March, 1928] agricultural experiments 1927 15 



TWIN-BEARING SHEEP MORE PROFITABLE 



Fecundity in Sheep 



Experiments with sheep are now under way to increase the inherent 

 fertihty of the flock so that twins will be the rule rather than the exception ; 

 in other words, to estabhsh a twin-breeding strain. This work has been 

 conducted by E. G. Ritzman with the co-operation of C. B. Davenport, di- 

 rector of the Department of Genetics, Carnegie Institution of Washington. 



Much argument has always been advanced in favor of single lambs; 

 but the results of the breeding work have definitely established the fact 

 that twins will grow as rapidly and to as large a size as singles provided 

 they are born healthy and receive an abundance of milk from their dam. 



Relation of Milk Yield to Growth of Lambs 



These studies indicate that ewes kept for the purpose of raising market- 

 able lambs should be selected with as much care as cows kept for the dairy. 



The difference in weight increase between lambs from high-milking 

 ewes and good-milking ewes was found to be 16 per cent; between high 

 and fair-milking ewes, 38 per cent; and between high and poor-milking 

 ewes, 79 per cent. The increase in weight of lambs was almost directly 

 proportional to yield. In four-week periods the greatest rate of increase 

 was found in the second period for all groups. A slight dcchne in the rate 

 of growth was found in the third period, except in the low yield, and this 

 was followed by a still further dechne in the fourth period. Since all 

 lambs had free access to a liberal amount of grain and hay, which they ate 

 greedily as they grew older (third and fourth periods) , the controlling fac- 

 tor must have been the milk. 



Fat content of milk was found secondary to quantity, since lambs 

 increased correspondingly in weight on milk varying in fat from 2 to 10 

 per cent. The increase from the milk richest in fat was no greater than 

 that from the milk which had the lowest fat content. In fact, the 

 highest gains (37 pounds) were on 2 to 3 per cent milk, and the lowest 

 gains (18 pounds) on milk testing 10 per cent or over. The hmiting 

 factor in this case was apparently the quantity of milk, as the high gains 

 were made from good to high milk yield, and the low gains from poor to 

 fair yield. 



Inheritance of Size and Conformation in Cross-Bred Sheep 



Experiments to determine the nature of inheritance of size and of 

 conformation or proportion of parts indicate that a new race, or type, can 

 be established in a relatively few generations of breeding. In a size cross 

 of sheep the mean body measurement of Fi and F2 offspring are both be- 

 tween the mean size of their parentage. 



A greater range of sizes exists in F2 than in Fi; but neither exceeds the 

 size of either the large or the small foundation parent and both differ from 

 the foundation parent, and even from each other in the proportion of parts. 



The results obtained here on size inheritance with sheep accord on the 

 whole very closely with those of other investigators on size inheritance with 

 mammals generally. Perhaps the most outstanding feature, in so far 

 as it may throw some light on the practical side of animal breeding, is that 

 observed by other investigators to exist in humans, in albino rats, and in 



