Abstract
Background:
Improving protein utilization in dairy diets requires an appropriate balance between rumen degradable protein (RDP) and rumen undegradable protein (RUP), along with sufficient availability of limiting amino acids such as lysine (Lys) and methionine (Met). Tropical legumes offer an alternative protein source, but some species (Indigofera and Moringa) contain high RDP, necessitating protection to increase bypass protein. Meanwhile, Leucaena had high RUP but also high levels of total phenolic compounds. In addition, Lys and Met are involved in protein synthesis, and their supplementation may enhance protein utilization. However, studies on protected tropical legumes, as well as on Lys & Met supplementation in relation to rumen fermentation and post-ruminal digestibility, remain limited.

Aim:
This study aimed to evaluate the effects of lysine and methionine supplementation and protected protein from tropical legumes on rumen fermentation and post-ruminal nutrient digestibility.

Methods:
Post-ruminal digestibility was analyzed using a three-step in vitro method. Protein protection wa applied via a moist-heat method to enhance bypass protein availability. The experiment employed a randomized block design with five dietary treatments and four blocks based on rumen fluid. The treatments consisted of R0 (control diet), R1 (R0+ Lys Met supplementation), R2 (R0+15% protected indigofera), R3 (R0+15% protected moringa), and R4 (R0+15% protected leucaena). Parameters measured included rumen pH, ammonia (NH3) concentration, total and partial volatile fatty acids (VFA), protozoal population, MPS, and degradation and digestability of dry matter (DM, organic matter (OM), and crude protein (CP). Data were analyzed using ANOVA followed by Duncan's test.

Results:
Treatments did not affect rumen fermentation characteristics or protozoa populations. Treatment R2 significantly decreased butyrate proportion. No significant effects were observed on total DM and OM digestibility, whereas significant differences (P<0.05) were found in degradable (D) and undegradable (UD) OM fractions and total CP digestibility.

Conclusion:
In conclusion, lysine and methionine supplementation, or inclusion of 15% protected tropical legumes, maintained fermentation characteristics and nutrient digestibility, with protected Moringa showing the most optimal performance among protected legume treatments by improving post-ruminal digestibility.

Key words: Amino acid; In vitro; Moist heating; Protein; Tropical forage.