Interactions among tannins, supplementation and polyethylene glycol in goats given oak leaves: effects on digestion and food intake

Effects were studied on food intake and diet apparent digestibility of giving to goats once daily a basal diet rich in tannin from inclusion of leaves of Quercus calliprinos either supplemented with a high carbohydrate or a high protein food. Also interactions with polyethylene glycol (PEG) were investigated. The results of the present work confirmed our presumption that the content of tannin in some Mediterranean browse is so high that it may negatively affect the utilization of protein in supplementary foods. Neutralizing the tannins with once-daily provision of PEG proved to be an effective means of preventing the negative effect. Providing 10 g/day PEG to goats given Quercus calliprinos leaves ad libitum and supplemented with 300 g/day concentrates containing 160 g crude protein per kg DM increased digestible crude protein intake by 50 g/day. When the concentrate food was given to goats, leaf dry-matter intake decreased significantly (from 664 to 565 g/day) and the goats lost weight rapidly. Therefore, supplementing tannin-rich leaves with concentrate food is recommended only if done in combination with PEG. High protein supplementation increased leaf (from 664 to 844 g/day) and digestible protein intakes (from 4·8 to 92·3 g/day) but a considerable portion of the protein supplementation was wasted due to interaction with tannins. PEG may allow economies in the use of such high-cost foods due to the greater efficiency of protein utilization (digestible crude protein intake increased from 92·3 to 122 g/day) of the supplementary food and to increased intake and protein utilization of the basal leaf diet (from 844 to 1023 g/day).