PART 1 - SMALL PLOT EVALUATIONS 



Introduction 



Pieterse et al . (1980) reported a ten-fold increase in the sen- 

 sitivity of waterhyacinths to 2,4-D due to a synergistic effect of 2,4-D 

 and gibberellic acid. These investigations were conducted under 

 greenhouse conditions in 200 x 100 x 50 cm concrete reservoirs. Plants 

 were first treated with an atomizing spray system with concentrations of 

 2,4-D (amine salt) ranging from to 1000 g/ha at a volume rate of 

 200 1/ha. GA was then applied to control plants and plants which 

 received 0, 50, 100, and 200 g/ha of 2,4-D. Concentrations of 

 GA, utilized were 0, 2, 4, 6, and 8 g/ha at a volume rate of 200 1/ha. 

 Results of these experiments indicated that combinations of GA and 2,4-D 

 at 6 to 8 g/ha and 100 g/ha, respectively, caused death of the plants 

 within one week. The same response was noted in plants which received 

 only 1000 g/ha 2,4-D. Pieterse and Roorda (1982) reported similar 

 results when the 2,4-D and GA were applied simultaneously in the same 

 solution at an extremely low volume rate of 40 1/ha. 



Based on results of these two studies, it was hypothesized that (1) 

 such a large reduction in the quantity of 2,4-D might lower the costs of 

 control programs even though costs of GA is relatively high (Pieterse et 

 al . , 1980) and (2) a decrease in the 2,4-D concentration would lower the 

 risk to nearby native vegetation or crops (Pieterse and Roorda, 1982). 

 The objective of this part of the overall study was to evaluate this 



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