Effect of row spacing and herbicide application on the growth indices, yield and yield components of rice in direct seeding

Abstract
Background and objectives: Change in the method of crop establishment from traditional manual transplanting of seedlings to direct‐seeding has occurred in many Asian countries in the last two decades in response to rising production costs. Change of rice establishment methods as well as water management practices, tillage and weed control strategies in direct seeded-rice can leads to change in the composition and diversity of the flora of weeds. Also, Greater number of weed species in direct‐seeding of rice can reduce the effectiveness of strategies to manage weeds. Therefore, this work was conducted with the objective of evaluating the performance of some post-emergence herbicides and different row spacing for weed control in direct-seeded rice.
Materials and methods: Experiment was conducted in a factorial arrangement using a randomized complete block design with three replications during 2011-2012 growing season. Factors consisted of crop spacing between the rows in three levels of 15, 25, and 35 cm and seven weed control methods including application of six herbicide treatments (pendimethalin, bentazon, propanil + cinosulfuron, oxadiargyl, clodinafop propargyl + "bromoxynil + MCPA" + bentazon and sethoxydim + "bromoxynil + MCPA" + bentazon) and weed-free treatment. The plots were divided into two sections, treatments were applied in a section and the other part of each plot was considered as the control of treated section.
Results: The loss of maximum leaf area index, total dry matter and maximum growth rate of rice caused by the use of bentazon (an average of 68%) and oxadiargyl (an average of 84%) herbicides were significantly greater than the application of pendimethalin (an average of 47%) and herbicides mixtures (an average of 41%). Also, the lowest reduction of rice growth indices in competition with the weeds was observed in 15 cm row spacing (an average of 44%). Row spacing significantly influenced the yield and yield components of rice. As, the lowest rice yield reduction was recorded in 15 cm row spacing (83%). Effects of none of the herbicide treatments on rice yield were similar to the weed-free treatment. However, the rice yield losses with herbicides mixtures of sethoxydim + "bromoxynil + MCPA" + bentazon (56%), propanil + cinosulfuron (59%) and clodinafop propargyl + "bromoxynil + MCPA" + bentazon (59%) were significantly lower than obtained yield reduction with the application of other herbicides (an average of 86%).
Conclusion: Application of herbicides mixtures evaluated in this study caused lowest differences between growth indices and yield of rice in comparison to weed-free control treatment. In this study, the rice yield loss in weed-infected plots was about 98 percent. Considering the yield loss in the absence of weed control, it is appears that to be satisfactory the yield loss obtained by only once application of herbicides mixtures during the growing season.