Physical Properties of Soil and Glyphosate Residue as a Function of Cassava Weed Management by Cover Crops in the Amazon Ecosystem

Abstract

Cassava production in Amazonas state deserves to be highlighted due to its great historical, social, and economic importance. Weed competition severely constrains cassava production in Amazonas. The use of cover crops is safe and very efficient at eliminating weeds while keeping the soil covered. The objective of this study was to evaluate physical properties of soil and glyphosate residues in storage roots as a function of the weed management in cassava. The experiment was carried out in a randomized complete block design with five treatments and five repetitions. The treatments were biological control with two species of cover plants (Brachiaria ruziziensis and Mucuna pruriens), chemical control, mechanical control, and treatment with no weed control. The cover crops characteristics evaluated were dry weight, the percentage of cover, and rate of decomposition of plant residues. In the soil, the bulk density and total porosity were determined. The contamination of the storage roots was evaluated based on the analysis of glyphosate residue. Brachiaria ruziziensis presented more dry weight and higher percentage of cover compared with M. pruriens, and both cover crops showed very similar decomposition rates. The physical properties of soil were unaffected by any treatment evaluated. There was no detection of glyphosate and its metabolite, aminomethylphosphonic acid (AMPA), in any treatment evaluated. Chemical control with glyphosate is not able to contaminate cassava storage roots.

Seasonal variations in the levels of glyphosate in soil, water and crops from three farm settlements in Oyo state, Nigeria

“…Also [ 86 ], investigated the availability of glyphosate and AMPA in a total of 243 samples of various fruits, and reported that all samples were within the maximum permissible limit. However [ 87 ], reported that glyphosate and its metabolite, AMPA, were not detected in cassava within the cover crops in the Amazon ecosystem even with five applications per year. While glyphosate levels in soybeans were reported to be 1.8 mgKg −1 for three times applications of glyphosate which is above the cRfD allowable dose, it was concluded that glyphosate and AMPA concentrations were higher when sprayings were done later in the growing season, close to time of blooming [ 83 ].…”