Indian Journal of Weed Science 46(3): 229–233, 2014 Effect of herbicides on soil microorganisms in direct-seeded rice Simerjeet Kaur*, Surjit Singh and R.P. Phutela Department of Agronomy, Punjab Agricultural University, Ludhiana, Punjab 141 004 Received: 29 July 2014; Revised: 5 September 2014 ABSTRACT The use of herbicides in direct-seeded rice may affect the biological equilibrium of the soil and thus influence the nutrient status, health and productivity of the soil. To study the effect of herbicides on soil microbial population of direct-seeded rice, field experiment was conducted at Punjab Agricultural University, Ludhiana (Punjab) during Kharif 2009 and 2010. Sixteen weed control treatments, viz. pendimethalin 0.75, butachlor 1.50, thiobencarb 1.50, anilofos 0.375, pretilachlor 0.75, oxadiargyl 0.09 and pyrazosulfuron-ethyl 0.015 kg/ha applied as pre-emergence and each followed by bispyribac 0.025 kg/ha at 30 days after sowing; two hand weedings and unweeded control were tested. The results revealed that viable microbial population was influenced to varying degrees with different weed control treatments during both the years. The herbicides, viz. pendimethalin, butachlor, thiobencarb, anilofos, pretilachlor, oxadiargyl and pyrazosulfuron-ethyl as pre-emergence and bispyribac as post-emergence were safe for soil microbial populations at recommended rate. Key words: Actinobacteria, Bacteria, Direct-seeded rice, Fungi, Herbicides, Non-target organisms To meet the global rice demand, it is estimated that about 114 million tonnes of additional milled rice need to be produced by 2035 which is equivalent to an overall increase of 26 per cent in the next 25 years (Kumar and Ladha 2011). To sustain present food self- sufficiency and to meet future food requirements, In- dia has to increase its rice productivity by 3 per cent per annum but the possibility of expanding the area under rice in the near future is limited. There has, how- ever, been stagnation in rice productivity in recent years and long-term experiments showed a declining trend in rice yield. Due to receding water table, rising costs of labour for transplanting of paddy and the adverse effects of puddling on soil health; direct-seeded rice (DSR) is gaining popularity. But, weeds are the main constraint for farmers practising direct- seeding so use of herbicides both pre- and post-emergence is required for good crop. An unintended consequence of the ap- plication of herbicides is that it may lead to significant changes in the populations of microorganisms and their activities thereby influencing the microbial ecologi- cal balance in the soil (Min et al. 2002, Saeki and Toyota 2004) and affecting the productivity of soils. When herbicides are applied in soil, they may exert certain side effects on non-target organisms. There- fore, there has been considerable interest on the influ- ence of herbicides on the soil microflora and microbially mediated processes. The effects of these chemicals on certain variables are associated with mi- crobial biomass and their activity (Wardle and Parkinson 1991). The increasing reliance of rice culti- vation on herbicides has led to concern about their eco- toxicological behaviour in the rice field environment. Soil health and microbial diversity have become vital issues for the sustainable agriculture. Loss of micro- bial biodiversity can affect the functional stability of the soil microbial community and soil health. Gener- ally, there are some negative effects of herbicides on the population level or composition of species. The impact of applied herbicides on the soil microbial populations were studied which included analysis of bacteria, actinobacteria and fungi counts. In Punjab, seven pre-emergence herbicides are being used in di- rect-seeded rice for chemical weed control, therefore, this work was carried out to estimate the counts of these microbes at different period of crop growth after their application. MATERIALS AND METHODS The experiment was conducted at Students’ Re- search Farm, Department of Agronomy, PAU, Ludhiana during Kharif season of 2009 and 2010. Ludhiana is situated in Trans-Gangetic Agro-Climatic zone, representing the Indo-Gangetic Alluvial plain at 30°56' N latitude, 75°52' E longitude and at an alti- tude of 247 m above mean sea level. The meteorologi- cal data recorded during rice growing season showed that the overall weather during 2009 remained good for the rice crop but it was not as conducive for rice during 2010. The average minimum temperature dur- ing the crop growing season of 2010 was higher by 1 °C than normal. The total rainfall of 818 and 627.6 *Corresponding author: simer@pau.edu