WEED HARROWING-DRIVING SPEED AT DIFFERENT STAGES OF DEVELOPMENT

The effects of weed harrowing on oats and annual weeds were studied in field experiments for 3 years in central Sweden. Harrowing was performed once, when the crop had developed 1, 2 or 3 leaves. The driving speed was 5, 9 or 13 km/h. The effects on the crop and the weeds differed between years and was dependent on the driving speed and the time of harrowing. The negative effect on the crop was strongest when the harrowing was done at the 1-leaf stage of the crop, probably caused by the high degree of soil cover on the crop. No significant change in the yield in relation to untreated plots was found in treatments harrowed at the 3-leaf stage in 1986 and 1987. Harrowing at the 1-leaf stage tended to give the weakest effect on weeds since few weed plants had become established at this time. Harrowing conducted at the 2- or 3-leaf stages of the crop led to greater reductions of the number of weed plants. The effect on the weeds and also on the grain yield was best correlated with the soil coverage on the crop plants. The soil covering degree is an important parameter for measuring of the weed harrowing intensity.