions, the 

 inbreds have undergone a genetic decUne in vigor in all characteristics. The 

 decUne in fertility was most marked. The inbreds were less resistant to tuber- 

 culosis than the controls. Sex ratios were not affected by inbreeding. 2. Marked 

 improvement immediately appeared in the progeny coming from crossing two 

 inbred strains. This improvement was most pronounced through two genera- 

 tions of crossing inbred lines. Size of Utter, body weight, disease resistance, 

 livability and fertility were all greatly improved. 



Extensive studies on the effects of inbreeding albino rats are reported by 

 King (1918a, 1918b, 1918c). This worker found that after 15 generations of 

 brother-sister matings, body weight averaged greater in inbreds than for controls. 

 No decline in fertility or in vigor as measured by length of life was observed. 

 High fecunditj'^, early sexual maturity and vigorous growth all appeared to be 

 inherited as a group in the inbreds. Sex ratio declined from 110 to 100 in 25 

 generations of inbreeding. 



With reference to inbreeding poultry, published results are less exhaustive than 

 those of Wright and King. Since the primary interest in this report is the effects 

 of inbreeding upon characteristics concerned in fecundity in poultry, reference 

 will be made to but a few reports on such phases of work. 



Dunn (1923) reports the results of brother-sister matings in several hnes using 

 White Leghorns for a period of three years. He notes a decline in hatchabiUty 

 and in fecundity. Mortality rate both for chicks and laying pullets rose. Growth 

 was slower and sexual maturity was delayed. His inbred flock was checked 

 against similar unselected stock not inbred. The decline in reproductive power 

 of his inbreds was so great as to preclude maintaining some lines. No selection 

 is reported except on the basis of size of family. Dunn notes that the birds 

 used in this experiment came from very restricted ancestry. 



Hays (1924) reports some results from inbreeding Rhode Island Reds for 



