Effect on time of application of post-emergence herbicides on weed seed bank in transplanted rice (Oryza sativa L.) at Kyamore plateau and Satpura hill zones of Madhya Pradesh

The experiment was conducted during kharif seasons of 2010 and 2011 at Jabalpur (Madhya Pradesh), India. The study was done to compare weed seed bank at different soil depths (0 – 5 cm, 5 cm – 10 cm and 0 - 15 cm). The results indicated that weed seed count differed significantly in all treatments with the application of the post emergence herbicides over control. The day time application of post emergence herbicides on weed seed bank was found to be non significant. The highest weed seed counts was in the control plot 624.31 kg⁻¹ of soil at 0 - 5 cm of soil depth at all the successive stages, which declined linearly in different soil layers 427.70 kg⁻¹ of soil at 5 - 10 cm and 233.71 kg⁻¹ of soil at 10 cm - 15 cm, respectively. The decline may be due to exhaustion of weed seeds in successive stages, lesser availability of food material and presence of anaerobic condition. Among different post emergence herbicides, application of bispyribac sodium recorded maximum weed seed counts at 21 d, 35 d, 60 d and harvest days after herbicidal application (DAHA) (24.97 - 119.89 kg⁻¹ of soil) except at 7 DAHA (11.54 kg⁻¹ of soil) up to 0 cm – 5 cm. Weed seed counts were also the maximum with the application of bispyribac sodium in 5 cm - 10 cm (27.67–65.88 kg⁻¹ of soil) and 10 cm - 15 cm (22.02–36.62 kg⁻¹ of soil) of soil depth at successive stages. Minimum weed seed counts were recorded with the application of pyrazosulfuron-ethyl in 0 cm - 5 cm (3.89 - 88.55 kg⁻¹ of soil), 5 cm - 10 cm (8.91 - 32.74 kg⁻¹ of soil) and 10 cm - 15 cm of soil depth (5.58 - 14.33 kg⁻¹ of soil) at all the successive stages, respectively. The highest relative weed seed density was that of Fimbristylis miliaceae L. Vahl. (11.23%), followed by Cyperus iria L. (11%) and Alternenthra sp. (9.07%). The lowest weed seed density was observed with Cynodon dectylon (L.) Pers. (5.49%) and Paspalam distichum L. (5.01%)