In ruminants, consumption of diets high in readily fermentable carbohydrates, such as starch, may result in a rapid increase in rumen acid load leading to a drop in ruminal acidosis. The aim of the present experiment was to determine the effect of various semi-arid native plant essential oils (EO) including thyme (TH) and peppermint (PE) leaves, and cumin (CU) and fennel (FE) seeds on in vitro medium pH, acidogenicity value (AV) and dry matter disappearance (DMD) of a low forage basal diet (BD) containing 20% alfalfa hay and 80% concentrate. Experimental treatments were BD, BD+TH, BD+CU, BD+FE and BD+PE. Approximately, 1 g of the basal diet was placed into a 125 ml bottle, as alone or combined with the EO evaluated, then 30 ml of buffered rumen liquor [60% diluted buffer, and 40% rumen liquor] was added (n=4). Each essential oil was applied as 1 ml/100 ml of the medium. The rumen fluid was collected, before the morning feeding, from three rumen cannulated sheep (body weigh; 45±2 kg) and filtered through four layers of cheesecloth. Animals were fed 1.4 kg alfalfa hay and 400 g concentrate (CP; 165 g kg-1 of DM), twice daily (0800 h and 2000 h). Bottles were incubated for 24 hours under anaerobic condition in a water bath at 38.6 °C. After the incubation period, medium pH was measured immediately. Each bottle contents were filtered through a 45 µm filter, then unfiltered residual was dried using an air-forced oven at 65 °C for 48 h, then weighed to determine the dry disappearance. A sample (2 ml) was withdrawn from each bottle filtered content, then transferred to a tube containing 50 mg (excess) of CaCO3 powder, then centrifuged at 4000 RPM for 10 min and supernatant was removed; next by means of atomic absorption device its calcium concentration was determined. The AV was calculated as the product of Ca concentration (from the analysis) and fluid volume (30 ml) divided by the sample weight (1 g). The experiment was carried out in a completely randomized design. Statistical analysis was done by SAS software using GLM procedure. Thyme leaf EO caused a significant (P< 0.05) increase in the pH of the medium (6.13) and a significant decrease in the AV (1.1) compared with the BD as control (4.67 and 12.7, respectively). Therefore, the potential of this essential oil to optimize both pH and AV of a rich concentrate ruminant diet might be an important finding of the present study.

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