Abstract
Cuttings from poplar (Populus) and willow (Salix) trees are used increasingly as supplementary feed for livestock in summer‐dry and drought‐prone regions of New Zealand. The present experiment aimed to measure in vivo organic matter digestibility and digestibility of organic matter in the dry matter in tree fodder and investigated whether the in vitro system of Roughan and Holland can be used to predict in vivo organic matter digestibility and digestibility of organic matter in the dry matter in poplar and willow tree fodder, which contains high concentrations of secondary compounds, including condensed tannin. In vivo work showed that the digestibility of tree fodder declined from late spring to autumn (p < 0.05) and that this decline was much smaller than the decline in digestibility of grass‐based pastures in New Zealand over the same time period. Mean metabolizable energy concentrations and digestibilities were generally higher for willow than for poplar. The in vitro enzymatic system of Roughan and Holland can be used to predict in vivo digestibility of tree fodder. A standard curve using in vivo values determined with animals fed tree fodder would be preferable, owing to the very different chemical compositions of pasture and tree fodder, particularly the greater concentration of secondary compounds in willow and poplar. However, the accuracy and the range of prediction need to be improved. Willow and poplar had similar ME concentrations to high‐quality lucerne hay; willow cuttings also had similar ME and CT concentration to vegetative Lotus corniculatus, a high‐quality forage legume. Copyright © 2005 Society of Chemical Industry