Pesticide residues contributing to the contamination of forage and feed may influence microbial function of the rumen and fermentation parameters. Phosalone is an organophosphate compound that frequently uses as a pesticide in iran and may contaminate the animal feed with it. Adequate risk management in the animal feeds depends on knowledge of absorption, metabolism, carry-over and toxicological profile of these substances and theirs effects on rumen microorganisms. Hence, a study was conducted for evaluation of some levels of phosalone on in vitro fermentation parameters. The results indicated that dry matter disappearance (DMD) after 24 h incubation), cumulative gas production parameters, significantly reduced by increasing the levels of phosalone rather than control group (p < 0.05), although level of 100 ppm had no significant effect for NH3-N and DMD (96 h incubation) rather than control group, but there was a significant effect for other levels of phosalone (250 and 500 ppm) between treatments rather than control group (p < 0.05). There was no significant effect for pH on the culture media. So it seems phosalone had an adverse effect on some of the DMD and rumen fermentation parameters under in vitro procedure at concentration > 100 ppm.

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