Effect of microencapsulated alachlor on South African sunflower (Helianthus annuus L.) cultivars

Abstract

The microencapsulated formulation of alachlor is becoming increasingly popular for use on sunflower in South Africa, where it is registered for use at a rate of 1.54 to 1.92 kg ai ha−1. Some injury symptoms, apparently herbicide induced, have been observed on sunflower where this herbicide has been applied in the field. The response of 22 commercially available sunflower cultivars was tested in a glasshouse experiment using both a sand (10% clay, 0.24% C) and a sandy loam soil (20% clay, 0.52% C). Five rates of alachlor (0, 0.5, 1, 2 and 4 times the recommended application rate for each of these soils) were applied and leached into the soil after planting. At harvest plant height, above-ground biomass and seedling survival rate were determined. Results indicated that cultivars differed with respect to alachlor tolerance, but that behaviour was modified by soil type. All effects were far more pronounced on the sandy soil, despite a lower recommended application rate. Plant height was affected by increasing application rate to a far greater degree than dry mass. Although the application rate for this herbicide was lower on the sandy soil, plant growth was significantly reduced at the recommended rate of application for this soil (1.536 kg ai ha−1). Seedling survival rates were reduced at the recommended application rate on both soils, although the effect was far more pronounced on the sand soil. The results indicated that despite the lower recommended application rate of microencapsulated alachlor on sandy soils a far greater degree of herbicide damage to sunflower can be expected on these soils due to increased availability to crop roots. These findings in part explain some of the problems found on sunflower where this formulation of alachlor has been applied.