Estimation of the economic threshold of Leptochloa chinensis (Chinese sprangletop) in direct-seeded fine grain rice (Oryza sativa)

: Leptochloa chinensis (L.) Nees (Chinese sprangletop) is a weed that is becoming a serious threat in upland and lowland rice. A field study was conducted at the Agronomic Research Farm, University of Sargodha, Punjab, Pakistan, during the summer seasons of 2018 and 2019 to evaluate the effect of L. chinensis density on the yield of direct-seeded fine rice. Treatments comprised of L. chinensis densities of 0, 5, 10, 15, 20 and 25 plants m-2, and the experiment was laid out in a randomized complete block design with four replications of each treatment. The results suggest that the presence of L. chinensis significantly hampered the grain yield of direct-seeded rice. Weed infestation caused 63% and 69% yield losses where 25 L. chinensis plants m-2 were sustained in 2018 and 2019, respectively. Yield reduction was due to the reduction in 1000-grain weight (22.9 and 29.1%), number of tillers m-2 (65.8 and 60.0%), and number of grains panicle-1 (53.3 and 60%) in 2018 and 2019, respectively. The highest weed infestation (25 plants m-2) produced the highest weed dry biomass (687 and 669 g m-2), N uptake (19.3 and 19.3 kg ha-1), P uptake (1.92 and 2.32 kg ha-1), and K uptake (20.53 and 20.27 kg ha-1) in 2018 and 2019, respectively. The lowest weed infestation (5 plants m-2) produced minimum weed dry biomass (47 and 85 g m-2), N uptake (1.6 and 2.9), P uptake (0.3 and 0.5), and K uptake (1.7 and 30 kg ha-1) in 2018 and 2019, respectively. The economic threshold of L. chinensis as estimated to cause 6.73% and 6.08%  yield loss by the prediction model was 1.70 and 1.73 plants m-2 during 2018 and 2019, respectively. It can be concluded that L. chinensis is a serious weed in direct-seeded rice and it should be controlled when its density reaches 1.70-1.73 plants m-2 to avoid significant yield losses.