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Estimates of Genetic and Phenotypic Parameters of Weanling and Yearling Traits in Range Rambouillet Ewes^{1, 2,}

Montana Agricultural Experiment Station, Bozeman^,5 and Texas A&M University College Station^,6

Abstract

Heritability and phenotypic and genetic correlations were estimated for bodyweight, face score, neck score, type score, condition score and staple length for Rambouillet ewe lambs. Estimates were made at both weanling and yearling ages using sheep that were unselected until afer yearling data were obtained. Least-squares analyses were made to determine the influence of identifiable sources of variation. Estimates of heritability and correlations were made using both data which were not adjusted for any identifiable source of variation and data which were adjusted.

Heritability estimates were higher at yearling age as compared to weanling age for all traits except neck score. Adjustments for significant identifiable sources of variation improved the parameter estimates only little if at all.

Heritability estimates for bodyweight were 0.37 at weanling age and 0.85 at yearling age for unadjusted data. Data adjusted for significant sources of variation gave estimates of 0.38 and 0.82.

Face score estimates of heritability at weanling and yearling ages for both unadjusted and adjusted data were high, ranging from 0.72 to 0.80.

Estimates of heritability of neck score were much lower at yearling age than at weanling age. Heritability at weanling age was estimated at 0.62 and 0.67 from unadjusted and adjusted data, respectively. Similar estimates at yearling age were 0.38 and 0.36.

Adjustment for significant identifiable sources of variation lowered the estimates of heritability of type score at both weanling and yearling ages. Yearling estimates of 0.61 and 0.57 were considerably higher than weanling estimates of 0.17 and 0.08.

Estimates of heritability of condition score were low at both weanling and yearling ages. The heritability estimate for this trait at weaning was 0.05 from unadjusted data and 0.00 (–.07) when adjusted. At yearling age the two estimates were 0.11 and 0.05.

Staple length estimates of heritability were 0.43 and 0.00 (–.01) at weanling age as compared to 0.49 and 0.46 at yearling age. The zero estimate obtained from adjusted weanling data is not readily explainable and is inconsistent with the other estimates.

Phenotypic and genetic correlations were calculated for all traits at both ages and using both adjusted and unadjusted data. Genetic correlations tended to be higher than phenotypic correlations for the same traits.

¹ Montana Agricultural Experiment Station Journal Article No. 748.

² This paper represents part of the Ph.D. dissertation, Texas A&M University, of the senior author.

³ Animal Science Department, Texas A&M University, College Station, Texas.

⁴ Department of Animal and Range Sciences, Montana State University, Bozeman, Montana.

⁵ Appreciation is expressed to the personnel of the U.S. Sheep Experiment Station, Dubois, Idaho who cooperated in obtaining the data used in this study.

⁶ The Data Processing Center, Texas A&M University made available the computer time which made this study possible.