The effects of proanthocyanidins from Calliandra calothyrsus and the alkaloid sparteine on in vitro fiber digestion

Tropical legumes accumulate anti-nutritive compounds whose impact on the nutritive value of livestock feeds is difficult to evaluate. Many studies have focused on the effects of a single compound while animals simultaneously cope with several. In vitro measurements of gas production were used to study the roles of proanthocyanidins (PA) and alkaloids (AL) on nutrient use by ruminants. Napier grass (Pennisetum purpureum (Schum) cv Bana: NG) and its isolated NDF (iNDF) were incubated with four levels (0, 0.14, 0.28 and 0.42 mg ml−1) of PA from Calliandra calothyrsus (Meissner), a tropical leguminous shrub, and four levels (0, 0.25, 0.50, 0.75 mg ml−1) of sparteine, a quinolizidine alkaloid. Coefficient of neutral detergent fiber digestibility of intact NG was lower (P < 0.001) than that of the iNDF (0.431 versus 0.598) and the tannin–alkaloid interaction was not significant. Sparteine depressed NDF digestibility by 0.054 and PA by 0.115. Neutral detergent fiber digestibility of both iNDF and intact NG was inversely related to PA (y = −0.0424x + 0.705; R2 = 0.99 and y = −0.0326x + 0.513; R2 = 0.98, respectively). Sparteine did not affect (P > 0.05) final gas volume or its rate of production. Proanthocyanidins at 0.42 mg ml−1 decreased final gas volume from 13.4 ml of gas per 100 mg of substrate (control) to 11.4 ml and the rate of gas production from 0.053 to 0.037 h−1. However, there was an interaction (P < 0.01) between AL and PA when rate of gas production was the dependent variable. Proanthocyanidins increased lag time (P < 0.05) in NDF fermentations linearly (y = 1.92x + 2.2; R2 = 0.9942) while sparteine had no effect on lag (P > 0.05). Microbial protein synthesis estimated from purine yield was suppressed (P < 0.01) by AL (from 34.1 to 24.7 μg), but low levels of PA (0.14 mg ml−1) increased purine yield compared to the control and higher PA levels. Simulations with the Cornell Net Carbohydrate and Protein System (CNCPS) showed that PA decreased rate of NDF digestion, thus reducing metabolizable energy and protein allowable milk. The effects of tannins, alkaloids and other secondary compounds should be included in ration formulation models.