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CERTAIN ASPECTS OF FERTILITY IN THE COW [Vol. XXI, Part II,
heifers 2.4 services were required per conception. The highest proportion of sterility
upto nine years of age was reported in the two-year old group and after ten years of
age the percentage of sterility was found to increase rapidly with increasing age.
Graves [1936] reported that in one Holstein herd 3.38 services were required for the
first conception. The same investigator in a study of registered Jerseys found that
4.33 services were required for the first conception, 3.37 for the second and 1.67
for the third to sixth conception. Morgan and Davis [1938] found from a study of the
University of Nebraska herd that virgin heifers required more services per conception
than any other age group from two to thirteen years with the exception of those in
the ten-year old group. Bowling, Putnam and Ross [1940] found that the first
conception of the heifers required 2.79 services as compared with 1.8 services for
later conceptions. Baker and Quisenberry [1942] reported that fertility was lowest
at four years and highest at nine years as determined by the percentage of calf crop
in range beef cattle. In a study of fertility in range beef cattle, Lasley and Bogart
[1943] found that fertility in heifers between two and three years of age was lower
than in cows of other ages. While the heifers required 2.37 inseminations per calf,
fertility was highest in five and six year old cows, which required 1.36 inseminations
per calf. Beginning at the sixth year of age fertility gradually declined with increas-
ing age until the ninth or tenth year, when the cows required 2.09 services per calf,
the decline in fertility presumably being due to the decline in vigour with advancing
age. From the data presented by Miller and Graves [1932] for the Beltsville herd
it is found that abortion-negative heifers required 3.31 services per conception while
abortion-negative cows required 2.51 services per conception. Similarly abortion-
positive cows required 3.03 services per conception as against 3.92 services for abor-
tion-positive heifers. This low fertility in heifers appears to be a rather general
feature than an exception in most herds.
The objects of the present investigation are to determine the effect of the incid-
ence of contagious abortion on fertility in cows, the effect of abortion on the number
of services required for conception and also to study the effect of age on fertility.
MATERIALS AND METHODS
For the present investigation, dairy herds which were under the supervision
of the Animal Husbandry Department of the University of California were chosen
and their herd records analyzed. For the study of the effect of contagious abortion
on fertility, the herd of Norman Neal was chosen and two three-year periods were
selected. In the first period from 1 January 1934 to 31 December 1936, con-
tagious abortion was prevalent in the herd but every one of the animals was not
tested for Brucellosis. Since that herd consists of animals that were positive for
Brucellosis and other non-tested animals which were probably non-infected for the
most part, that herd was called the mixed herd. The second period was from 1
January 1943 to 31 December 1945 and the herd was free from contagious abortion
during this period. A gap of six years was given between the two periods under
study with the view of avoiding any of the animals in the first group being carried
over to the second period. The herd under study in the second period is called the
healthy herd. Only those services were included in the study the results of which
were known during those periods.