Growth characteristics of several clover species and their suitability for weed suppression in a mixed cropping design

Weed management without herbicides is a challenging undertaking and requires reliable alternative strategies, particularly in poorly competing crops, where weeds can cause severe yield losses. Adding a companion crop is one of the means to enhance the weed suppressive ability of the crop canopy. In this regard, clover is often referred to as  an  interesting option as, apart from weed suppression, clover species enhance the soil nitrogen status and contribute to pest suppression. Earlier experimental work showed however that clover can easily become too competitive, hindering the growth and development of the main crop. It thus remains questionable whether clover can be added as a companion cover crop for weed suppression without substantially harming the main crop. In this context, the relevance of the choice of clover species was investigated in the current research project. Next to field experiments, experiments in containers and pots were used to determine various traits of a range of  clover species and to relate these traits to competitive and weed suppressive ability. Differences between clover species were large and it was shown that plant height was the main determinant of the competitive effect on crop plants, while rapid early growth was the trait correlating most strongly to weed suppressive ability. Persian clover was a good example of a species that suppressed weeds successfully, but also competed too fiercely with the main crop. The performance of the short growing subterranean clover was unsatisfactory. The slow early growth of this species resulted in poor weed suppression and in addition, the species competed more strongly with the crop than expected based on plant height. This last was shown to be due to a relatively strong ability to compete for below ground resources. It was concluded that successful introduction of a companion clover crop for enhanced weed suppression will not be possible unless additional management  to reduce the competitive damage to the main crop is conducted. Of all the investigated clover species white clover held most promise as a weed suppressing clover species in a mixed cropping design.

Key words: Clover, subterranean clover, weed suppression, cover crop, living mulch, smother crop, yield loss.