In this study, dehydrogenase activity increased from the second to the sixth week of treatment. This might be due to increase in microbial populations with the capability of utilizing the herbicides as carbon source. Under laboratory conditions, a normal dose of glyphosate inhibited dehydrogenase activity by 5 - 10% (3 weeks after herbicide application) (Nada and Mitar, 2002). A tenfold dose of glyphosate affected negatively, the activity of this oxide-reducing enzyme by 5% (11 weeks after herbicide application) (Schuster and Schroder, 1990).