Effect of replacing soybean meal with marula nut meal on rumen dry matter and crude protein degradability

In situ rumen degradability of dry matter (DM) and crude protein (CP) in ruminant diets composed of graded inclusion levels of marula nut meal (MNM) as a protein source to replace soybean meal were evaluated. The five diets denoted as diet 1 (0.0 g kg^-1 MNM: control), diet 2 (45.5 g kg^-1 MNM), diet 3 (91.7 g kg^-1 MNM), diet 4 (138.4 g kg^-1 MNM), and diet 5 (185.9 g kg^-1 MNM) were tested. Three cannulated mid-lactating Holstein cows were used to incubate triplicate subsamples of each diet for 2, 4, 8, 16, 24, and 48 h in polyester bags. The soluble fractions of DM and CP increased (P < 0.05) with increasing levels of MNM (138.4-185.9 g kg^-1 MNM). However, the insoluble but degradable fractions of DM and CP increased (P < 0.05) with decreasing levels of MNM (45.5-0.0 g kg^-1 and 91.7-0.0 g kg^-1 MNM, respectively). While the degradation rate of fraction and the effective degradability of DM increased (P < 0.05) with decreasing levels of MNM (91.7-0.0 g kg^-1), those of CP increased with increasing levels of MNM (138.4-185.9 g kg^-1 MNM) in the diet. These results suggest that dietary inclusion of MNM up to 91.7 g kg^-1 in ruminant diets can provide undegradable protein to be absorbed in the small intestines and used for metabolic function, while an inclusion of up to 185.9 g kg^-1 can be degraded in the rumen for microbial protein synthesis.