Comparative Evaluation Of Efficacy Of Different Formulations Of Glyphosate And Monitoring Of Glyphosate-Resistant (or Tolerant) Weeds

Chemical weed control has been of vital importance in global agriculture modernization. Due to much more frequent use of chemical herbicides, more and more weeds have involved in resistance to herbicides. The burgeoning of herbicide-resistant weed populations has become a new problem to comprehensive weed control system mainly composed of chemical control. The issue of glyphosate-resistant weeds has attracted extensive attention since glyphosate became the most important nonselective herbicide in the world. Owing to unique characteristics of glyphosate such as functional mechanism,metabolization, chemical structure and no soil activity, up to 1990, people have not been aware of weeds resistant to glyphosate. After 15 years' use of glyphosate,glyphosate-resistant L. multiflorum as the first case was discovered in Australia, South Africa and California in USA in 1998. Since then, glyphosate-resistant cases happened worldwide frequently. In 1999, farmers report from central Chile, the resistance of L.multiflorum was 5-6 times after 8-10 years' use of glyphosate in orchard. The resistance of Conyza canadensis was 10 times after glyphosate-tolerant transgenic soybean continuously cropping three years in North America in 2000. Glyphosate resistance is becoming a problem which should not be neglected along with the popularization of glyphosate-tolerant transgenic crops. China is one of main countries in use and production of glyphosate. The monitor of weeds resistant to glyphosate is helpful for early forecasting of emergence of resistant weeds populations and establishing systems of rational use of herbicide and comprehensive weed control, also can provide basic information of national weed control, herbicide manufacturing, extention and application as well as macroscopical control system of extention and application of glyphosate-resistant transgenic crops.We bioassayed comparatively the efficacy of seven kinds of glyphosate formulation by greenhouse and field trials. The results showed that the efficacy of these different formulations with various adjuvants and salts against weeds were different significantly. Roundup produced by Monsanto Corporation performed best in both toxicity and efficacy. Roundup also had stable efficacy against different weeds. It is the perfect herbicide to be used in the selection of glyphosate tolerant weeds.We screened the glyphosate-tolerant weeds in Naming. The weeds growing in various habitats and different seasons were treated by Roundup in concentrations: 250ml/667m²,500 ml/667m², 1000 ml/667m². The results showed that the perennial weeds of Compositae,Vitaceae and Gramineae had high glyphosate tolerance. The tolerance of adult weeds was higher than at seedling stage. The ED₉₀ values of the weeds at different tolerance levels also were tested. The results showed that the lethal concentration of sensitive weeds was 150ml/667m²-250ml/667m². That of low tolerant weeds was 250 ml/667m²-400 ml/667m²,that of middle tolerant weeds was 400 ml/667m²-600ml/667m² and the lethal concentration of high tolerant weeds was more than 600ml/667m².The bioassay method was established to monitor glyphosate resistance of weeds. The sampling seeds were sown onto the mixed soil surface of shallow tray, and were cultured to be seedling. Single seedling at 5-7 leaves stage was transplanted into separate 5×5cm plastic cup containing mixed soil. Then the isopropylamine salt of glyphosate at the rate of 0, 35, 70, 140, 280, 560, 1120, 2240, 4480, 8960 g ai/ha were applied when the seedling grew to 15-20 leaves stage. The first observation was carded out three days after application and herbicide injury was recorded based on visual assessment standard. The same work was consistently conducted every four days until the injury symptom did not develop further, and fresh and dry weight of plant above the ground was weighed. All data were fitted to a log-logistic regression model..Through comparison of the results of two-time experiments, it was concluded that three values: herbicide injury index, fresh weight and dry weight all could reflect the relationship between herbicide injury and glyphosate dosage, but the herbicide injury index was the best one by which the results could be repeated very well. And we use it to test weather the weeds collected by ourselves and Syngenta Company could have involved in resistance to glyphosate. But we did not find high glyphosate-resistant level weeds, the highest resistance ratio to susceptible population is about 2. The reason may be the location where we got the weeds was used glyphosate for too short time.We tested the resistance level to glphosate of two horseweed populations (001G, 002G) collected from Zhejiang Province by using bioassay and shikimate acid test. The results showed that both of those two horseweed populations have obvious resistance to glyphosate. The ED₅₀ of 001G was 2513.043 g ai/ha and that of 002G was 2412.682 g ai/ha,and the highest resistance ratios to susceptible population are 8.28 and 7.95. The results of shikimate acid test also showed that the increased quantity of shikimate acid in two resistant populations is obviously lower than that in susceptible population.