effects of pre-harvest application of parquat on grain moisture reduction, grain yield and quality of rapeseed (brassica napus l.) cultivars

pre-harvest desiccation of rapeseed crop may allow a more timely harvest as well as increase the speed and harvest operation efficiency as a second crop after rice. an effective desiccant which could be used as a harvest-aid with no negative effects on rapeseed yield and quality would be advantageous for oil seed crop producers and may reduce yield losses with no interferential effects on rice cultivation. field experiment s were carried out in a split plot design at rice research institute of iran (rrii) in the 2004-2005 cropping season. paraquat was applied to hyola308, hyola401, hyola420 and rgs003 rapeseed cultivars as main plots at the rates of 0.2, 0.4, 0.8 lit. based on 20% ai.ha-1 and a non-treated plot (sub-plot) was used as a control. paraquat treatments were applied based on grain moisture content of 40% to one third of bottom siliques of inflorescence on the main stem in late april to late may. results showed that the effect of paraquat application was significant on grain moisture reduction rate, grain yield and oil content. paraquat at 0.8 lit.ha-1 promoted daily grain moisture reduction (2.94% day-1) compared to the control (1.75% day-1) and harvest time (5-7 days earlier than control) with no adverse effects on grain yield and oil content. grain yield of treated plants were the same or higher than the control. the highest grain yield was obtained with 0.4 lit.ha-1 paraquat (2370 kg.ha-1 in hyola420 vs. the control; 2159 kg.ha-1). results also showed that paraquat application had no adverse effect on oil content and fatty acids profile. oileic acid content in hyola308 at 0.2, 0.4 and 0.8 lit.ha-1 of paraquat and the control were 64.93, 64.22, 64.25 and 65%, respectively. mean residues of paraquat in the grains for the medium to low application rates were 5.62 and 2.61 mg.kg-1, respectively and all of its rates were lower than the toxicity threshold. it can be concluded that paraquat in reduced doses, can be used as a proper desiccant in rapeseed cultivation.