Nutritional uniformity of crude protein fractions in some tropical browse plants estimated by two in vitro methods

Twenty eight plant species consisting mainly of leaves from leguminous browse trees collected in Tanzania, Colombia and Vietnam were analysed for total crude protein (CP), total soluble phenolic compounds, neutral detergent fibre (NDF) and CP in neutral detergent fibre (NDF-CP). Two in vitro methods were used to determine digestibility of the dry matter (IVDM) and the CP. Both methods involved a 48 h incubation with rumen liquor followed either by a 16 h neutral detergent extraction (IVND) to isolate the fibre fraction or a 16 h HCl-pepsin digestion (IVPEP) to mimic abomasal digestion. True CP digestibility was estimated as the slope of the line in a Lucas test when disappearance per unit of sample DM was regressed upon sample content. Crude protein contents ranged from 105 to 309 g/kg DM with 6 to 72% bound to the NDF. The mean IVDMD and digestible CP (DCP) for the IVND method were 73.9% (SD ± 13.1) and 17.9% (SD ± 5.9), respectively and were significantly (p < 0.05) higher than for the IVPEP method of 67.3% (SD ± 15.4) and 16.0% (SD ± 6.5), respectively. As the proportion of NDF-CP in the total CP increased, CP digestibility (CPD) decreased linearly (p < 0.05) for both in vitro methods. There were no significant (p > 0.05) relationships between the content of phenolic compounds and either CPD or DCP of the browse species. The Lucas test revealed that CP did not behave uniformly in either of the in vitro methods, reflected by high standard deviations of the regression slopes and regression slopes more than 100%. The estimated true digestibilities of the CP fraction were 105% (SD ± 13.6%) and 105% (SD ± 10.2%) for the IVPEP and the IVND methods, respectively. The metabolic CP fractions were 6.89 (SD ± 3.04) and 5.13 (SD ± 2.29) g/100 g DM feed for the IVPEP and IVND methods, respectively. True digestibility of NDF-CP for the IVND method estimated by the Lucas test was 40% (SD ± 9.3%) with an intercept of 0.03 (SD ± 0.71) g/100 g DM. Tannins were considered to be the main factor responsible for the non-uniformity of the CP fractions in the browse species tested. It was concluded that digestibility of CP in tropical browse may not be predicted by the content of CP or phenolic compounds.