Maleic hydrazide (MH) residues resulting from applications at three times of day and three MH rates
The tobacco industry evaluates Maleic hydrazide (MH) residues based on established CORESTA guideline residue levels (GRL) of 80 ppm. Over the last 25 years, changes in application techniques and amounts of MH applied have resulted in lower residue levels. There still appear to be factors in addition to the application rate of the chemical that affect resulting MH residues. The objective of this study was to determine the effects of application time of day and rate on MH residues. Trials were conducted near Tifton, Georgia in 2014 and 2015 to evaluate MH application time of day (7 am, noon, 4 pm) and MH rates (1.25 kg/ha, 2.5 kg/ha, 3.78 kg/ha). MH residues were determined from repeated sampling of green leaves from the fourth leaf from the top of the tobacco plants starting the day after application and following each successive rainfall event. Results of the MH residue samples show a strong correlation between early day application and lower residue levels at each of the MH application rates throughout the harvest period. Residues were reduced by more than half with the first rainfall regardless of the application time and amount applied (Application at 7 am of the three MH rates resulted in next day determined MH levels of 107, 302, and 445 ppm. These were reduced to 39, 81, and 45 ppm respectively for samples taken after 6 mm of rainfall). Successive rainfall events reduced the MH residue level further. These trials confirm that MH residues are directly related to application rate, that rainfall reduced MH residues, that application rates in excess of 1.25 kg/ha often resulted in MH residues in excess of the 80 ppm target, and application early in the day resulted in lower MH residues than at noon or 4 pm.