Cover Cropping Under Temperate Conditions: Influence of Growth Period and Incorporation Time.

Cover crops (CC) are generally followed by spring sown crops which limits the use of winter cereals in a crop rotation. A change from winter cereals as e.g. winter wheat (Triticum aestivum L.) to a spring sown crop as barley (Hordeum vulgare L.) often results in a reduction in grain yield. To encourage increased use of CC and to lessen the consequences on choice of main crop new innovative ways of using CC should be considered. This study tested the potential for using CC that could allow for repeated winter wheat growing and still permit CC in breaks between crops. Cruciferous CC (Raphanus sativus L., Sinapis alba L.) spread in a growing winter wheat crop in July and incorporated in September (Autumn CC) before sowing the following winter wheat was compared with the same CC cultivars sown after harvest and incorporated in spring (Winter CC). The cruciferous CC were compared with Winter CC of spring sown ryegrass (Lolium perenne L.) and a reference without CC. The study included comparable field and lysimeter experiments where N uptake of Autumn and Winter CC were measured together with their influence on nitrate leaching. An incubation study determined the decomposability of the CC material as affected by developmental stage at incorporation and soil temperature. Cruciferous Autumn CC sown by spreading seeds in a growing winter wheat developed well provided that wheat harvest took place at normal time in mid August. Winter CC sown after harvest was likewise highly influenced by harvest time of the main crop which determined time of their sowing. The reduction in nitrate leaching with Autumn CC was generally about half of the reduction caused by Winter CC. Preliminary results suggest that the decomposability of CC is not highly affected by the developmental stage reached by the CC before incorporation.