- Data from 592 litters of pigs farrowed and raised at the
Oregon Agricultural Experiment Station at Corvallis from the spring
of 1961 through the spring of 1968 were analyzed for effects of year
and season of birth, breed, sex, inbreeding and age of dam on
measures of fecundity, growth rate and carcass merit. Hierarchal
analysis of variance was done to find the effects of population structure
and inbreeding on the components of phenotypic variance and
heritability for measures of litter size and average daily gain.
The Oregon State University swine herd is composed of a
partly closed line of Berkshires, a partly closed herd of Yorkshires,
reciprocal crosses between these two breeds and a line developed
from inter-se mating of F₁ and later generations. Significant
differences among the various breed groups were found for number
of pigs born alive, number born alive plus dead, number born plus
mummified fetuses, number weaned per litter, average daily gain,
carcass length, average backfat thickness, loin eye area and ham
weight. Differences among breeds were not significant for loin
The level of performance did not increase, over the period
studied, for any of the traits studied except loin eye area. The lack
of increase in level of performance may be due to a lack of sufficient
selection pressure, detrimental environmental effects or the depression
in performance due to inbreeding.
Fall-farrowed pigs had lower average daily gains than spring-farrowed
pigs but, in general, had more desirable carcasses. The
differences in performance and carcass traits are attributed to the
smaller proportion of males performance tested from fall-farrowed
litters. Males gained 0.096 pounds per day faster than females but
carcasses were 0.264 inches shorter, had 0.067 inches more
average backfat, had ham and loin weights that were 0.798 and 0.516
pounds less respectively and had loin eye areas that were 0.382
square inches smaller.
Negative regressions for the effects of inbreeding of the dam
on litter size at birth and weaning and positive regressions for the
effect of inbreeding on litter size at birth were found but most
effects were not statistically significant. The number of mummified fetuses and stillbirths was not affected by inbreeding but mortality
between birth and weaning increased significantly when inbreeding of
the dam rose above a threshold of ten percent.
Both linear and quadratic components of age of dam had highly
significant effects on the litter size at birth and on the number of
pigs weaned with a peak in sow productivity occurring at about three
years of age. Age of dam effects were thought to be due almost
entirely to differences in ovulation rate or early embryo survival as
the number of mummified fetuses, stillbirths or pre-weaning deaths
were not related to age of dam.
Environmental variation of litter size and average daily gain
increased with increasing levels of inbreeding, indicating that inbred
individuals were unable to buffer themselves against fluctuations in
environment. Heritability of average daily gain decreased with inbreeding
because of lower genetic variance and higher environmental
variance. Heritability estimates of litter size were quite variable
due to the limited number of sire groups available but, in general,
were quite low and generally not significantly different from zero.
Estimates of heritability of average daily gain were more than twice
as high as most estimates reported in the literature. These high
estimates were thought to be biased due to the cumulative effects of
a number of characteristics of the population and of the data.
A line developed from a Berkshire-Yorkshire crossbred
foundation should have more alleles segregating and therefore should
have a higher additive genetic variance than the parental breeds.
However, no differences between breed groups were observed in the
magnitude of genetic variance, environmental variance or heritability