Relationship between speed, soil movement into the cereal row and intra-row weed control efficacy by weed harrowing

Summary in the original language of the document

Field trials were made at a single Danish and two Spanish locations. In Denmark winter wheat was sown at 24 cm spacing allowing hoeing in the inter-row area. Hoeing speeds of 2, 5 and 8 km h-1 were tested at the end of tillering, at the beginning of stem elongation and at both times. The crop was harrowed immediately afterwards at the same speed. In the Spanish locations the winter barley was sown at a spacing of 12 cm and harrowed only at either pre-emergence plus post-emergence or once post-emergence at mid-tillering at 2, 4, 6 and 8 km h-1. The depth of the soil layer thrown into the cereal row was measured. This layer ranged between 0.4 cm and 1.4 cm depending on the site and on the treatment, but was generally higher with a single treatment at all sites. The soil layer tended to increase with higher speed only at the Danish location. On a more sandy soil and on a soil rolled prior to treatment, less soil was thrown into the cereal row. When two hoe + harrowing treatments were made, a finer soil structure was achieved. However, this did not affect the weed control. In the Danish location, initial intra-row efficacy based on plant number before and 7 d after treatment, was found to be low (24-40%) but increased to 74-80% when assessed after 45 d. Partial burial and bending of B. napus, together with crop competition, probably suppressed weed plant growth and enhanced final mortality. Uprooting was probably a more important mortality cause for S. media. In the Spanish locations, efficacy on P. rhoeas was similar, ranging between 58-83% and was already achieved few days after harrowing. A thicker soil layer did not result in a higher efficacy of weed kill. It was therefore suggested that burial alone could not be the main factor responsible for weed control in any of the cases studied. No reduction in wheat biomass measured at the end of May was found with increasing speed, nor with repeated passes of the harrow. The results suggested that faster harrowing, which is economically more attractive for farmers, could be recommended. The soil layer thrown into the row was not found to be a useful parameter to predict the weed control efficacy in the present cases.