Evaluation of four tropical browse legumes as nitrogen sources: Comparison of in vitro gas production with other methods to determine N degradability

The value of the tropical browse legumes Acacia cornigera (ACA), Albizia lebbekoides (ALB), Enterolobium cyclocarpum (ENT) and Leucaena leucocephala (LEU) as ruminal N sources was examined. The N content (g/kg dry matter) was 38.0, 28.6, 35.1 and 46.7, and acid detergent insoluble N (ADIN; g/kg dry matter) was 9.0, 10.7, 18.7 and 19.8 for ACA, ALB, ENT and LEU, respectively. In vitro gas production at 24 h ranked ACA > ENT > LEU > ALB (P < 0.001), except that there were no differences between ENT and LEU after 12 h of incubation. The N degradability was estimated by the in vitro relationship between gas production and ammonia N concentration (NDg), N disappearance from incubation residue (NDd), and incubation with Streptomyces griseus enzymes (NDe). To determine NDg, legume samples (601 ± 0.54 mg) were incubated with 0 mg (n = 3), 150 mg (n = 2) or 300 mg (n = 2) starch for 24 h in two experiments. Regression of ammonia (y, mg) on gas produced (x, ml) for each browse in each experiment (n = 7) yielded r2 coefficients between 0.974 and 0.997. Average estimated NDg were 0.32, 0.22, 0.63 and 0.22 for ACA, ALB, ENT and LEU, whereas NDd had N digestion coefficients of 0.40, 0.17, 0.70 and 0.36, and NDe 0.41, 0.24, 0.44 and 0.36, respectively. NDg was lower than NDd and NDe for LEU, but higher than NDe for ENT (P < 0.05). However, methods correlation ranged from 0.73 to 0.92. In vitro estimated N digestion suggested that 0.52, 0.17, 0.30 and 0.41 of undegraded N from ACA, ALB, ENT and LEU was intestinally digestible.