Variation in Puberty Phenomena in Inbred Gilts

Abstract

Data on age at puberty, ovulation rates at different heat periods, percentages of gilts conceiving at first breeding and litter size at farrowing were obtained on 205 inbred gilts of Chester White and Yorkshire ancestry in a two year period. The average ages at puberty were 236 days in 1948 and 251 days in 1949. Corresponding average weights at puberty were 176 and 201 pounds. Line and year differences in both age and weight at puberty were highly significant but the line × year interaction was significant for weight at puberty only. The pronounced tendency for the same lines to be either early or late maturing in both years would appear to indicate that the line differences were due mainly to genetic differences of the lines. Within the lines body weights at all ages between 56 and 168 days were highly correlated negatively with age at puberty but the lines with the highest average 154 day weights tended to be the slowest to reach puberty. The correlations between rate of gain in successive two week intervals between 98 and 168 days were highly correlated negatively with age at puberty but the lines with the highest average 154 day weights tended to be the slowest to reach puberty. The correlations between rate of gain in successive two week intervals between 98 and 168 days and age at puberty were lower than those between body weight at the different ages and age at puberty. Multiple correlations of body weight and rate of gain at the different ages with age at puberty were very similar to the simple correlations between weight and age at puberty. There was an average increase of 0.8 eggs in ovulation rate at second heat in 48 gilts in which the ovulation rates at both first and second heats could be determined. These data together with data on single ovulations in a limited number of animals slaughtered at other heat periods indicate a steady rise in ovulation rate at each succeeding heat period from first to fourth heat. Line differences in ovulation rate at first heat were highly significant. Gilts conceiving at the third heat period farrowed an average of 1.4 pigs more than those conceiving at second heat and 2.5 more than those conceiving at first heat. These differences approached significance. There was not a significant difference in the number of pigs farrowed by gilts of different lines. Conception rates at different heat periods ranged from 55% to 80% but the differences were not significant. The average fertilization rate in 63 gilts bred within 24 hours after the onset of estrus was 82%. Only 63% of the gilts had no fertilized eggs and 87% had 50% or more fertilized eggs indicating that a complete lack of fertilization is only a minor cause of repeat breeding in gilts. A comparison of the ovulation and fertilization rates in slaughtered gilts with the conception and farrowing rates of their sisters which were saved for breeding indicates that embryonic death is an important cause of “repeat-breeding” in gilts and also of the discrepancy between ovulation rate and litter size. The data presented in this paper indicate that reproductive efficiency of gilts as measured by ovulation rate and fertilization rate increases with advancing sexual age. Copyright © . .