Abstract

Forty crossbred (Dorset Down x (Border Leicester x Romney)) overfat (ultrasonic GR 13.3 ± 0.5 mm), ewe lambs aged approximately 6 months and weighing 40.4 (±0.8) kg live weight were randomly allocated to either an initial slaughter group or one of three dietary treatment groups on the basis of live weight and ultrasonic GR measurement. Dietary treatments consisted of three pelleted diets that contained 101 g (MP), 189g (HP) and 274g (SP) crude protein with estimated metabolizable energy contents ranging from 7.1 to 7.8 MJ per kg dry matter. Lambs were fed ad libitum for 42 days. Throughout the experiment, daily live weight gain was significantly higher for MP than HP or SP lambs (P<0.01). However, this was solely due to greater gut fill for MP lambs compared to HP and SP lambs (P<0.05). There were no significant treatment effects on mean carcass weight. However, all treatments significantly (P<0.05) reduced GR tissue depth (measured on hot carcass at slaughter) by 20%, 19% and 24% for MP, HP and SP lambs, respectively. Chemical fat content in the carcass significantly decreased (8%, 12% and 16%, respectively (P<0.05)). Differences between dietary treatments were not significant for these traits. M. longissimus dorsi area and protein content of the carcass were not significantly altered by dietary treatments (P>0.05). It is concluded that carcass fat in overfat lambs can be successfully reduced by feeding diets low in energy and supplemented with medium to high levels of protein without adverse effects on carcass protein. High levels of dietary protein do not appear to be necessary for this to occur.

AH, Liu, and MJ Young

Proceedings of the New Zealand Society of Animal Production, Volume 54, , 193-196, 1994