Weeds often form a major problem in weakly competitive vegetable crops, particularly in low input systems. Undersown cover crops can be used to suppress weeds, but often put too high a competitive pressure on the main crop. Cover crop selection is one of the potential means that can be used to design or optimize these intercropping systems. The objective of the current research was to investigate the variability among a range of clover species in morphological and physiological traits that are considered relevant for interplant competition. To this purpose, field experiments with pure stands of eight clover species (2001) and a selection of three clover species (2002) were conducted, in which regular observations and periodic harvests were taken. Clear differences in the time in which full soil cover was obtained, total accumulated biomass, growth duration, height development and N-accumulation were observed. Persian clover (Trifolium resupinatum L.) and subterranean clover (T. subterraneum L.) were the two most contrasting species in this study, particularly differing in the period in which full soil cover was obtained. Persian clover's faster soil cover could not be attributed to a single trait, but resulted from a number of intrinsic characteristics, like light extinction coefficient, light use efficiency and specific leaf area that together determine the relative growth rate. The study also demonstrated the importance of differences in relative starting position, caused by, for instance, seed size, seeding rate and fraction establishment, for the analysis of early growth characteristics. Alsike clover (T. hybridum L.), berseem clover (T. alexandrinum L) and crimson clover (T. incarnatum L.) developed slower than Persian clover, but all produced a higher amount of accumulated dry matter, due to a longer growing period. Clear differences in height and height development between species were observed. These differences were not associated with dry matter accumulation, as the tallest (red clover; 80 cm) and the shortest species (subterranean clover; 12 cm) produced similar amounts of dry matter. A strong positive correlation between early soil cover development and N-accumulation was observed. The large variability among clover species indicates that species selection is a very important aspect of the development of cropping systems that include clover as a cover crop.
- corn zea-mays