Relationships between growth and productivity of range ewes.

The relationships between various measures of growth and productivity of range sheep were investigated, utilizing records of 1,109 range ewes sired by 269 rams. Body weights and degree of maturity of body weight at birth, weaning, 12 mo, 18 mo, 30 mo and absolute growth rate, absolute maturing rate and relative growth rate over various age intervals were studied relative to their relationship with productivity characters. Measures of ewe productivity were average annual production for the 4-yr period, 2 through 5 yr of age, for grease fleece weight, number of lambs born, number of lambs weaned and weight of lambs weaned. Heritability estimates were .31 +/- .11 for grease fleece weight, .42 +/- .12 for number of lambs born, .08 +/- .10 for number of lambs weaned and .03 +/- .10 for weight of lambs weaned. All production characters had positive phenotypic correlations (.04 to .22) with body weight at all ages. Both number of lambs born and weight of lambs weaned had small positive phenotypic correlations with growth rates over the 12- to 18-mo age interval. The genetic correlations between ewe productivity and weights at different ages were variable, ranging from -.71 between weaning weight and grease fleece weight to values greater than 1.00 for correlations between weight of lambs weaned and weights at birth, weaning and 18-mo. Degree of maturity at 12 mo had positive genetic correlations with all production characters. Estimated genetic correlations between number of lambs born and absolute growth rate, relative growth rate and absolute maturing rate over the 12- to 18-mo age interval were positive.

An analysis of body weights and maturing patterns in western range ewes.

The objectives were to determine the association of maturing patterns with growth rates and body weights and to estimate heritabilities and genetic and phenotypic correlations among these characters of sheep. Records of 1,109 range ewes from the Montana Agricultural Experiment Station Red Bluff Research Ranch at Norris were analyzed. Body weight and degree of maturity of body weight at birth, weaning, 12 mo of age, 18 mo of age and maturity (body weight only), and absolute growth rate (AGR), absolute maturing rate (AMR) and relative growth rate (RGR) over various age intervals were examined. Mature weight, required to calculate degree of maturity and AMR, was estimated by the average of the fall weights taken at 42, 54, 66 and 78 mo of age. Heritability estimates were .53 +/- .12 for mature weight and from .26 to .46 for immature weights. Genetic correlations among body weights at all ages were positive and generally large between immature weights and mature weights. Heritability estimates for degree of maturity ranged from .63 +/- .12 at 12 mo of age to .19 +/- .11 at 30 mo of age, at which time maturity was being approached. Genetic correlations between degree of maturity and body weight at the same age were positive; however, degree of maturity at all ages was negatively correlated with mature weight. Animals more mature at any age or stage during growth tended to be more mature at later stages, to be lighter at maturity, and to grow faster and weigh more up to 12 mo of age. Heritability estimates for AGR, RGR and AMR were moderate to high and were similar for the same age intervals. Selection for any one of the measures of growth rate would tend to expand the shape of the growth curve toward heavier weights and lower degree of maturity for any interval.