Reduction of tannin level in a tropical legume (Desmodium ovalifolium) with polyethylene glycol (PEG): effects on intake and N balance, digestion and absorption by sheep

Abstract

Three feeding trials were conducted to determine the effect of reducing the concentration of condensed tannins (CT) in Desmodium ovalifolium Wallick ex Gagnep (CIAT 350) on voluntary feed intake, N digestion, absorption and balance in sheep. Polyethylene glycol (PEG) was used as a tannin-binding agent to reduce extractable CT. In Trial 1, 12 growing lambs were assigned at random to one of four treatments: ad libitum feeding of Centrosema macrocarpum Bentham (CIAT 5713), C. macrocarpum plus PEG (50 g kg-1 DM), D. ovalifolium, or D. ovalifolium plus PEG (50 g kg-1 of DM). Extractable CT values were 46 and 17 g kg-1 for D. ovalifolium and D. ovalifolium plus PEG, respectively. Forage of C. macrocarpum contained 1.7 g kg-1 extractable CT and was not affected by PEG addition. Voluntary DM intake did not differ (P > 0.05) between C. macrocarpum treatments, but was higher (P<.10) for D. ovalifolium low in CT than for D. ovalifolium high in CT (23 vs. 19 g kg-1 BWd-1). When D. ovalifolium high in CT was fed fecal N excretion (4.4 vs 2.8 g d-1) was higher and N retained (1.3 vs. 4.3 g d-1) was lower (P<.10). In Trial 2, three ruminally and duodenally fistulated sheep were used in a 3 x 3 latin square design. Treatments were restricted feeding of D. ovalifolium (Control, 41 g kg-1 CT), D. ovalifolium plus 3.5% - PEG (17 g kg-1 CT) or D. ovalifolium plus 7.0% - PEG (16 g kg-1 CT). Mean rumen NH3-N concentrations (mg dL-1) were 5.2, 12.2, and 12.7 for the control, 3.5 - PEG and 7.0 - PEG diets, respectively. Fecal N was lower (P<.05) for the low CT diets, but apparent N absorption increased (P<.10) with increasing levels of extractable CT in the diet. In Trial 3, three sheep, fistulated at the rumen, duodenum, and ileum, were used in a switch-back design. Treatments were restricted fedding of D. ovalifolium (Control, 46 g kg-1 CT) and D. ovalifolium plus 5.0% - PEG (18 g kg-1 CT). Sheep fed forage high in extractable CT had higher (P < 0.01) N flux to the duodenum than those fed forage low in CT (8.3 vs. 5.9 g d-1). Nitrogen absorption in the small intestine was also higher (P<.05) with the diet high in CT (3.0 vs 3.8 g d-1). High levels of extractable CT in this tropical legume reduced voluntary intake and protein degradation in the rumen (i.e. less rumen ammonia), but in combination with feed restriction, increased N flow and absorption in the small intestine.