Effect of protozoa and urea level on in vitro starch disappearance and amylolytic activity of ruminal microorganisms

In vitro experiments were conducted to study the effects of protozoa on starch fermentation and amylolytic activity of ruminal bacteria and protozoa. The first experiment consisted of a randomised complete block design with a 3 × 2 × 5 factorial arrangement of treatments. Treatments were grain source (corn, high moisture corn or grain sorghum), inoculum source (a mixed ruminal population or one free of protozoa) and urea addition (0, 5, 10, 20 or 30 mg g−1 of grain). An interaction (P < 0.01) existed between inoculum source and urea level on in vitro starch digestion (IVSD) and ruminal ammonia N concentration. Ammonia N (quadratic, P < 0.10) and IVSD (quadratic, P < 0.01) increased when urea was added in the absence of protozoa. In vitro starch digestion did not respond to urea when protozoa were present. The total concentration of volatile fatty acids (VFA) was greater (P < 0.01) with protozoa than without them (66.8 vs. 58.7 mM). In incubations with protozoa, the molar proportion of propionate was lower (P < 0.01) than for the mixture of bacteria and protozoa (20.9% vs. 25.2%). The IVSD (35.44–40.17%) and ammonia N (8.37–32.86 mg per 100 ml) increased linearly (P < 0.01) with the level of urea added. In the second experiment, ruminal fluid from steers fed grain was used to determine the amylolytic activity. No difference (P > 0.01) in amylase specific activity (units ml−1 of protein) was observed for protozoa (0.0802), bacteria (0.0472), and a mixed population (0.0435). The contribution of protozoa on the in vitro starch digestion may be dependent on the ammonia N concentrations.