Ogräskonkurrerande egenskaper hos vårkorn och vårvete

This report presents the results of two projects focused on how spring barley and spring wheat can be used to suppress seed weeds in organic farming. The projects studied more specifically how the weed situation was affected by variety choice, plant height, seed rate, plant stand, sowing time and false seedbeds. The weed studies were carried out in the national trials variety testing of spring wheat and spring barley in organic farming, in the trials "Spring barley-variety-sowing time" and at SITES, Lönnstorp SLU Alnarp. Eight trials were studied in 2019 and 2020. In parallel with the studies of the effect of cereals on weeds, two light measurement methods were evaluated: the crop leaf area index (LAI) and the crop shading effect, i.e. the photosynthetically active radiation at the ground surface (PAR). This was done to assess whether these methods can complement alternatively replace traditional assessment of weed parameters such as weed cover and plant height in the cereal crop. When we studied the effect of plant height on the weed situation in spring barley and in spring wheat, higher varieties generally had better weed-suppressing ability than shorter varieties. However, some short varieties had good weed-suppressing properties, that possible could be an effect of allelopathy. When we studied the effect of the seed rate on the weed situation in spring barley, it turned out that the weed weight increased, 4 to 5 times, and the number of weeds increased, about 2 times, when the seed rate was reduced from 400 to 200 seeds per m². Furthermore, the weed cover increased by about 2.5 times, when the seed rate was 50 % of normal. A delayed seeding date, with two to three weeks, in spring barley resulted in a reduced amount of black grass (Alopecurus myosuroides Huds.). At the same time, the broad-leaved weeds decreased in one of the experiments and increased in another. When we studied a delayed seeding date in combination with the effect of false seedbeds, on the weed situation in the spring cereals, we could not see any effect of the false seedbed. The lack of effect was probably due to the fact that the soil was too dry when the false seedbeds were made two to three weeks before the delayed seeding date. When we compared the ground cover of the cereal crops with the weed occurrence, we found, as expected, a negative correlation between the ground cover of the crop and the weed weight. We also found a negative correlation between the ground cover of the crop and the weeds. This means that the higher the ground cover that the cereals have, the lower the weed weight resp. weed ground cover. Furthermore, when we compared the height of the cereals with its ground cover, we found as expected, a positive correlation between these. It turned out that the weed-suppressing ability of different varieties of spring barley and spring wheat can be largely explained by its leaf area index (LAI), the amount of PAR light at the ground surface, the ground cover of cereals and the height of the crop. It also turned out that the larger the leaf area index (LAI) that the cereal had, the less was the weed ground cover. It was also possible to show that there was a positive correlation between PAR at ground surface and the weed cover. This means that the more PAR at ground surface, the greater the weed cover will be. When it comes to assessing the weed-suppressing ability of spring barley and spring wheat using LAI and PAR at ground surface, our study shows that these can probably complement ocular assessment of the ground cover of cereals and manual measurements of plant height. Furthermore, it turned out that an increased LAI in spring barley and spring wheat gave an increased yield. In order to assess the weed-suppressing ability of spring barley and spring wheat with the help of LAI or PAR at ground surface, measurement of these parameters should take place no later than at the heading stage, according to our assessments. However, in one of our studies in spring wheat, we got a better correlation between LAI and the weed ground cover when LAI was measured when the flag leaf was visible (BBCH 37) compared with the measurement at the end of heading (BBCH 59). The ground cover of the weed was assessed ocular in the experiment at heading stage. More experiments to determine the optimal time for measuring LAI or PAR in relation to the weed's ground cover are needed, in order to more precise in the use for these parameters. Due to LAI is easier to register in the field and the result is easier to statistically analyze, in comparison with measuring PAR at ground surface, we propose to use LAI as an objective method to measure the weed-suppressing properties of spring barely and spring wheat.