Glyphosate [N-(phosphonomethyl)glycine] was readily bound to kaolinite, illite, and bentonite clay and to charcoal and muck but not to ethyl cellulose. Fe+++ and Al+++-saturated clays and organic matter adsorbed more glyphosate than Na+ or Ca+-saturated clays and organic matter. Glyphosate appears to be bound to the soil through the phosphonic acid moiety as phosphate in the soil competed with 14C-glyphosate for adsorption sites. Glyphosate mobility in the soil was very limited and was affected by pH, phosphate level, and soil type. The 14C-glyphosate was biodegraded in soil to 14CO2 possibly by co-metabolism. Potentiometric titrations of the compound gave pKa values of 2, 2.6, 5.6, and 10.6.

[1]The log-logistic model is a popular candidate for modeling dose response in agronomy. One of its limitations is monotonicity: the where E[Y|x] represents the average response at dosage response is either continuously increasing or decreasing with changes x, and ␣ and ␦ are the upper and lower asymptote of in dosage. In herbicide dose response investigations, authors have response, respectively. The parameters and ␤ are repreviously noted an increase in biomass or growth for subinhibitory lated to the rate of change and point of inflection of dosages. This hormetic effect, if ignored, can lead to substantial bias in estimates of effective dosages and can distort inferences about the curve. the differential effects of experimental treatments. We present an Equation [1] is only one of many different expressions extension of a model capable of modeling hormetic effects that allows (parameterizations) of the log-logistic model. Since the inference about effective dosages. Furthermore, the model can be used model is nonlinear in its parameters, it is fit to data to compare effective dosages among different treatments if hormetic by nonlinear regression techniques. Statistical packages effects are present for some and absent for others and provides approcapable of nonlinear model fitting will typically report priate estimates of rates of change. In a designed experiment where parameter estimates, along with their standard errors velvetleaf (Abutilon theophrasti Medikus) and barnyardgrass [Echiand confidence intervals. This enables convenient statisnochloa crus-galli (L.) P. Beauv.] were treated with glyphosate [isotical inference, if the quantities of interest are paramepropylamine salt of N-(phosphonomethyl)glycine] ϩ (NH 4 ) 2 SO 4 or ters of the model. A common, alternative expression glufosinate [2-amino-4-(hydroxymethylphosphinyl)butanoic acid] ϩ (NH 4 ) 2 SO 4 at various rates, hormetic effects were found for barn-(see Seefeldt et al., 1995) for Eq. [1] is yardgrass treated with glyphosate but not with glufosinate. Procedures for a valid comparison of effective dosages between the two herbicides E[Y|x] ϭ ␦ ϩ ␣ Ϫ ␦ 1 ϩ exp[␤ ln(x/)] [2]are presented. Software code to perform the analyses is given in an appendix.

Radioactive glyphosate [N-(phosphonomethyl)glycine] is rapidly absorbed with a large portion of the 14C translocated to the rhizomes and untreated shoots of quackgrass [Agropyron repens (L.) Beauv.]. The adjuvant used with glyphosate was important in determining its phytotoxicity to quackgrass. In other perennial weeds and annual species, glyphosate also moved to the areas of highest metabolic activity. In Canada thistle [Cirsium arvense (L.) Scop.], bentazon (3-isopropyl-1H-2,1,3-benzothiadiazin-(4) 3H-one 2,2-dioxide) at 2.24 kg/ha applied prior to treatment with 14C-glyphosate reduced 14C translocation. Iron or nitrilotriacetic acid (NTA) did not appear to effect glyphosate activity on wheat (Triticium aestivum L. ‘Avon’). The respiration of quackgrass treated with glyphosate was significantly reduced 9 days after treatment. Glyphosate reduced total photosynthesis more in quackgrass than in wheat.

Hard-water cations, such as Ca+2and Mg+2, present in the spray solution can greatly reduce the efficacy of glyphosate. These cations potentially compete with the isopropylamine in the formulation for association with the glyphosate anion.14C-Glyphosate absorption by sunflower was reduced in the presence of Ca+2. The addition of ammonium sulfate overcame the observed decrease in14C-glyphosate absorption. Nuclear Magnetic Resonance (NMR) was used to study the chemical effects of calcium and calcium plus ammonium sulfate (AMS) on the glyphosate molecule. Data indicate an association of calcium with both the carboxyl and phosphonate groups on the glyphosate molecule. Initially, a random association of the compounds occurred; however, the reaction progressed to yield a more structured, chelate type complex over time. NH4+from AMS effectively competed with calcium for complexation sites on the glyphosate molecule. Data suggest that the observed calcium antagonism of glyphosate and AMS reversal of the antagonism are chemically based.

Adsorption of14C-imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} and imazethapyr [2-(4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl)-5-ethyl-3-pyridinecarboxylic acid] to soil increased as soil pH decreased from 8.0 to 3.0 in laboratory studies. Significantly more imazethapyr3(AC-263,499) than imazaquin was adsorbed at soil pH levels 3.0 and 5.5, while the greatest difference in adsorption behavior between the two herbicides was observed at a soil pH of 5.5. In greenhouse studies, phytotoxicity to corn (Zea maysL.) was greater for imazaquin than AC-263,499 applied at 26 and 53 g ai/ha. There were significant pH by herbicide and pH by rate interactions, but in trend analysis only a small proportion of the corn response (r2= 0.01 to 0.35) was attributed to increasing soil pH values. In field studies where imazaquin was applied to soil pH levels of 4.2 to 4.8, 5.4 to 5.5, and 5.8 to 6.2, injury to corn across all pH levels decreased as the time delay between herbicide application and corn planting increased. There was no significant effect of soil pH on imazaquin injury to corn planted in July or August. Decreased injury from imazaquin was observed in 1985 on corn planted in June on the soil pH of 5.8 to 6.2. Imazaquin injury was less for June-planted corn in 1984 than in 1985, across all soil pH levels.