Calcium chloride in the spray carrier antagonized the toxicity of diethanolamine 2,4-D and sodium 2,4-D, dimethylamine MCPA, sodium bentazon, dimethylamine dicamba and sodium dicamba, sodium acifluorfen, imazamethabenz, ammonium imazethapyr, and isopropylamine glyphosate to kochia in greenhouse experiments. Diammonium sulfate overcame calcium chloride antagonism of the above herbicides, except for glyphosate and imazethapyr. Diammonium sulfate or ammonium nitrate adjuvants overcame calcium chloride and sodium bicarbonate antagonism of dicamba toxicity to kochia and enhanced toxicity of sodium dicamba to nearly equal that of dimethylamine dicamba.

Research was conducted to determine the influence of salts on 2,4-D toxicity to kochia. Calcium, magnesium, sodium, potassium, and iron salts except for sulfate and phosphate salts of calcium and sodium were antagonistic to 2,4-D diethanolamine. None of the ammonium salts antagonized 2,4-D control of kochia. Effects of individual ions generally antagonistic to 2,4-D were additive when in mixture. 2,4-D generally controlled kochia better when mixed with various acids than with their ammonium salts in distilled, sodium bicarbonate, or ferric sulfate water carriers, relating to the lower pH with the acids. However, low pH was not essential in overcoming salt antagonism of 2,4-D for kochia control, nor was 2,4-D always effective with low pH. Sulfate and monobasic phosphate anions were most effective in overcoming sodium bicarbonate and calcium chloride antagonism of 2,4-D. The concentration of diammonium sulfate needed to overcome sodium bicarbonate antagonism of 2,4-D increased with sodium bicarbonate concentration. Diammonium sulfate at 2% (w/v) overcame 1200 mg L–1 sodium as sodium bicarbonate. Nonionic surfactants and oil adjuvants also overcame antagonism of 2,4-D caused by water from several sources.

Glyphosate toxicity to wheat was antagonized more by calcium chloride than sodium bicarbonate. Mixtures of the salts at greater than 100 mg L−1 sodium bicarbonate and 200 mg L−1 calcium chloride were additive in antagonism of glyphosate in the greenhouse experiments. Surfactant and oil adjuvants did not overcome sodium bicarbonate or calcium chloride antagonism of glyphosate. Oil adjuvants were generally antagonistic to glyphosate. An equation is presented that determines the amount of diammonium sulfate required to overcome glyphosate antagonism based upon the sodium, potassium, calcium, and magnesium cations in the spray carrier.

Diammonium sulfate often is used as an adjuvant with glyphosate. Experiments were conducted in the greenhouse to determine the effectiveness of diammonium sulfate in overcoming calcium antagonism of glyphosate toxicity to wheat, sunflower, kochia, and soybean. Each species was a separate experiment and treatments varied with the experiment. Diammonium sulfate at 0.5% (w/v) in a spray carrier containing calcium chloride (500 ppm calcium) overcame calcium antagonism of glyphosate toxicity to wheat, overcame antagonism and enhanced toxicity to sunflower, but only partly overcame calcium antagonism of toxicity to kochia and soybean. With glyphosate at 100 g ae ha–1, diammonium sulfate at 2% (w/v) in a distilled water spray carrier enhanced toxicity to sunflower from a 11% to a 55% fresh weight reduction but was antagonistic to glyphosate toxicity to kochia and soybean.

