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 forage rape (Brassica napus) or irrigated perennial pasture was examined from December to April 1991-92, as was the effect on meat quality assessed by objective and subjective means. During the first 6 weeks the groups of lambs showed similar liveweight gains, but thereafter those on the irrigated pasture treatment had a slower growth rate until first slaughter. The first slaughter, of all lambs weighing >48 kg, took place 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) than the pasture-fed lambs (43.8 +/- 0.79 kg); consequently, significantly (P<0.001) more of them were slaughtered. Rape-fed lambs had significantly (P<0.001) higher fat scores (mean +/- s.e.) before the first slaughter 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 12.3 +/- 0.36 mm, respectively, when adjusted to a common carcass weight 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-fed lambs had significantly (P<0.05) higher a* values (indicating relative redness), but there was no significant difference for the M. longissimus thoracis et lumborum (LL). There was a tendency (P = 0.09) for the LL and SM muscles from the pasture-fed lambs to be tougher as indicated by shear values, but there was no significant difference between groups for cooking loss. For both the LL and M. biceps femoris (BF) from rape-fed lambs, flavour was considered significantly (P<0.05) stronger 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, with no difference for the LL. We conclude that provided a management procedure is in place to prevent overfatness, grazing entire male lambs on forage rape can produce heavy lean lambs over the summer period, whereas under normal dryland pasture conditions, the finishing time would be significantly longer. Based on objective assessments, the meat from entire lambs finished on forage rape will be of quality similar to that from lambs finished on perennial pasture and may have a more acceptable meat colour. From subjective assessments of quality, however, some Australian consumers may detect a stronger, less acceptable flavour from the meat of rape-fed lambs. The importance of these flavour effects to consumers requires further study.