Weed shift by long term use of dinitroaniline herbicides and glyphosate in cotton

Abstract

Adoption of chemical based weed control for crop cultivation has been one of the most important revolutions in the history of agriculture. In certain regions application of non-selective glyphosate is commonly followed since many years under crop situation by farmers by physically covering the plants. Weed survey was conducted in mono cropped cotton farm to find out weed shift. The survey was conducted at CICR Farm, Coimbatore (Tamil Nadu) to assess continuous use of pendimethalin and Rasipuram (Salem Dt, Tamil Nadu) and Namakiripettai (Namakkal DT, Tamil Nadu) to assess intensively glyphosate used farms during the year of 2013-2014. The farm had adopted manual weeding for weed control, was kept as control plot. Weed analysis carried out in cotton field applied with dinitroaniline for more than 30 years revealed that Trianthema portulacastrum (Itsit) is the major one (RD, 40.5%), (RF, 18.5 %), (IV, 59.0) and (SDR, 29.5). It is closely followed by Cyperus rotundus (Motha) (with 35.5 % RD, 23.0 % RF, 58.6 IV and 29.3 SDR). On the contrary, control plots is infested with dominant species of Panicum repens with RD of 46.4 %, RF of 20.76 %, IV of 67.1 and SDR of 33.5. Thus, over the years, weed species has shifted from grassy weeds to broad leaved & sedges by application of pendimethalin. Pot culture experiment was carried out by using the seed materials of Trianthema portulacastrum collected from the continuous herbicide applied and control field and tested with different levels of pendimethalin (0, 0.55, 1.0, 2.0 and 3 kg/ha) to assess weed control efficiency. The results were nor varied between continuous herbicide applied field and control field. Hence the study concluded that the dominance of Trianthema portulacastrum under the specific situation is more probably due to weed shift and there is no evidence for development of resistance to pendimethalin. Absolute weed density of 429.8 and 279/m2 were counted in control plot respectively at Rasipuram and Namakiripettai. Echinochloa colona was the predominant and observed with absolute density of 369.8/ m2, relative density of 86.1 per cent, important value index of 109.2 and summed dominance ratio of 54.6 in control plot at Rasipuram. The same species was pre dominant species in Namakiripettai also found with absolute density of 64.0/ m2, relative density of 22.9 per cent, important value index of 31.8 and summed dominance ratio of 15.9 in control plot. Intensive glyphosate used farms (more than 20 years) were surveyed revealed that weed population was significantly reduced and reached the absolute density of 12.0 and 19.1/ m2 respectively at Rasipuram and Namakiripettai. In glyphosate used farm Cyperus rotundus was predominant (60.78 per cent) followed by Commelina benghalensis (21.57 per cent) at Rasipuram. In Namakiripettai, Cyanotis cucullata recorded the highest relative density of 31.4 % and followed by Cyperus rotundus (relative density of 24.2%) in glyphosate applied plots. Thus, over the years weed species shifted from grasses to sedges and broad leaved weeds by continuous use of glyphosate.