Greenhouse and field experiments were conducted near Knoxville, TN, during 2002 and 2003 to investigate the effects of calcium and magnesium ions on the performance of three glyphosate formulations with and without diammonium sulfate (AMS). Weed species investigated in the greenhouse were broadleaf signalgrass, pitted morningglory, Palmer amaranth, and yellow nutsedge. Three glyphosate formulations (isopropylamine salt, diammonium salt, and potassium salt) and two glyphosate application rates (0.42 and 0.84 kg ae/ha) were applied to weeds in water fortified with either calcium or magnesium at concentrations of 0, 250, 500, 750, and 1,000 ppm. In all comparisons, there were no differences in the three glyphosate formulations. Glyphosate activity was reduced only when cation concentration was >250 ppm, and this antagonism was not observed when 2% w/ w AMS was added to the spray solution. A chemical analysis of the calcium and magnesium concentrations in water collected from farmers indicated that water samples from eight different producers contained relatively low amounts of cations, with calcium at <40 ppm and magnesium at <8 ppm. In the field results using these and other waters as the herbicide carrier, broadleaf signalgrass control was greater with the 0.84 kg ae/ha than 0.42 kg ae/ha glyphosate rate regardless of water source or addition of AMS. Pitted morningglory responded similarly to glyphosate with water from all farms and with AMS added, and the addition of AMS gave similar results for both glyphosate rates. In 2003, common cocklebur was evaluated and control was >93% regardless of glyphosate rate, water source, or AMS addition. Based on these results, the addition of AMS-based adjuvants to many glyphosate applications may not be warranted.

Field studies were conducted to assess two sulfur-containing additives for use with glyphosate applied postemergence to glyphosate-resistant cotton for the control of sicklepod and yellow nutsedge. Neither diammonium sulfate (AMS) nor ammonium thiosulfate (ATS), both applied at 2.24 kg/ha, increased control of either species. Effective control of both species was dependent on glyphosate (isopropylamine salt) rate alone, with optimum control at 1.26 kg ae/ha. Plant-mapping data further indicated that sulfur-containing additives generally had no effect on either cotton fruiting patterns or yield. However, applying glyphosate at any rate did increase seed cotton yield in 2 of 3 yr vs. no glyphosate. In addition, applying glyphosate at any rate resulted in an increase in the number of bolls vs. no glyphosate in the following plant-mapping responses: total number of bolls per plant, number of abcised bolls per plant, bolls at the top five sympodial nodes, and bolls at positions 1 and 2 on the sympodia. Glyphosate absorption and subsequent translocation, as influenced by the addition of the sulfur-containing additives, was evaluated using radiotracer techniques. Glyphosate absorption after 48 h was 86, 63, and 37% of amount applied in cotton, sicklepod, and yellow nutsedge, respectively. Absorption by sicklepod and yellow nutsedge was not affected by the addition of either of the additives. Absorption by cotton was reduced by ATS but was not affected by AMS. In yellow nutsedge and cotton, glyphosate concentration in the treated area and adjacent tissue was not affected by either additive. A greater portion of glyphosate was translocated away from the treated area in sicklepod with glyphosate plus AMS (32%) than with glyphosate plus ATS (21%). AMS and ATS may be used in glyphosate-resistant cotton without the risk of either crop injury or yield reduction. However, their use for increased control of annual weed species, such as sicklepod and yellow nutsedge, may not be warranted.

Greenhouse and field research was conducted to determine the effect of glufosinate, glyphosate, and glyphosate plus additional adjuvant on yellow nutsedge control and tuber production. Glyphosate at 0.84 kg/ha reduced yellow nutsedge dry weight 64%, whereas glufosinate at 0.4 kg/ha reduced dry weight only 22% when averaged over diammonium sulfate (DAS) and spray volume. Furthermore, yellow nutsedge dry weight was reduced 53% in the presence and 34% in the absence of DAS; however, dry weights were similar when spray volumes of glufosinate or glyphosate ranged from 140 to 1038 L/ha. Yellow nutsedge control with glyphosate and glufosinate increased to 88 and 68%, respectively, when the herbicides were injected into the plant. In the field, glufosinate at 0.4 kg/ha and glyphosate at 0.84 kg/ha controlled yellow nutsedge 19 and 53%, respectively. Glyphosate reduced yellow nutsedge tuber density 51%, tuber fresh weight 59%, and tuber sprouting 17% 42 wk after treatment in the field. The addition of nonionic surfactant, methylated seed oil, or crop oil concentrate to glyphosate plus DAS did not increase yellow nutsedge control with glyphosate in the greenhouse or field.