Weed seed bank dynamics in soil under different agronomic practices and its implications in maize (Zea mays L.) field.

Abstract

Weed and crop management practices can modify the weed seed bank and hence the weed flora. Studies on the weed seed bank dynamics under different management practices in autumn planted maize were conducted at the Agronomic Research Area, Department of Agronomy, University of Agriculture, Faisalabad during 2013 and 2014. Experiment-1 comprised of four tillage approaches (viz. cultivator only, rotavator + cultivator, moldboard (MB) plough + cultivator and chisel plough + cultivator) and three weed management practices (viz. atrazine @ 1 kg a.i. ha-1, hand hoeing at 20 days after sowing (DAS) and 40 DAS and no weeding). Soil samples from 0-5, 5-10, 10-15, 15-20 and 20-25 cm were collected before sowing and after harvesting to record the status of the weed seed bank prior to and after treatment applications. Experiment-2 comprised of three sowing methods (viz. ridge sowing, bed sowing and flat sowing; R × R = 75 cm, P × P = 25 cm) and three mulching techniques (viz no mulch, polythene [clear plastic] mulch and sorghum straw mulch @ 5 t ha-1). In Experiment-1, the results showed 16 weed species to be represented in the soil seed bank with an abundance of horse purslane, jungle rice, johnsongrass and false amaranth. Maximum weed seed germinability (30%) was found in the 0-5 cm soil cores, while in the lowest 20-25 cm soil core only 8-10% viability was found. The top soil layer (0-5 cm) contributed more than 35% of the total weed seed bank sampled. The MB plough proved to be the most effective in burying weed seeds up to a 20 cm depth. Beyond 20 cm depth, only the chisel plough caused reduction in weed seed bank. Hand hoeing proved (> 16%) to be as effective as atrazine in controlling the deposition of weed seed bank. No weeding throughout the season added as much as > 65% to the existing weed seeds to the bank over 2 years. Reduced tillage using a cultivator alone reduced maize crop height up to 13% as compared to MB ploughing. Weed management practices also significantly (>30%) enhanced plant height compared to no weeding. Yield contributing factors (viz. as number of grains per cob 15-17%;1000-grain weight 15%; and grain yield 17-22%) were all enhanced by MB ploughing and hand weeding in 2013, but in 2014, their interaction (tillage × weeding) also showed a significant effect on yield contributing factors. In Experiment-2, clear plastic mulch and sorghum straw mulch restricted weed growth and decreased the weed seed bank, possibly through solarization, promoting biological activity to enhance seed decay and through allelopathy. Both types of mulch reduced the weed seed bank up to 13% in the top soil layer (0-5 cm depth). Below the 10 cm soil depth, neither sowing method nor mulching technique had any effect on decreasing the weed seed bank. Insignificant changes may have been caused by soil setting, seed decay and rodent herbivores. Increases in crop height (18-26%), caused by using the ridge sowing methods and by using mulches (24-32%), were observed as compared to the non-mulched plots. Maximum grains produced per cob (564.6 and 603.6 grains), 1000-grain weight (256.6 and 240.1 g) and grain yield (7.31 t ha-1 and 7.26 t ha-1) were observed in the ridge sown maize during 2013 and 2014, respectively. Clear plastic mulch and sorghum straw mulch enhanced crop height up to 25 and 18%, grains per cob up to 23 and 18%, 1000-grain weight up to 25 and 13% and grain yield up to 28 and 21%, respectively. Sorghum straw mulch, being an organic and cultural technique, produced the greatest benefit-to-cost ratio (1.05 to 1.34). Deep ploughing using a MB plough in combination with hand hoeing at 20 and 40 DAS gave promising results in terms of grain yield and weed control. Sorghum straw mulch @ 5 t ha-1proved to be the more effective method for reducing the weed seed bank and it was also an economically feasible approach for farmers to use.