Mixing the correct nitrogen source and rate with 2,4-D increases efficacy in hard and soft water

Abstract

Hard water antagonism of 2,4-D dimethylamine (DMA) is avoided with the inclusion of ammonium sulfate (AMS) as a water conditioning agent in mixtures. In addition to functioning as an adjuvant, AMS is also a nitrogen (N) fertilizer. Two experiments evaluated the efficacy of 2,4-D DMA when mixed with alternative N sources and rates. The first experiment sought to determine the influence of alternative N sources and N rates on 2,4-D DMA efficacy in comparison to AMS. The second experiment more closely evaluated the influence of AMS rate on 2,4-D DMA efficacy. Nitrogen sources included AMS, ammonium nitrate (AMN), potassium nitrate (KNO3), urea–ammonium–nitrate (UAN), and urea. Each N source was tank-mixed with 2,4-D at both an adjuvant rate (20 g L−1 or 25 ml L−1) and a fertilizer rate (24.4 kg N ha−1) in both soft (0 mg CaCO3 L−1) and hard (600 mg CaCO3 L−1) water. Efficacy of the mixtures was assessed on dandelion (Taraxacum officinale F.H. Wigg.) in the greenhouse. Without consideration to water hardness level or N rate, 2,4-D DMA efficacy with the N sources was: AMN = UAN > AMS = Urea > KNO3. Ammonium sulfate reversed hard water antagonism of 2,4-D DMA at the adjuvant rate, but not at the fertilizer rate. When applied at the adjuvant rate in hard water, dandelion control rankings from 2,4-D mixes were generally: AMN = UAN = AMS > Urea > KNO3. It is not advised to mix KNO3 with 2,4-D applications based on reduced efficacy. The second experiment determined that 2,4-D efficacy decreased as AMS rate increased above the commonly used adjuvant rate (2% w/w, 20 g L−1), likely from salting out. Applicators desiring to use AMS as a foliar fertilizer should apply AMS separate from 2,4-D DMA to prevent potential antagonism.