A comparison of methods for estimating the area of clipped wool patches on the mid-side of sheep

Abstract

A study was conducted comparing 3 methods of measuring the skin area of a clipped patch on sheep to determine the most appropriate technique in terms of its precision, repeatability and ease of measurement. The methods were: 1) using callipers to take 3 measurements of both dimensions, 2) tracing the patch outline onto plastic, and 3) photographing the patch with an object of known area. Two clipped patches on the mid-side of 20 weaner sheep at each of 2 geographic locations were measured 2-3 times by each of the 3 methods.Linear relationships, and high correlations (0.83-0.89), between the patch area estimates by each of the methods indicate consistency between the methods in discriminating relative differences in patch size. The mean patch area was larger (P<0.01) when measured using callipers (21.0 ± 3.2 cm²) than when estimated by the tracing technique (18.5 ± 2.7) or photography (18.7 ± 2.7), although the overall coefficients of variation (CV) were similar (14.7-15.1%). The between-patch variance was also larger with callipers than tracing or photography, having between-patch CVs of 14.3, 13.6 and 13.9 %, respectively. However, the within-patch CVs of callipers and photography were lower (4.7 and 5.0%) than that for tracing (5.9%). All methods were associated with a high degree of repeatability. Callipers provided the highest level of repeatability (t=0.90), followed by photography (0.88) with tracing the least repeatable (0.84).