Controlling weeds with natural phytotoxic Substances (NPS) in direct seeded soybean

Summary

Field trials on weed control in direct seeding with the use of natural phytotoxic ingredients (NPI) were carried out from 2011 to 2013 at the Agricultural Research Institute of Paraná state, Brazil (IAPAR).
Soybean was direct seeded into residues of precrop black oats (\textit{Avena strigosa)}. In a split-plot trial the whole-plot treatment consisted of two different weed densities. In the eight subplot treatments the effects of "natural phytotoxic ingredients" (NPI) on weed control was evaluated.
The two NPI formulations used in the experiment contained 50 L/ha Pine Oil (formulation ``Pine Oil'') or 50 L/ha d-Limonene (formulation ``Limonene''), both combined with 50 kg/ha NaCl. These formulations were applied once, twice, or three times in a weekly interval. A weedy and a manually hoed clean plot was used as control treatment.
The single application treatments of Limonene and Pine Oil scored 41 \% and 52 \%, two applications 53 \% and 56 \% and three applications 61 \% and 67 \% respectively, with a 70 \% rating being considered an acceptable result in conventional herbicide testing.
The grain yield for the 'clean' control treatment was highest with 4.07 t/ha and lowest for the untreated control treatment with 2.61 t/ha. The limonene treatments yielded 3.07, 3.22, 3.57, and the pine oil treatments 2.68, 3.00 and 2.96 t/ha with one, two and three applications respectively. The lower grain yields of pine oil despite slightly better performance in the weed control rating can be explained by phytotoxic effects of applied product which came in contact with soybean crop during application.