The growth performance and carcass attributes of Merino and Poll Dorset × Merino lambs fed rumen-protected methionine (Smartamine™-M)

This study investigated the effect of 5 levels of rumen-protected methionine (0, 1, 2, 3, or 5 g/head.day) on growth rate, feed conversion efficiency, carcass composition, and wool growth in lambs fed a production diet ad libitum. Merino and Poll Dorset × Merino (crossbred) wether lambs (120 of each genotype) were housed in group pens each of 6 sheep. After a 1-week adjustment period, all lambs were fed a pelleted diet of lupins, cereal grain, and hay ad libitum. Rumen-protected methionine was added to the diets in the form of Smartamine™-M. The crossbred lambs were slaughtered after 28 days on the treatment diet and the Merino lambs after 42 days.Increasing the level of methionine supplementation did not lead to an increase in growth rate, daily feed intake, feed conversion efficiency, wool growth, skin thickness, or final liveweight and condition score. The Merino lambs had a lower growth rate, thinner skin, faster rate of wool growth, and were less efficient at converting feed into liveweight gain than the crossbred lambs (P < 0.05). There was no interaction between breed and methionine treatment.Increasing the level of methionine treatment also did not improve hot carcass weight, dressing percentage, tissue depth over the rib, longissimus dorsi (LD) cross-sectional area, meat pH, or meat colour. The only beneficial effect of methionine supplementation on carcass attributes was a decline in fat depth over the deepest part of the LD as methionine level increased (P < 0.05).There were no differences in tenderness, juiciness, or flavour intensity between the breeds or as a main effect with methionine supplementation. However, there was an interaction between breed and methionine for flavour intensity, such that methionine supplementation reduced the flavour intensity of Merino meat but not that of the crossbred meat (P < 0.05).This work suggests that there are unlikely to be production gains in terms of liveweight gain, feed conversion efficiency, or wool growth in supplementing lambs with protected methionine if they are being fed a high performance diet ad libitum. However, there were 2 benefits that may have scope for further investigation. The first was a reduction in fat cover over the loin as the level of methionine supplementation increased. The second was a reduction in the intensity of flavour found in Merino lamb when the lambs were supplemented with methionine. The study also provided some useful guidelines for the differences in growth and carcass characteristics between Merino and crossbred lambs when they are finished under identical conditions to an even fat cover, and demonstrated that Merino lambs can produce carcasses of very acceptable eating quality when well finished.