EFFECTS OF RAPESEED-MEAL AND FISH-MEAL SUPPLEMENTATION OF MAIZE SILAGE-BASED DIETS UPON THE TISSUE-GROWTH AND BODY-COMPOSITION OF STORE LAMBS

Thirty-six individually penned lambs (mean live weight 32.4 (s.d. 2.27 ) kg) were offered maize silage ad libitum and one of three concentrat e mixes, two of which contained extracted rapeseed meal (control and H R) and the other fish meal (FM). The concentrates were given according to live weight and in sufficient quantities to provide proportionatel y about 0.4 of the dry matter (DM) intake of the lambs. The dietary co ncentrations of the nitrogen (N) g/kg DM were 22.4, 27.4 and 27.5 and of the rumen undegradable N 6.6, 7.3 and 11.6 for the control, HR and FM diets, respectively. All lambs were slaughtered at 45 kg live weigh t and chemical composition of the empty body and some of the component parts determined. A further 12 lambs were slaughtered at the beginnin g of the experiment to establish body composition before the dietary t reatments were imposed. The HR lambs had lower fleece-free empty body (FFEB) gains than either control or FM lambs (P < 0.05). This reduced gain of HR lambs was particularly associated with a reduction in fat d eposition (P < 0.01) such that the FFEBs contained less fat than contr ol and FM lambs (P < 0.01). The efficiency of conversion of metaboliza ble energy for growth (kg) was worse than both the control (P < 0.01) and the FM (P < 0.001) diets. The FFEBs of HR lambs also contained mor e ash (P < 0.05) than the lambs receiving the other diets. The FM diet was associated with greater gains of fat and energy in the guts compa red with the control diet (P < 0.05) and FM lambs had a better kg valu e than control lambs (P < 0.05). There was no evidence that FM lambs h ad better N retention than lambs on the other two treatments.