ABSTRACT This study aimed to determine the effects of fermentation parameters of sunflower oil, vegetable oil blend, palm oil and chicken oil addition to sunflower meal, wheat grain, alfalfa herbage and corn silage at different rates (4% and 6%, dry matter basis). The oil supplementation up to 6% rate to the alfalfa herbage and corn silage did not adversely affect the level of in vitro gas production, metabolic energy (ME), net energy lactation (NEL), organic matter digestion (OMD) and molarities of acetic, propionic, butyric, iso-valeric, iso-butyric and valeric acids and number of total protozoa (P > 0.05). The oil supplementation to alfalfa herbage increased molarities of butyric, iso-valeric, iso-butyric and valeric acids and number of total protozoa (except sunflower oil) in rumen fluid (P < 0.05). Palm oil decreased the in vitro gas production, ME, NEL and OMD values of sunflower meal (P < 0.05). Supplementation of vegetable oil blend to wheat grain increased ME, NEL and OMD values and butyric acid molarity in rumen fluid (P < 0.05). The in vitro ruminal ammonia–nitrogen concentrations of alfalfa herbage and corn silage increased by vegetable oil blend, palm oil and chicken oil up to 6% (P < 0.05). As a result, the nutrient content of the feedstuffs used changed the in vitro ruminal fermentation values of the oil additive to be added. The addition of sunflower oil, vegetable oil blend, palm oil and chicken oil at 4% and 6% rates to corn silage, alfalfa herbage, sunflower meal and wheat grain feed found to differ on in vitro ruminal fermentation parameters.