Selection for yearling fleece weight and its effect on fleece shedding in New Zealand Wiltshire sheep

A flock of New Zealand Wiltshire sheep was divided into two lines and selected for increased greasy fleece weight or decreased greasy fleece weight as yearlings. Wiltshires shed their fleece annually, and although these yearlings may have expressed shedding as lambs in the preceding summer, they were shorn in autumn at 5 months and again in spring at 12 months of age to determine fleece weight before the subsequent shedding. A rapid separation in fleece weight was observed within 8 years, with ewe (0.89 kg) and ram yearlings (0.92 kg) selected for increased fleece weight producing significantly more wool (P < 0.001) than ewe (0.43 kg) or ram yearlings (0.39 kg) from the line selected for decreased fleece weight. There was no significant difference between sexes, and the sex x line interaction was not significant. Clearly this was very low wool production, but fleece weight was heritable (h(2) = 0.57 +/- 0.04). Shedding was scored from 0 (not shed) to 5 (completely shed) for all animals (n = 2505 records) and was found to be heritable in December (h(2) = 0.39 +/- 0.04) and again in January (h(2) = 0.52 +/- 0.05), but less so in September at shearing (h(2) = 0.24 +/- 0.03). Fleece weight in spring was negatively correlated with shedding score in January, both phenotypically (-0.52 +/- 0.02) and genetically (-0.81 +/- 0.04). Selecting yearlings for low fleece weight developed a shedding sheep with negligible fleece, which may not require shearing. Selecting for shedding of lambs in January would likely achieve a similar outcome.