Genetic analysis of fibre characteristics in adult Targhee ewes and their relationship to breeding value estimates derived from yearling fleeces

Fleeces from 847 adult (2-7-year-old) Targhee ewes enrolled in the U.S. National Sheep Improvement Program were evaluated using an optical fibre diameter assessment system (OFDA2000; BCS Electronics Pty Ltd., Ardross, WA, Australia). Measurements recorded included the average, S.D. and CV for fibre diameter, comfort factor (percentage of fibre below 30 μm), spinning fineness, staple length, average curvature, and uniformity (S.D.) along the staple. Significant ewe age effects were observed for all measurements except the CV of fibre diameter and uniformity along the staple. Fleeces became coarser as ewes aged, but staple length and curvature were maximized at 2 and 3 years of age, respectively. Heritability estimates (h2) for OFDA measurements exceeded 0.50 for average fibre diameter, spinning fineness, and average curvature and were below 0.30 only for uniformity along the staple (h2 = 0.19). Associations between OFDA measurements of fleece fineness in adult ewes and estimated breeding values (EBV) for average fibre diameter derived from yearling fleeces were very strong. The regression coefficient relating the OFDA average fibre diameter measurement in adult ewes to fibre diameter EBV in yearling fleeces of 1.08 ± 0.07 μm/μm was consistent with its expected value of 1.00 and suggests that selection on yearling fibre diameter EBV would nearly maximize selection response in adult fleeces. The EBV for staple length in yearling fleeces was closely associated with the OFDA measurement of staple length in 2-year-old ewes (regression coefficient of 1.21 mm/mm), but this relationship weakened as ewes aged, perhaps as a result of negative associations between adult staple length and the number of lambs produced, and the regression coefficient averaged 0.65 ± 0.10 mm/mm across all ewe ages. In general, relationships between EBV for growth, maternal, and reproductive traits and OFDA measurements in adult ewes were consistent with those observed in yearling fleeces.