Effects of competition on compensation point and phenological development in Sonchus arvensis L.

Abstract: Sonchus arvensis L. is a perennial weed which has become more common in Scandinavian countries owing mainly to the proliferation of organic farming which limits the use of her-bicides. When soil tillage is used for controlling perennial weeds, farmers are usually recommended that mechanical disturbance should be done at the developmental stage when the plant is most sensitive for disturbance which coincides with minimum below ground dry weight. Håkansson (1969b) has shown that S. arvensis reaches its minimum dry weight of underground regenerative structures at 5-7 true leaves. The time taken to reach dry weight minimum depends on environmental factors like temperature but may also be influenced by competition from other plants. The aim of the study was to evaluate if the phenological stage, at which minimum dry weight of below ground biomass occurred in S. arvensis (i.e. compensation point), was affected by competition from a crop. A greenhouse box experiment was performed at Ultuna, close to Uppsala, Sweden (59º 48’N, 17º39’E) during January - April 2012. The plant material used in the experiment was Sonchus arvensis L. (perennial sow-thistle) and Hordeum vulgare L. var Judit (spring barley). The experiment consisted of two treatments: S. arvensis grown (1) with (C+) and (2) without (C-) competition from spring barley. The compensation point in competing plants occurred at leaf stages 4 and 5, while it occurred at the 4 leaf stage in plants without competition. For given phenological stages, weight loss of below-ground structures did not differ between the C+ and C- plants, apart from the 5 leaf stage, at which the C+ plants had lost more in weight that the C- plants. The initially planted root parts of C+ plants lost more weight than the C- plants. Phenological development of S. arvensis, in terms of development of numbers of leaves over time, was delayed by competition with spring barley. For agronomy practice, these results imply that S. arvensis should be controlled earlier than previously was recommended, and that competition from a crop can be used in addition to greatly deplete the below-ground resources in S. arvensis.