Effect of climatic factors on the potency of ioxynil and bromoxynil

Abstract

Outdoor pot experiments, carried out between 1967 and 1971, mainly with Polygonum lapathifolium L. and Stellaria media (L.) Vill., showed that herbicidal potency of ioxynil and bromoxynil salts and esters was affected by climatic conditions, particularly solar radiation and relative humidity. Multiple regression equations were frequently obtained which accounted for 80% or more of the observed variation in effective dose (ED90). By contrast, experiments in growth cabinets suggested that air temperature was the most important climatic variable; the reasons for this disagreement are discussed. It is concluded that climatic factors affect the potency of ioxynil and bromoxynil via their influence on leaf uptake; ester formulations (emulsions) were much less affected than aqueous salt solutions.

Influence of environmental factors on the activity of ioxynil salt and ester applied to Stellaria media

“…The results suggest that ioxynil activity was inversely correlated with all but the highest levels of light intensity. Savory et al (1975) also concluded that ioxynil activity was inversely correlated with light intensity, although humidity was also inversely correlated with light intensity in their experiments and may therefore have been the more influential factor. The increased activity of ioxynil at lower light intensities may reflect a reduced rate of photosynthesis in leaves and shoots unaffected by the herbicide which would determine the extent of new growth in the plant.…”

“…Atmospheric humidity has previously been cited as influencing ioxynil activity, and was considered by Savory et al, (1975) to be of major importance in this respect. However, in preliminary controlled-environment experiments by the author, humidity appeared to have no significant effect on the activity of ioxynil against S. media.…”

“…Carpenter et al (1964) found that when natural daylight was reduced to 40% of full intensity using wire gauze screens the activity of ioxynil salt applied to S. media was significantly reduced. Although the work of Savory et al (1975) suggested an inverse correlation between solar radiation and ioxynil activity, high levels of light were usually associated with low humidity. Both Savory et a/.…”

“…Effects of temperature over the 14 days after spraying were inconclusive. Savory, Hibbitt & Catchpole (1975) studied environmental effects outdoors by correlating measured variables with ioxynil activity. They concluded that temperature did not significantly influence activity.…”