The active ingredient listed on the label of an herbicide may refer to an amine, an ester or a salt. Amines and esters are derivatives of an acid. The plant converts these derivatives into the parent acid that interferes with the plant's growth and eventually kills it. Amines are obtained from ammonia; ester derivatives are made by condensing an acid with alcohol or carboxylic acid. Salt herbicides kill by pulling water from the plant.
Most fats and oils are esters; those with a low molecular weight are found in essential oils and pheromones used in fragrances. Esters are soluble in organic solvents and oils but not water. Acid equivalent is that portion of an ester herbicide that can theoretically be converted back into the plant-killing acid.
Ester herbicides are soluble in lipids, meaning they dissolve in fats and waxes such as those that cover the surface of plant leaves. Amine and salt herbicides are less efficient at moving through the cuticle or protective covering of a leaf and into the plant. In general, ester herbicides kill more weeds than those containing amine derivatives or salts.
Volatility is when a liquid ester herbicide evaporates, forming a mist or vapor that can drift onto plants that you don't want to kill. There are esters with short chains of molecules and esters with long chains of molecules. Short-chained esters are called volatile; esters with long changes are nonvolatile. While amine herbicides are nonvolatile, they can react with calcium and magnesium found in fertilizer solutions and hard water to form insoluble salts that will not penetrate the plant. Both relative humidity and temperature can cause spray droplets of high-volatile ester herbicides to evaporate and drift. Agronomists at North Dakota State University say that a wind of 3 miles per hour can blow very fine particles from 367 yards to a few miles.
Effect of Humidity
Vapor produced by high-volatile ester herbicides will drift more in low humidity. However, plants that grow in low humidity resist penetration of amine and salt herbicides because they produce a thicker cuticle, or surface protection, than those growing in high humidity.
Effect of Temperature
Researchers at North Dakota State University say that when the temperature increases from 60 to 80 degrees F, the formation of vapor of a high volatile ester of 2,4-D nearly tripled. The vapor drift of low volatile esters of 2,4-D did not damage tomato plants at 70 to 75 F but did damage them at 90 to 120 F. Amine herbicides are essentially nonvolatile and did not damage plants with vapor drift even at high temperatures.