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Two field experiments were carried out in 2016 with a focus on weed control characteristics and the biomass production from various summer intermediate crops. One of the experiments was located at Norra Åsum, Kristianstad in Skåne and the other at Lilla Böslid, Laholm in Halland.

The summer intermediate cover crops with the best weed competitive characteristics at both experimental sites were oilseed radish, facelia and buckwheat, while sudan grass had the lowest ability to compete with weeds. Hemp and hemp intercropped with hairy vetch had also as good weed competitive effect besides oilseed radish, facelia and buckwheat, at Norra Åsum.

Half of the trial plots were fertilized with nitrogen at both experimental sites. In Norra Åsum generally more weeds were found in the fertilized plots. The opposite was true at Lilla Böslid where the weed flora was dominated by Corn Spurrey. In the fertilized plots in Norra Åsum a weak correlation between weed dry weight and summer cover crops dry weight was found.

Summer cover crops, which were sown July 20 at Norra Åsum, gave the biggest biomass yield in October 13 (85 growing days). Sudan grass in combination with hairy vetch, however, gave higher yield on November 7 (110 growing days).

In the fertilized plots at Norra Åsum (41 kg of ammonium nitrogen per ha, as biogas digestate) hemp produced the significant highest amount of biomass (4.2 t DM per ha), while fertilized sudan grass, respectively buckwheat gave the lowest biomass (2.9 t DM per ha) on October 13. Fertilized oilseed radish compared to unfertilized oilseed radish increased biomass yield from 1.4 to 3.3 t DM per ha, that is, an increase of 2.3 times. In the unfertilized plots, hemp in combination with hairy vetch gave the significantly highest biomass yield (3.8 t DM per ha).

The conclusion from the experiments in 2016 on weed control characteristics from summer cover crops are: Oil radish, facelia, buckwheat and hemp and hemp combined with hairy vetch have the potential to compete well against weeds on light sandy soil. This assumes that the cover crop establish well and they have a good growth during the summer and fall. Fertilization of summer cover crops by about 40 and about 50 kg of nitrogen per hectare generally resulted in more weeds in Norra Åsum, and fewer at Lilla Böslid, where the weed flora was dominated by Corn Spurrey. This result shows that further experiments with the weed competing properties from summer cover crops need to be conducted to determine the fertilization effect on weed effect and biomass production under different years, soil conditions, etc.