Effect of Thyme Essential Oils (Thymus hyemalis and Thymus zygis) and Monensin on in Vitro Ruminal Degradation and Volatile Fatty Acid Production

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Abstract
The effect of the essential oils of thyme on the in vitro ruminal degradability of a barley seed/alfalfa hay substrate was studied. Two essential oils were used, one from Thymus hyemalis (TH), rich in carvacrol, and the other from Thymus zygis (TZ), rich in thymol. Four experimental treatments of in vitro degradability, using the Daisy II200/220 incubator, were conducted including a negative control (CO), a positive control at 7.5 μg/mL of monensin (MO), and two treatments with essential oils (TH or TZ) at 1.35 μL/mL. The material was incubated at 39.5 °C for various lengths of time. At each time, the disappearance of dry matter, crude protein, and neutral detergent fiber was measured. Volatile fatty acids (VFAs) were determined after 48 h of incubation. CO and MO provided (p < 0.01) higher values of potential degradability (a + b) of DM than the TH and TZ treatments (72.6 and 70.8 vs 53.2 and 48.2%, respectively). Also, crude protein degradability was lowest in the essential oil treatments. The CO treatment showed the highest potential degradability of NDF. The values of VFA production obtained (p < 0.001) with CO and MO treatments were higher than those obtained with TH and TZ treatments (21.0 and 19.1 vs 11.2 and 10.1 mM). The essential oils decreased the molar proportion of propionate, increasing the acetate/propionate ratio. In conclusion, the effects of essential oils at assayed doses would not be nutritionally beneficial to the ruminal energetic metabolism. Keywords: In vitro degradability; monensin; Thymus hyemalis; Thymus zygis; essential oils

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