The influence of gypsiferous water on the behaviour of selected herbicides

Abstract

Gypsiferous water, i.e water containing high levels of calcium sulphate, is used to irrigate crops such as maize and wheat on a commercial scale, in some parts of South Africa due to shortage of irrigation water. Gypsiferous water is saline due to the presence of Ca2+, S042- and sometimes Mg2+ in solution. These salts may have a detrimental effect on herbicide performance. Possible herbicide injury to crops has been reported in areas irrigated with gypsiferous water, which suggested interaction between herbicides and gypsiferous water. Bioassay experiments were conducted in the greenhouse to assess whether the bioactivity and persistence of the selected herbicides were affected by the presence of gypsum in the soil. The biological activity of atrazine was significantly increased in the presence of gypsum, and in the case of 2,4-D the same effect was even more pronounced. In contrast, the activity of metolachlor was reduced in the presence of gypsum. These differential effects on herbicide activity would have important practical consequences for herbicide performance, i.e weed control efficacy, selectivity, and behaviour in soil. In the persistence experiment, the activity of all herbicides decreased with time. However, the presence of gypsum had a significant effect on the rate of dissipation of only atrazine. In the presence of gypsum the rate of loss of atrazine was significantly less compared to its rate of loss in soil that did not contain gypsum. The rate of loss of metolachlor was not significantly affected by the presence of gypsum, and in the case of 2,4-D the trends were not clear at all. A further experiment was conducted to determine the leaching potential of metolachlor and atrazine in soil irrigated with either gypsiferous water or uncontaminated water. In the leaching experiment metolachlor showed a significant growth reduction of 39% in the first 10 cm column zone in soil irrigated with gypsiferous water and no further losses were observed in deeper column segments, and this indicated that there would be no threat of groundwater contamination. However, the results showed that weeds with shallow roots would be effectively controlled in this column. Similar results were observed in soil without gypsiferous water irrigated. The findings of this experiment appear to contradict the findings of Chapter 4 for metolachlor, however it is to be considered that in Chapter 4 the "close system" was used and in the leaching experiment an "open system" was used. In addition, the calcium sulphate concentrations for the two experiments were different. In the atrazine experiment it was found that 0.1 mg kg-1 would pose a serious threat to groundwater contamination and sensitive follow-up crops. There is a need to reduce the atrazine dosage rate to 0.05 mg kg-1 because no threat to groundwater was observed at this rate. However, weed control efficacy at this low rate maybe poor.