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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® and Touchdown IQ®) 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·ha -1 volume at 152 kPa. Spraying was done at 14 day intervals during 2002 and 2003 in a plot size of 18 × 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·ha -1 provided significantly better control of broadleaf weeds than lower rates; however, 1.26 kg·ha -1 provided similar control of grass weeds compared with 1.68 kg/ha -1 . Citrus is an important crop of Florida agriculture with 748,555 acres planted (http://www.nass.uda.gov/fl; 17 Sept. 2004), and contributing over a billion dollars to the local economy. Young citrus trees can be negatively impacted by high weed populations that compete for moisture, nutrients, and space. Florida weather provides ideal conditions for growth of weeds throughout most of the year. The total annual cost of weed control in Florida citrus was estimated at 5150 million (Singh, 2000). Weed control is the single largest component of citrus production costs. Production costs have increased significantly in recent years and efforts have . been made to lower these costs by improved decisions and adoption of latest management tools (Jackson and Davis, 1999). Mechanical and manual weeding is limited and expensive. Trunk and root damage by hand or mechanical hoes or other cultivation equipment further compounds the problem. Herbicide application is a common cultural practice in citriculture for weed control. Weed flora varies within a grove depending upon the weather conditions of the growing season. Grass weeds are more prominent with the onset of the summer rainy season, whereas broadleaf weeds and sedges generally emerge early in the season. Glyphosate is one of the most commonly used herbicides for post-emergence weed control. Application of glyphosate at different times during the growing season may have different effects on weeds depending upon their growth stage (Futch, 2004). Effect of glyphosate has been found to vary among weed species at different growth stages under greenhouse conditions (Singh and Singh, 2004). Glyphosate at 0.84 kg ae/ha provided 36, 55, and 63% mortality of ivyleaf morningglory, strangler vine (milkweed), and tall (annual) morningglory when applied to six leaf stage plants growing in the greenhouse compared to 92, 79, and 92% mortality, respectively, for the four-leaf stage plants under similar growing conditions. Information is limited on efficacy of glyphosate for citrus weeds when applied to weeds of different growth stages under field conditions. The decreasing citrus prices press Florida growers to maximize efficacy through an understanding with post-emergence herbicides that can serve to lower the cost of weed management. Recently, there has been increased interest to use inorganic salts, particularly ammonium sulfate (AMS) to increase herbicide efficacy on a broad spectrum of weeds. AMS lowers the pH of leaf surface and increases glyphosate movement into the plant, thereby neutralizing the antagonistic effect of hard water. Effects of ammonium sulfate may vary with weed species and herbicide formulations (Maschoff et al., 2000; Young et al., 2003). The present field investigation was carried out to evaluate efficacy of two formulations of glyphosate tank mixed with 2% ammonium sulfate applied at different times in a block of young citrus trees Material and Methods Glyphosate formulations were sprayed on weeds growing in soil in a Haines City, Fla. grove (Candler fine sand, 1.5% organic matter and a pH of 7.3) during 2002 and 2003. The grove had young citrus trees [Valencia orange ( Citrus sinensis ) on Benton] 3to 4-years-old that were actively growing at spraying. Herbicides were sprayed using a tractor mounted sprayer fitted with 80015 Teejet nozzles and an off center OC04 flat spray tip, delivering 190 L·ha -1 volume at 152 k P a pressure. The Teejet nozzles were directed downwards, whereas the off center nozzle was directed sideways and crossed areas on both sides of the tree line. Glyphosate (Roundup UltraMax® and Touchdown IQ®) at 0.84, 1.26, and 1.68 kg ae/ha was applied by tank mixing with 2% ammonium sulfate. A control treatment, where no herbicide was sprayed, was maintained for comparisons. There were four replicate plots of 18 × 3 m with four trees per plot, arranged in a randomized block design. There was a dense cover of weeds growing in the plots and no individual count of weeds was made. The dominant weeds infesting the grove were Brazil pusley ( Richardia brasiliensis ), Texas panicum ( Panicum texanum ), carpetweed ( Mollugo verticillata ), American nightshade ( Solanum ameri*Corresponding author; e-mail: msingh@crec.ifas.ufl.edu