Critical period for alligator weed (Alternanthera philoxeroides) in puddled transplanted rice

Alligator weed (Alternanthera philoxeroides), an invasive plant in Pakistan due to its amphibious growth habit, has become a problematic weed for puddled transplanted rice (PTR). To assess the critical timing of removal (CTWR) and rice yield losses caused by this weed, a two-year field investigation was conducted. The treatments included varying alligator weed-crop competition durations for the initial 4 weeks following transplanting (WFT), 6 WFT, 8 WFT, and 10 WFT. The whole season, weedy and weed-free treatments were also kept. During both years, a gradual decline in paddy yield and rice quality parameters occurred with the prolongation in weedy period due to an increase in alligator weed’s dry weight (up to 196%), N-uptake (up to 114%), P-uptake (up to124%), K-uptake (up to 134%), Cu-uptake (up to 101%), Fe-uptake (up to 21%), Zn-uptake (up to 81%), and Mn-uptake (up to 58%). Consequently, the full-season alligator weed competition resulted in the highest reductions in paddy yield (21.6%-22.3%), fertile tillers per hill (10.5%-11.5%), kernels per panicle (3.1%-3.7%), 1000-kernel weight (15.4%-21.2%), kernel amylose content (12.9%-13.5%), and protein content (12.5%-21.6%) during the first and second years’ experiments, respectively. The alligator weed infestation increased in percentages of sterile spikelets (56%-69%), opaque kernels (27%-28%), and chalky kernels (17%-18%) of rice. The logistic model estimated the CTWR of the alligator weed in puddled transplanted rice (PTR) to be 6.1 WFT and 5.7 WFT in order to prevent 5% yield losses during the years 2011 and 2012.