Sunn hemp (Crotalaria juncea) weed suppression and allelopathy at different timings

The utilisation of plant allelopathy is an important tool in a diversified weed management strategy. Allelochemicals are released by some plants, such as sunn hemp (Crotalaria juncea L.), to inhibit the growth of neighbouring plants. This species was tested for use as an allelopathic plant for weed control in cropping systems. The allelopathic properties of sunn hemp were tested under field conditions at different timings of 4, 8, 12 and 16 weeks after planting (WAP). The results showed that sunn hemp at 8, 12 and 16 WAP significantly decreased the total number of weeds compared with the control weedy check. The similar trend of decreasing weed number was observed in grass and broadleaf weeds, as well as sedge. The allelopathic activity of sunn hemp was further investigated in the soil planted with sunn hemp and water extracts on tested plants. Phytotoxicity of the soil, planted with sunn hemp at different timings, did not significantly decrease germination, shoot length, root length and dry weight of tested plants. However, the water extracts caused significantly decreased germination and plant growth, especially the shoot extracts exhibited the inhibitory effects more than the root extracts. Interestingly, the extracts from sunn hemp at 4 WAP had higher inhibitory effects than the extracts from other stages. These results suggest that sunn hemp growth stage is an important consideration for maximising biological weed control in cropping systems.