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Genetic analysis of reproductive performance of Rambouillet sheep
Data from a purebred Rambouillet range flock between the years of 1919 and 1985 were analyzed to determine: the relationship of age, body weight, birth type, sex of natal twin on lamb and fleece production, and longevity; an accurate method of calculating the estimated breeding value (EBV) for reproduction of ewes with different production records. Genetically stable years for genetic parameters estimation were determined by ANOVA and testing for differences between years (P<.01). Data utilized for genetic parameter estimation included the years 1959 to 1985 with 6017 ewes mated to 293 rams. EBV was attained by modifying a standard EBV equation for repeated measures, improved by the use of repeatability and heritability estimates for number of lambs born within each age group. Heritability estimates increased linearly from age 2 to 7 yr (.30, SE =.10 to.64, SE =.20), except for age 5 yr (.41, SE =.08), suggesting an age or parity effect on gene expression. Frequency of multiple births increased between the ages of 2 and 6 yr (4.7 to 21.0%). Selection of twin born ewes over single born as breeding replacements was suggested by their higher frequency of multiple births (35.8 vs 33.6) and number of lambs born to age 8 yr (8.4 vs 8.0), without any significant negative effect on fleece weight (P<.01). No freemartin effects on twin born ewes were observed on pre- and postnatal growth for birth weight, fleece weight, number of lambs born and body weight (P<.05). Fleece weights decreased as the number of lambs reared increased: 4.2, 4.0, 3.9 and 3.8 kg for ewes with 0, 1, 2, 3 lambs, respectively. The genetic correlation between fleece weight and the average number of lambs born was $-.18$. The relationship of a ewe's body weight to lamb production was identified as curvilinear with lighter and extremely heavier ewes having lower rates of reproduction. A body weight range of 54.5 to 64.0 kg was found to have the highest frequency of multiple births and average number of lambs born. Selection for mature breeding weight could be accomplished with a high degree of accuracy at the age of 3 yr (.88). A genetic correlation of.26 was derived for average breeding weight and average number of lambs born. Longevity was not influenced by birth type of the ewe (P<.05). Longevity decreased significantly (P<.01) as the average number of lambs born and fleece weight increased.
Ph. D. in Animal Breeding Texas A & M University 1987
Includes bibliographical references (leaves 96-103).