MEAT QUALITY, CARCASS FATNESS, AND GROWTH OF SHORT SCROTUM LAMBS GRAZING EITHER FORAGE RAPE OR IRRIGATED PERENNIAL PASTURE

The growth of 136 short scrotum male crossbred lambs fed on either for age rape (Brassica napus) or irrigated perennial pasture was examined from December to April 1991-92, as was the effect on meat quality asse ssed by objective and subjective means. During the first 6 weeks the g roups of lambs showed similar liveweight gains, but thereafter those o n the irrigated pasture treatment had a slower growth rate until first slaughter. The first slaughter, of all lambs weighing >48 kg, took pl ace when >50% of the rape-fed lambs exceeded this liveweight, at which time they were significantly (P<0.001) heavier (49.3 +/- 0.82 kg) tha n the pasture-fed lambs (43.8 +/- 0.79 kg); consequently, significantl y (P<0.001) more of them were slaughtered. Rape-fed lambs had signific antly (P<0.001) higher fat scores (mean +/- s.e.) before the first sla ughter than pasture-fed lambs (3.8 +/- 0.10 v. 2.9 +/- 0.09). The mean (+/- s.e.) GR tissue depths for the 2 groups were 15.2 +/- 0.36 and 1 2.3 +/- 0.36 mm, respectively, when adjusted to a common carcass weigh t of 21.4 kg. Carcasses from pasture-fed lambs had significantly (P<0. 05) higher pH values. Meat from rape-fed lambs was significantly (P<0. 05) lighter in colour; the M. semimembranosus (SM) from the pasture-fe d lambs had significantly (P<0.05) higher a values (indicating relati ve redness), but there was no significant difference for the M. longis simus thoracis et lumborum (LL). There was a tendency (P = 0.09) for t he LL and SM muscles from the pasture-fed lambs to be tougher as indic ated by shear values, but there was no significant difference between groups for cooking loss. For both the LL and M. biceps femoris (BF) fr om rape-fed lambs, flavour was considered significantly (P<0.05) stron ger than for the same muscles from pasture-fed lambs, as was the aroma of the LL. Overall, the BF from pasture-fed lambs was significantly ( P<0.05) more acceptable to panellists than BF from rape-fed lambs, wit h no difference for the LL. We conclude that provided a management pro cedure is in place to prevent overfatness, grazing entire male lambs o n forage rape can produce heavy lean lambs over the summer period, whe reas under normal dryland pasture conditions, the finishing time would be significantly longer. Based on objective assessments, the meat fro m entire lambs finished on forage rape will be of quality similar to t hat from lambs finished on perennial pasture and may have a more accep table meat colour. From subjective assessments of quality, however, so me Australian consumers may detect a stronger, less acceptable flavour from the meat of rape-fed lambs. The importance of these flavour effe cts to consumers requires further study.