Effect of application time and glyphosate formulations on weed control efficacy in young citrus groves

Weeds can compete with young citrus trees for space, nutrients, and moisture. Herbicide rates and application timings, in relation to weeds growth stage, may affect the effectiveness of weed control. The objective of this work was to maximize weed control using appropriate rates of glyphosate applied at various growth stages of weeds. Two glyphosate formulations (Roundup UltraMax (R) and Touchdown IQ (R)) at 0.84,1.26, and 1.68 kg ae/ha(-1) mixed with 2% ammonium sulfate were evaluated under field conditions. Treatments were applied by a tractor mounted boom sprayer delivering 190 L center dot ha(-1) volume at 152 kPa. Spraying was done at 14 day intervals during 2002 and 2003 in a plot size of 18 x 3 m with four replications. The field was infested with several broadleaf and grass weeds, but Brazil pusley and Texas panicum were dominant. Weed mortality was recorded at 2-week intervals for 10 weeks after spraying. Higher efficacy of glyphosate was recorded when plots were sprayed at the end of May compared to mid or early May applications. Efficacy was significantly reduced when sprays were delayed to mid June. Mortality of grass weeds was higher than 80% at all spraying dates, however, higher efficacy was observed when sprayed on 15 or 30 May. Maximum weed control was observed 6 weeks after treatment (WAT); a decrease of 10-20% was recorded at 10 WAT. No difference in efficacy was observed between the two formulations of glyphosate. Glyphosate at 1.68 kg center dot ha(-1) provided significantly better control of broadleaf weeds than lower rates; however, 1.26 kg center dot ha(-1) provided similar control of grass weeds compared with 1.68 kg/ha(-1).