2 BULLETIN 9-96, U. S. DEPAETMENT OF AGEICULTUEE. 



FACTORS INFLUENCING SIZE OF LAMB CROP. 



The size of the lamb crop is dependent upon two things — the num- 

 ber of dry ewes (those not having lambs) and the number of twins 

 and triplets. Under ordinary farm-flock conditions the proportion of 

 dry ewes is insignificant. In range flocks, however, it is a principal 

 cause of lower lamb yields, and it is often impossible to furnish the 

 feed necessary to put the ewes into condition to make sure of their 

 getting in lamb. 



The advantages of flushing are to be obtained principally through 

 an increased number of twins. It has not been proved that the sire 

 influences the number of twins occurring among his offspring. The 

 production of twins or triplets is determined chiefly, if not entirely, 

 by the ewe. Twins may result in either of two ways. First, two 

 developed ova (eggs) may be discharged from the ovaries during the 

 period of heat. Second, a single fertilized ovum may become divided 

 at an early stage and each part develop a fetus. The first is believed 

 to be the more common cause of twins. The production of a second 

 or third ovum is thought to be largely influenced by the condition 

 of the ewe and on this basis the connection between flushing and twin 

 births is rendered very clear. 



To show the connection between production of twin lambs and 

 maturing of extra ova, Marshall 2 slaughtered 55 Black-faced High- 

 land sheep shortly after breeding and examined the ovaries to learn 

 the number of ova that were produced. His findings were as follows: 



1 ruptured follicle in one ovary — 1 ovum produced 42 cases. 



1 ruptured follicle in each ovary — 2 ova produced 7 cases. 



2 ruptured follicles in one ovary — 2 ova produced 5 cases. 



2 ruptured follicles in one ovary and one in the other — 3 ova produced .... 1 case. 



In this case if the ewes had been kept and if all the ova had been 

 fertilized and all developed normally, the result would have been 42 

 single lambs, 12 pairs of twins, and one set of triplets, a total lamb 

 crop equal to 125.4 per cent of the number of ewes bred. The report 

 of this experiment states that this is higher than the ordinary returns 

 from flocks of the breed and that apparently under ordinary condi- 

 tions some of the ova do not produce lambs. 



It is a common observation that the twin lambs in a flock are pro- 

 duced chiefly in the early part of the lambing season. In 302 cases 

 of lambing in purebred Southdown ewes used in experiments con- 

 ducted by the Bureau of Animal Industry and extending over five 

 years, 78 per cent of the ewes dropping twins lambed during the first 

 half of the lambing period. 



The explanation of these facts must be found in one of two things, 

 either of which has an important relation to management for maxi- 



2 The CEstrous Cycle and the Formation of Corpus Luteum in Sheep. In Philosophical Transactions of 

 the Royal Society, Series B, No. 196. 



