TESTING HARROWS FOR MECHANICAL CONTROL OF ANNUAL WEEDS IN AGRICULTURAL CROPS

Selectivity, defined as the ratio between weed control and crop burial in soil as a result of harrowing, has been used to test different implements according to their degree of selective operation. For this purpose a concept considering qualitative and quantitative aspects of selectivity has been proposed. This makes it possible to determine whether differences in selective mode of operation were caused by construction or by application of implements. Analysis of covariance was used to make this separation. Only if implements affected parameters in a model describing the relationship between crop soil cover and weed control were they suggested to operate with different selectivities. Crop soil cover was assessed by visual score as percentage of crop leaf area covered by soil as a result of harrowing. Using this method it was shown that none of the five harrows tested could be separated on the basis of their selective mode of operation. Selectivity was shown to be influenced by day of treatment, composition of the weed flora, site characteristics and degree of weed control. Harrowing at late crop growth stages was more selective than expected.