Feed supplementation blocks for increased utilization of tanniniferous foliages by ruminants

Ruminant animals raised in arid, semi-arid and mountainous regions in various parts of the world rely heavily on shrub and tree species as the main diet component. The presence of secondary compounds – mainly tannins – in a wide range of these plant species constrains their fodder potential. Tannins may cause toxicity when present in the hydrolysable (HT) form, and may reduce considerably the nutritive value of browse and tree foliage when they are present in the condensed (CT) form. Tannins form complexes primarily with proteins, but also with carbohydrates, amino acids and several minerals, thereby reducing intake, digestion and animal growth. A number of methods have been tested to de-activate tannins in, or to remove them from, woody species. Amongst these, the use of polyethylene glycol (PEG; MW 4 000 or 6 000), which inactivates tannins by forming complexes with them, was shown to be a promising way to increase the nutritive value of tanniniferous browse and tree foliage. PEG was supplied to animals on tannin-rich diets in different ways (in concentrate or feed blocks, dissolved in water, or sprayed on feed). Feed block, a solidified blend of agro-industrial by-products, was found to be an efficient supplement for increasing intake, rumen fermentation, digestibility and daily weight gain in sheep or goats fed on shrub foliage high in tannins. These positive effects were further strengthened when PEG was incorporated into these blocks. The advantage of these supplements lies in the synchronized, fractionated and balanced supply 21 Institut National de la Recherche Agronomique de Tunisie, Laboratoire des Productions Animales et Fourragères, rue Hédi Karray, 2049 Ariana, Tunisia. E-mail: 22 Animal Production and Health Section, Joint FAO/IAEA Division, International Atomic Energy Agency, P.O. Box 100, Wagramerstrasse 5, A-1400 Vienna, Austria. E-mail: impaginato_25_11_2007.indd Sez1:185 25-11-2007 16:54:53 Increased utilization of tanniniferous foliages by ruminants 186 of main nutrients to rumen microflora and the host animal on tannin-rich diets, results attributable to the slow release of PEG on licking of the block by the animal. This slow-release characteristic provides an economic use of this relatively costly tannin-inactivating compound, and maximizes its positive effect on the fodder potential of tanniniferous browse or tree species. Information on the use of feed blocks as supplements for ruminant animals on tannin-rich shrub and tree foliage diets is scanty. The importance of these alternative supplements to livestock raised under agro-pastoral systems and the need to pay more attention to this promising technology are highlighted in this paper. An auxiliary role of feed blocks is their possible use as a carrier for additives: several specific minerals for increasing reproductive performance; anthelmintic medicines to control gastro-intestinal parasites in browsing animals; and rumen modifiers such as saponins to decrease protozoa in the rumen, leading to more efficient microbial protein production. Therefore research and development programmes must be undertaken, aimed at field evaluation, economic analysis, dissemination to a large number of farmers, and better integration of the block technology in feeding systems.