Abstract
Duroc and Yorkshire lines which had been selected for 10 or more generations for either thicker or thinner backfat were mated in all possible combinations. Data from 164 litters were analyzed to estimate genetic and maternal influence on litter size, total litter weight and average pig weight at birth and at 21 and 56 days of age. Pure lines differed significantly for most traits, with the Yorkshire low-fat line ranking first in all traits except pig weight at birth and at 56 days. The Duroc low-fat line performed best for the latter two traits and ranked second for the other seven traits. Line crosses averaged significantly above pure lines for all nine traits, indicating significant mean positive heterosis effects. Percent heterosis increased with age at measurement, ranging from 11 to 40%. Lines differed significantly in maternal influence on total litter weight and average pig weight at all three ages, in line crosses. Low-fat lines of both breeds excelled in this respect for each trait. However, lines evidenced little difference in maternal influence on litter size. Lines differed significantly in average breeding value (general combining ability, GCA) only for average pig weight at birth and at 56 days of age. The Duroc high-fat line was highest in GCA but lowest in maternal ability for litter size at any age. Durocs evidenced more GCA than Yorkshires for litter weight through 56 days. Significant specific heterosis (specific combining ability, SCA) was noted for average pig weight at each age and for litter weight at 21 and 56 days. Inter-breed crosses of similarly-selected lines evidenced most SCA in ranking above intea-breed crosses and inter-breed crosses of oppositely-selected lines in all traits except litter size at birth. Inter-breed crosses of low-fat lines exceeded crosses of high-fat lines for all nine traits, significantly so for litter and pig weights. Individual line crosses, including reciprocals, varied widely, both within and between traits. The study indicated that long-term selection based on backfat thickness produced significant changes in the lines in their maternal and general and specific combining ability. Also, further evidence was obtained for a negative correlation of additive genetic and maternal effects. Thus, both factors need to be considered when evaluating individual dams and lines, strains or breeds for crossing. Copyright © 1974. American Society of Animal Science . Copyright 1974 by American Society of Animal Science.