Effect of floor type and dietary molybdenum content on the liver copper concentration at slaughter and performance of intensively finished lambs

Abstract

A 3×2 factorial experiment was used to evaluate the effects of added molybdenum (Mo) at the rate of 0, 4 or 8 ppm in the diet and two floor types (straw bedding or galvanised expanded metal) on 92 Texel cross intensively fed lambs in relation to hepatic copper levels and lamb growth. The lambs had ad libitum access to a coarse commercial lamb concentrate. The mean crude protein (%), copper (Cu) concentration (mg/kg) and background Mo (mg/kg) of the diets as fed were 18.6, 9.7 and 0.92, respectively. The lambs were slaughtered when they reached 38 kg live weight. Following slaughter, the caudate lobe of the liver was removed for Cu analysis. There was no effect ( P>0.05) of Mo level or floor type on lamb growth rate (mean 411±7.2 g/hd/day) or kill-out proportion. The added Mo was more effective in lowering liver copper values when the lambs were housed on straw bedding especially at the 4 ppm inclusion level. For the 4 and 8 ppm Mo inclusion, the respective reduction in liver copper values was 21% ( P<0.05) and 43% ( P<0.001) when lambs were housed on expanded metal flooring and 52% ( P<0.001) and 62% ( P<0.001) when lambs were bedded on straw. There was an interaction between floor type and the level of molybdenum inclusion. Overall, when lambs were bedded with straw, the hepatic Cu liver value was 15% lower than on the expanded metal, while if the 0 level Mo inclusion treatments are excluded, the effect of the straw was to lower the liver copper value by 32%. At the 4 ppm Mo inclusion, there was a positive relationship between liver weight and liver Cu concentration, although the liver Cu concentration did not approach what might be considered potentially toxic values. In conclusion, high Mo inclusion did not affect lamb growth rate and there were significant differences in its effect on hepatic copper levels depending on inclusion level and floor type. For intensively fed lambs, liver Cu accumulation and risk from Cu toxicity may be reduced, without loss of performance, by housing lambs on straw bedding when Mo is added to the diet and it is also possible that the Zn coating, frequently applied to protect metal flooring, may also confer protective properties evidenced in the absence of Mo. These outcomes should be taken into account when designing strategies for minimising copper toxicity in intensively fed lambs.