Genetic effects in GrowBulk sheep.

GrowBulk sheep are a strain of long-wool type sheep growing high bulk wool developed by introgressing high-bulk genes from Texel and Dorset rams into Romney ewes. Progeny selection was planned to minimise decreases in clean fleece weight. Rams have been released to four multiplier flocks. Genetic effects were evaluated in the nucleus and multiplier flocks using a restricted maximum likelihood animal model. Heritability estimates were 0.46±0.05 for bulk and 0.30±0.05 for clean fleece weight with a genetic correlation of -0.48±0.09. The crossbred genotypes represented in the nucleus flock showed a range in average breeding merit for bulk of 27 to 31 cm/g, their weighted mean genetic superiority over contemporaneous Romneys being 5.3 cm/g. The first crop of commercial progeny by rams from this flock showed an average genetic superiority in bulk over their dams of approximately 4 cm/g. Genetic trends indicate that selection has been able to maintain the bulk advantage of the crossbreds while at the same time improving their fleece weight.