A. Rahman

Sorption characteristics of atrazine and imazethapyr in soils of New Zealand: importance of independently determined sorption data.

We investigated sorption characteristics of two commonly used herbicides, atrazine and imazethapyr, in 101 soils with allophanic and non-allophanic clays of New Zealand using the batch equilibration technique. Soil properties, such as organic carbon (OC) content, texture, pH, amount and type of clay, and cation-exchange capacity (CEC), were tested against the sorption coefficients (Kd) of these herbicides. There was a wide variation in the sorption affinities of the soils, as the Kd values of atrazine and imazethapyr ranged from 0.7 to 52.1 and from 0.1 to 11.3 L kg(-1), respectively. For atrazine, the sorption affinities for the allophanic set of soils (mean Kd of 8.5 L kg(-1)) were greater than for the non-allophanic set of soils (mean Kd of 7.5 L kg(-1)). However, no effect of allophanic status was found for imazethapyr sorption (mean Kd of 0.82 and 0.76 L kg(-1) for allophanic and non-allophanic, respectively). None of the measured soil properties could alone explain adequately the sorption behavior of the herbicides. The variation of OC soil sorption coefficients, Koc, was also larger for atrazine (mean Koc of 126.9 L kg(-1)) than for imazethapyr (mean Koc of 13.2 L kg(-1)). The prediction equations for atrazine and imazethapyr developed overseas failed to provide the acceptable values of sorption coefficients for the soils of New Zealand. The study highlights the danger of using sorption coefficient data from the literature for practical assessments of the herbicide leaching in New Zealand soils.

Dissipation of the Herbicide Clopyralid in an Allophanic Soil: Laboratory and Field Studies

Soil dissipation of the herbicide clopyralid (3,6‐dichloropicolinic acid) was measured in laboratory incubations and in field plots under different management regimes. In laboratory studies, soil was spiked with commercial grade liquid formulation of clopyralid (Versatill®, 300 g a.i. L− 1 soluble concentrate) @ 0.8 µg a.i. g− 1 dry soil and the soil water content was maintained at 60% of water holding capacity of the soil. Treatments included incubation at 10°C, 20°C, 30°C, day/night cycles (25/15°C) and sterilized soil (20°C). Furthermore, a field study was conducted at the Waikato Research Orchard near Hamilton, New Zealand starting in November 2000 to measure dissipation rates of clopyralid under differing agricultural situations. The management regimes were: permanent pasture, permanent pasture shielded from direct sunlight, bare ground, and bare ground shielded from direct sunlight. Clopyralid was sprayed in dilute solution @ 600 g a.i. ha− 1 on to field plots. Herbicide residue concentrations in soil samples taken at regular intervals after application were determined by gas chromatograph with electron capture detector. The laboratory experiments showed that dissipation rate of clopyralid was markedly faster in non‐sterilized soil (20°C), with a half‐life (t1/2) of 7.3 d, than in sterilized soil (20°C) with t1/2 of 57.8 d, demonstrating the importance of micro‐organisms in the breakdown process. Higher temperatures led to more rapid dissipation of clopyralid (t1/2, 4.1 d at 30°C vs 46.2 d at 10°C). Dissipation was also faster in the day/night (25/15°C) treatment (t1/2, 5.4 d), which could be partly due to activation of soil microbes by temperature fluctuations. In the field experiment, decomposition of clopyralid was much slower in the shaded plots under pasture (t1/2, 71.5 d) and bare ground (t1/2, 23.9 d) than in the unshaded pasture (t1/2, 5.0 d) and bare ground plots (t1/2, 12.9 d). These studies suggest that environmental factors such as temperature, soil water content, shading, and different management practices would have considerable influence on rate of clopyralid dissipation.

Post-emergence weed control through abrasion with an approved organic fertilizer

Corn gluten meal (CGM) is an approved organic fertilizer and pre-emergence herbicide that can be manufactured in the form of grit. This grit was tested for its ability to abrade seedlings of the summer annual weedy grass, Setaria pumila, when plants were in the 1- to 5-leaf stages of growth. CGM was propelled at air pressures of 250–750 kPa at distances of 30–60 cm from the plants. Established seedlings of S. pumila were controlled more effectively when grit was applied at 500 and 750 kPa than at 250 kPa, as well as when the applicator’s nozzle was 30 cm from the plants compared to 60 cm distance. Seedling growth and dry weights were greatly reduced by exposures to grit at 60 cm and 500 kPa for 2 s or less, and seedlings were nearly completely destroyed at 30 cm distance and 750 kPa. CGM, a soft grit, was as effective for abrading seedlings as fine quartz sand, a hard grit. CGM had little pre-emergence herbicidal effect on S. pumila. Although regrowth can occur in S. pumila after abrasion by grit, the initial grit-induced stunting is sufficient to allow competing crop plants, like maize, to escape competition and suppress the weed. Consequently, CGM may be an effective form of soft grit for post-emergence abrasion of seedlings of summer annual grass weeds in organic row crops, while simultaneously supplying the crop with fertilizer.