Spread of Paraffinic Oil on Leaf Surfaces of Johnsongrass (Sorghum halepense)

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Abstract
Paraffinic oil applied at 2.3, 4.7, and 9.3 L/ha to the surface of johnsongrass [Sorghum halepense (L.) Pers. # SORHA] leaves spread to provide complete coverage of the leaf surface. Soybean oil applied at the same volumes provided only 30 to 50% coverage of leaf surfaces. Water at 187 L/ha that contained 1.25% paraffinic oil adjuvant provided only about 30% coverage because spray droplets did not spread appreciably. Spread coefficients obtained when five different oil-soluble herbicides were applied to leaf surfaces in paraffinic oil were more than 100 times greater than when applied in water alone. Higher spread coefficients were obtained when paraffinic oil contained herbicides than when paraffinic oil was applied alone. Spread coefficients were also much greater for paraffinic oil alone than for soybean oil with or without herbicides. Stomata on johnsongrass leaves were distorted by paraffinic oil, suggesting that leaf surface wax was dissolved and this might be a mode of entry of herbicide into johnsongrass when applied in paraffinic oil. Stomata were unaffected by soybean oil or by water sprays that contained 1.25% soybean oil adjuvant or paraffinic oil adjuvant.