Aim : Methodology :Results : Interpretation :Mesosulfuron-methyl is one of the post emergence sulfonylurea herbicides and biotransformation is major process for its degradation. Persistence of mesosulfuron-methyl in soil mainly depends on temperature and microbial population. When herbicide is applied at different time of a day, it is exposed to different soil environmental factors. This interaction of mesosulfuron-methyl with soil environmental factors may affect microbial population differentially. Hence, the main objective of the study was to find how the day time application of mesosulfuron-methyl at different doses affects the microbial population of wheat rhizosphere.Five weed control treatments, comprising of three doses of mesosulfuronmethyl (10.0, 11.5 and 12.0 g ha ) including one hand weeding and weedy check as main plot treatments, were superimposed with three day times of herbicide application (8 a.m., 12 p.m. and 6 p.m.) as sub plot treatments and laidout in split plot design with four replications. Soil sample were collected at 5, 10, 30 and 80 days after herbicide application. The developed colonies were counted using serial dilution plate count technique and expressed as colony forming units (cfu) per gram dry soil.Bacterial population was significantly (14.20 and 16.14%) higher under 12 g ha mesosulfuron-methyl application over unweeded check at 10 and 30 days after, respectively. However, competition with dominant bacterial population and toxic effect of herbicide immediately after application inhibited fungal and actinomycete species. But with time, adapted fungal population was increased to 24.77 and 58.13% under 11.5 g ha , and 28.77 and 73.40% under12.0 g ha application at 30 and 80 days after application, respectively, over 10 days after application. While, the population of actinomycetes and was ever less under mesosulfuron-methyl in comparison to non-herbicidal treatments. However, fungal and actinomycetes population survived successfully in case of mesosulfuron application during noon hours even at higher doses due to less residue in soil. Population ofand actinomycetes (up to 80 days) as well as fungi (upto 10 days) was affected due to mesosulfuron doses except bacteria. Similarly, fungal and actinomycetes population was affected identically due to morning and evening application of mesosulfuron. On the contrary bacterial and population did not vary. herbicides to achieve higher weed control efficacy may also have ill effects on public health and environment. Whitcomb (1999) also reported that herbicides which inhibit acetolactate synthase enzyme, affect many species of higher plants as well as bacteria, fungi, yeasts and algae. Since photodegradation, photolysis and volatilization processes are not significant for mesosulfuron-methyl, hence instead of increasing its rate only, change in time of application during day hours (morning, noon and evening) could be taken into consideration so as to enhance activity of mesosulfuron-methyl against weeds even with lower dose. But when...

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