The study investigates the effects of tamanu kernel cake (TKC) as protein substitution in the dietary concentrate on ruminal digestibility and methane emission. TKC is a by-product of the plantation industry of tamanu oil. The dietary concentrate consisted of wheat pollard, rice bran, corn grain, palm kernel cake, and soybean meal. The concentrate was formulated to contain crude protein and total digestible nutrients of approximately 15% and 65%, respectively. In the present study, TKC was used to substitute protein sources at different levels, such as 0% (T0), 50% (T1), and 100% (T2). Another dietary treatment was also prepared by adding 0.5 mineral salt to T2 (T3). All dietary treatments were incubated in rumen buffer according to the method of Tilley and Terry for 48 h at 39°C. In the results, the digestibility of dry matter and organic matter from dietary T1, T2, and T3 were not different compared to T0. In ruminal fermentation, dietary treatment did not affect total VFA and ammonia. Dietary T2 and T3 resulted in lower methane emissions than dietary T0 (p<0.05). Additional mineral salt in dietary T3 did not affect methane emission compared to dietary T2. The present study concluded that the substitute of protein source with TKC at 30% reduced methane production effectively without negatively affecting ruminal digestibility and fermentation.
