Chemical control of ambrosia Artemisiifolia on non-crop areas: are there alternatives to glyphosate?

We compared glyphosate, glufosinate and metsulfuron-methyl to control Ambrosia artemisiifolia under non-crop conditions. A laboratory study showed that A. artemisiifolia is an easy-to-wet species and that glufosinate and glyphosate are quickly absorbed by its leaves (nearly 100% in 24 h). Metsulfuron-methyl absorption was slower (about 50% in 24 h) but was strongly promoted by terpenic alcohol and esterified rapeseed oil. In the greenhouse, all three herbicides were efficacious against A. artemisiifolia, with ED50s of <23, 23 and 0.8 g ha(-1) for glufosinate, glyphosate and metsulfuron-methyl, respectively. These results were confirmed on a non-crop area for glufosinate and glyphosate, which at half the registered dose reached high efficacies at both the 4 to 6-node and flowering stages of A. artemisiifolia. By contrast, metsulfuron-methyl showed no efficacy. However, after treatment at the 4- to 6-node stage, new emergence of A. artemisiifolia led to the presence of vigorous plants that bore numerous flowers and produced high levels of pollen. After treatment at the flowering stage, flower production by A. artemisiifolia was not significantly affected, but achene weight was decreased by 60 to 70% and seed viability was only 8 to 13% for the treated plants, as compared to 85% for the control. No significant difference was observed between the two herbicides and between the doses. It is concluded that glufosinate can be an alternative to glyphosate for the chemical control of A. artemisiifolia on non-crop areas. However, with both herbicides, it is difficult to attain the two objectives of reducing seed production and pollen production by means of only one treatment.

Glyphosate resistance in a Chilean Lolium multiflorum.

Lolium multiflorum (Italian ryegrass) has recently demonstrated itself to be poorly controlled with glyphosate in cereal crops of South Chile. The concentration of glyphosate necessary to reduce shoot length by 50% (ED50) in seedlings, after eight days of root contact was 7.3-fold in the resistant Vil-1 than in the susceptible (S) biotype. The obtained spray retention values were higher on S than the resistant (Vil-1) biotype. Contact angles measured on the adaxial surface of S and Vil-1 were similar. However, on the abaxial surface contact angles were of 63 degrees on Vil-1 as compared to 42 degrees on S. A greater glyphosate uptake was observed through the abaxial surface of S. Regarding translocation, glyphosate accumulated mainly in the tip of the treated leaf of Vil-1, 24 h after treatment. It was afterwards also well distributed to the rest of the leaves and roots, as in the susceptible biotype. Nevertheless, 14C-glyphosate remained higher in the foliar apex of Vil-1. Hence, resistance to glyphosate by the Vil-1 Lolium multflorum biotype seems to involve a lower uptake through the abaxial leaf surface and a different migration pattern.

Influence of application volume on herbicide efficacy.

Herbicide doses used by farmers are often lower than registration doses. One of the reasons put forward is that field assays for registration are performed at relatively high application volumes (typically 300 L ha(-1)), whereas farmers use lower volumes (down to 75 L ha(-1) or even lower). The resulting concentration of the active ingredient(s) and the formulants is supposed to enhance efficacy. To test this hypothesis in the case of specific graminicides, we compared the efficacy of clodinafop-propargyl and fenoxaprop-ethyl on Avena sativa at two application volumes. Fenoxaprop-ethyl was more efficacious when applied in 75 L ha(-1) as compared to 300 L ha(-1); ED95 were 20.0 and 26.0 g ha(-1), respectively. By contrast, clodinafop-propargyl exhibited the same efficacy under both conditions. Studies of dynamic surface tension showed differences in behaviour between spray dilutions of the two herbicides, especially at concentrations corresponding to ED95s. After 100 ms, surface tension decrease was lower than 2 mN m(-1) for clodinafop-propargyl at both application volumes. By contrast, surface tension decrease was 7 mN m(-1) for fenoxaprop-ethyl at 300 L ha(-1), and 18.5 mN m(-1) at 75 L ha(-1). In the case of fenoxaprop-ethyl sprayed at doses used in the practice, the retention of active ingredient by Avena sativa was lower at high application volume. This study shows that in some cases, concentration of the spray dilution may increase efficacy, formulation being probably involved.

Adjuvant influence on glyphosate efficacy in the presence of CA2+.

Calcium ion in the spray water diminishes the efficacy of glyphosate. Ammonium sulphate (AS) is long shown to be the most efficacious remedy to this antagonism. However, it is sometimes claimed that acidified lecithins or terpenic alcohols can also be used to this aim, and even ethoxylated amines (EA) provided that calcium concentrations are not higher than 5 mM. We investigated these claims by examining dose-response curves of barley plants treated with glyphosate at various calcium concentrations in the presence of these adjuvants. Calcium ion increased the slope of the curves and ED50 values, indicating a decrease in efficacy. Acidified lecithins and terpenic alcohols did not modify the response of barley to glyphosate observed in the presence of calcium ion. AS restored the ED50 and slope values observed in the absence of calcium ion. It was concluded that the inhibition of glyphosate efficacy by calcium ion was not relieved by terpenic alcohols and acidified lecithins but was completely reverted by AS. EA decreased ED50 and ED90 values of the response curves established in the presence of calcium ion without influencing the slopes (which remained high). As a consequence, at calcium concentrations up to 5 mM the response curves obtained with AS and EA intersected. It followed that in the presence of EA, ED50 values were higher than AS, whereas ED90 values were lower. By contrast, at 10 mM calcium the response curves did not intersect and consequently, in the presence of EA, ED50 and ED90 values were both higher than AS. Since ED90 relates to efficacies sought in the field, our experiments explain why at low calcium concentrations EA may appear as a better remedy than AS. Glyphosate formulations containing ammonium sulphate or a higher concentration in surfactants than usual glyphosate formulations were also studied. In the presence of calcium ion they behaved in a way which was in agreement with the above observations.