MEDULLATION AND AVERAGE FIBER DIAMETER VARY INDEPENDENTLY IN THE WOOLOF ROMNEY SHEEP

The proportion of medullation was less in wool from a line of Romney s heep 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 diame ter tram hoggets 35.2 versus 33.1 mu m, P < 0.001; ewe hoggets 35.1 ve rsus 31.8 mu m, P < 0.001). Rams from the selection lines were fed die ts containing high and low levels of protein and energy. During the tr eatment period, the high protein diets increased average fibre diamete r by 14% (P < 0.001) and the proportion of medullation to 5.6% compare d with 3.8% for the low protein diets (P < 0.001). During the post-tre atment period, fibre diameter was 12% higher in the groups fed the hig h protein diet relative to the low protein diet (P < 0.001), but the p roportion of medullation fell to 4.9% for the high protein diets, incr eased to 4.4% for the low protein low energy diet, and fell slightly f or the low protein high energy diet (P < 0.067). The results of these experiments indicate that the proportion of medullation in Romney wool can vary independently of average fibre diameter.