Effects of Conservation Agriculture on Weed Abundance and Herbicide Residues in a Humic Nitisol in Kabete Sub-county, Kenya

Abstract

Conservation agriculture has often been fronted as a sustainable practice that minimizes soil degradation while improving crop yields and soil fertility through its principles of minimum tillage, mulching and crop rotation. However, the reduction of tillage operations has led to the challenge of weed control under CA. This has also led to the increased dependence on herbicides for weed control. However, these herbicides have a higher chance of persisting in the soil due to their chemical properties and soil properties. This study was conducted for two seasons (May- September 2021) and (October 2021- February 2022). Its main objective was to assess the effects of conservation agriculture on weed abundance and herbicide residues in soil as influenced by herbicide application under different tillage methods. It specifically assessed the effect of different tillage methods and herbicide application on the abundance and diversity of weeds, the distribution of Atrazine, S-metolachlor, and Nicosulfuron herbicides in soil under different tillage methods and determined the levels of Atrazine, S-metolachlor, and Nicosulfuron herbicides left in the soil under different tillage methods. The three treatments were a combination of herbicides and the type of tillage methods. They were: (i) ripping + S-metolachlor 290 g l-1 + Atrazine 370 g l-1 + Nicosulfuron 240 g l-1, (ii) jab planter + S-metolachlor 290 g l-1 + Atrazine 370 g l-1 + Nicosulfuron 240 g l-1 and (iii) hand hoeing with no herbicides applied (control).The treatments were replicated four times in a randomized complete block design (RCBD). The data was collected on weed abundance and diversity as affected by tillage methods and herbicide application as well as herbicide distribution and persistence as affected by ripping and jab planter tillage methods. The weed data was analysed using R statistical software while soil samples were analysed for Atrazine, S-metolachlor and Nicosulfuron using Gas Chromatography with Mass Spectrometry (GC-MS). The results showed that in the first season tillage methods combined with herbicide application had a significant effect (P < 0.0038) on species richness with ripping recording the highest average species number of 2.05 m-2, jab planter at 1.78 m-2 and hand hoe (control) at 1.53 m-2. Shannon diversity also significantly varied (P < 0.0072) with ripping recording the highest average value of 0.59, followed by jab planter at 0.47 and finally the hand hoe (control) at 0.37. In the second season, tillage and herbicide application had no significant effect on species richness and Shannon diversity. Atrazine, S-metolachlor and Nicosulfuron significantly leached down the soil profile based on the tillage methods applied. At 0-15cm in ripping and jab planter tillage methods, residues were 14.7 mg kg-1, 15.6 mg kg-1 for atrazine, 18.5 mg kg-1, 17.5 mg kg-1 for S-metolachlor and 5.6 mg kg-1 ,6.2 mg kg-1 for Nicosulfuron respectively. At 15-30cm in ripping and jab planter residues were 2.4 mg kg-1, 1.3 mg kg-1 for Atrazine, 2.4 mg kg-1, 1.3 mg kg-1 for S-metolachlor, and 1.4 mg kg-1 and 1.4 mg kg-1 for Nicosulfuron respectively. While herbicides are increasingly being embraced by farmers practicing conservation agriculture, their persistence and leaching in soil presents an ecological and human health challenge. To reduce these risks associated with herbicide residues persistence in the soil, it is imperative for the government of Kenya to make available biopesticides and for farmers to embrace alternative ways of weed control such as use of biopesticides, use of agronomic practices like mulch and crop residues, push pull technique, crop rotation, and intercropping of crops with allelopathic properties such as sorghum that inhibit growth of weeds.