The aim of this study was to investigate the effects of various doses of essential oils (EO) of thyme (TEO) or cinnamon (CEO) on in vitro rumen degradation of protein of a commercial total mixed ratio (TMR) con-taining 16.4% crud protein (CP). Treatments were TMR without any of EOs (control); control plus 25 μL of TEO (TEO25); control plus 50 μL of TEO (TEO50); control plus 25 μL of CEO (CEO25) and control plus 50 μL of CEO (CEO50). Rumen fluid was collected before the morning feeding from two rumen fistulated dairy cows. Approximately, 90 mL of buffered rumen fluid (BRF), 400 mg of feed sample plus carbohy-drates (maltose, xylose and starch) at four concentrations (100, 200, 300 and 400 mg) were added to screw-cap bottles. Gas production (mL) and ammonia nitrogen concentration (mg) in each bottle were measured at 4, 8, 12, 24 and 30 h post-incubation and in vitro crude protein degradation (IVDP) was calculated via a linear regression. Both TEO and CEO caused a significant reduction in IVDP values compared with those of control (P<0.05). The IVDP values for treatments of control, TEO50 and CEO50 after 30 h post incuba-tion were 0.56, 0.33 and 0.48, respectively. Amount of the readily fermentable fraction and the potentially degradable fraction of the feed protein were affected significantly by treatments (P<0.05). Also, the effec-tive crude protein degradability (EPD) was significantly affected by the EOs (P<0.05). These results sug-gest that the TEO and CEO might be used for manipulating the ruminal protein degradability.

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