Residual diazinon in brassica oleracea var. Acephala and influence of application conditions on its levels in Kimira-oluch smallholder farmers improvement project, Kenya

Abstract

Use of pesticides in agriculture often causes residual effects on crops with potential health risks. Diazinon, a synthetic pesticide whose dietary exposures have been associated with human health impacts is popular in the production of Brassica oleracea var. acephala at Kimira-Oluch Smallholder Farmers Improvement Project (KOSFIP), Kenya. Diazinon has relatively long pre-harvest interval (PHI) which farmers may not be observing. Levels of diazinon residues and associated health risks in farm-gate Brassica oleracea var. acephala are unknown. There is no available data on how application conditions influence the residue levels. This study quantified the residual diazinon levels in farm-gate baskets of the vegetable; influence of rate of application on residual levels at PHI; and influence of post-application duration on residual levels at different application rates. Cross-sectional survey based on purposive snowball sampling identified 40 of 45 farms from which samples were collected in triplicate. Randomized complete block design arranged in 5x8 split-plot controlled experiments were used to determine the influence of diazinon application rates and PHI conditions on residual levels. Main treatments were 0.0000, 0.6125, 1.2500, 1.8625 and 2.5000 litres ha-1, respectively, split for 0, 5, 10, 13, 16, 18, 20 and 21 days post application harvesting. Samples were prepared using QuEChERS method, and subjected to LC-ESI-MS/MS analysis. Farm-gate samples had residual diazinon levels (92.5%) of which 67.57% were above the Codex Maximum Residue Limits. Of the samples, 40% had Health Risk Index >1.0, higher than most previous studies. Malpractices against Good Agricultural Practices (GAPs) may be responsible for higher residual levels with consequent increased health risks to consumers. The results demonstrate the need for surveillance and increased farmer education to reduce diazinon residue levels in the farm-gate vegetables. Residual diazinon levels increased (P≤0.05) with increasing rates of application and were inversely proportional to increasing PHI in agreement with previous studies. Application rates ≥ 1.25 litres/ha required longer PHI than the labeled, suggesting that the recommended PHI is inappropriate for the vegetable in the study area. A review of PHI from 12 to 14 is recommended for rate of 1.25 litres/ha. The use of diazinon on Brassica oleracea var. acephala at KOSFIP should be discouraged and alternative pesticides be used according to GAPs.