Abstract The proportion of medullation was less in wool from a line of Romney sheep selected for High staple tenacity (HST) than in one selected for Low staple tenacity (LST) (1.15% versus 1.58% in 1991‐born ram hoggets, P < 0.076; 1.90% versus 3.10% in 1992‐born ewe hoggets, P< 0.001). This occurred despite the HST line having the greater mean fibre diameter (ram hoggets 35.2 versus 33.1 μm, P < 0.001 ; ewe hoggets 35.1 versus 31.8 μm, P < 0.001 ). Rams from the selection lines were fed diets containing high and low levels of protein and energy. During the treatment period, the high protein diets increased average fibre diameter by 14% (P < 0.001) and the proportion of medullation to 5.6% compared with 3.8% for the low protein diets (P < 0.001). During the post‐treatment period, fibre diameter was 12% higher in the groups fed the high protein diet relative to the low protein diet (P < 0.001), but the proportion of medullation fell to 4.9% for the high protein diets, increased to 4.4% for the low protein ...
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