2 BULLETIN 996, U. S. DEPARTMENT OF AGRICULTURE. 
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-nock 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 iu 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 pans 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 332 cases 
of lambing in purebred Southdown ewes used in experiments con- 
ducted by the Bureau of Animal Industry and extending over six 
years, 70 per cent of the- ewes dropping twins lambed during the first 
half of the lambing period. 3 
2 The CEstrous Cycle and the Formation of Corpus Lutem in Sheep. In Philosophical Transactions of 
the Royal Society, Series B, No. 196. 
s In 1920 and 1921 a portion in each lot did not receive the same treatment as the others and, therefore, 
was not included. 
