FERTILITY IN SHROPSHIRE SHEEP ' 



By Elmer Roberts 

 Illinois Agricultural Experiment Station 



Heape (4) ^mentions that in some breeds young ewes bear fewer twins than 

 older ewes. Carlyle and McConnell {2) reported some observations which 

 they had made on the efifect of age on fertihty in sheep from which they 

 concluded that ewes from 3 to 6 years old averaged a larger percentage of 

 lambs than younger or older ewes, and also that i -year-old rams were not 

 so prolific as those 2 or 3 years old. The same conclusions were reached 

 by Humphrey and KJeinheinz (<5) from a study of later records of the 

 Wisconsin flock. Recently Jones and Rouse (7) showed that in sheep the 

 percentage of twins increased with age until 5 years, when there was a 

 decided drop. 



The present paper gives the results of a study of the influence of age 

 and season upon fertility in American Shropshire sheep. 



The source of data is the American Shropshire Sheep Record (j). 

 Individuals with registry numbers between 325502 and 344869 have 

 been used, date of birth noted, whether bom as single, twin, or triplet, 

 and age of dams and sires looked up. 



AGE OF EWE AND FERTILITY 



Table I shows the percentage of lambs born as singles, twins, and trip- 

 lets from dams of various ages. Ewes under i year and 6 months are 

 grouped in the i-year class, those i year and 7 months to 2 years and 6 

 months in the 2 -year class, and so on. The percentage in multiple births 

 increases to 4 years and remains fairly constant through 8 years. For 

 the older groups the numbers are too small to draw conclusions. 



I Paper No. i6 from the Laboratory of Genetics, Agricultural Experiment Station, Urbana, III. 

 * Reference is made by number (italic) to "Literature cited," p. 234. 



Journal of Agricultural Research, Vol. XXII, No. 4 



Washington, D. C. Oct. 22, 192 1 



aae Key No. Ill.-ii 



(231) 



