The effect of different soil nitrogen concentration and water hardness on glyphosate efficacy in redroot pigweed (Amaranthus retroflexus L.) control

In order to study the effect of different levels of soil nitrogen concentration and water hardness on glyphosate efficacy in redroot pigweed control, an experiment was conducted as a completely randomized design in factorial arrangement with three replications in 2011 at the College of Agriculture, Ferdowsi University of Mashhad. Experimental treatments were included of soil nitrogen content at 5 levels (18, 50, 90, 200 and 300 mg/kg soil), glyphosate doses at 7 levels ( , 75, 100, 125, 150, 175 and 200 % of glyphosate recommended dose for redroot pigweed), and calcium carbonate concentration in water as an index of water hardness at 5 levels ( 0, 100, 300, 600 and 1200 ppm). Spray solution was applied at 8-10 redroot pigweed leaf stage and 30 days after spray plants survival and shoot dry matter was measured. Results showed, herbicide rate, soil nitrogen content and water hardness significantly affected (P≤0.01) on growth and survival of redroot pigweed. Increasing herbicide doses decreased redroot pigweed survival and growth in pure water spraying. Increasing water hardness decreased glyphosate efficacy in control of redroot pigweed and increased glyphosate ED50 parameter. Minimum (906 gr. a.i ha-1) and maximum 1606 gr. a.i ha-1) glyphosate ED50 for redroot pigweed was observed in 0 and 1200 ppm of water hardness. Increasing soil nitrogen content from 18 to 300 (mg/kg soil) increased glyphosate efficacy in redroot pigweed and glyphosate ED50 decreased from 1745.2 to 896.49 (gr. a.i ha-1).