Purple vetch

The purple vetch (Vicia benghalensis) is native to southern Europe, North Africa, and nearby islands, and it is utilized elsewhere in agriculture and may be present in the wild as an introduced species.

Purple vetch is an annual herb with a climbing stem which is coated in hairs, often densely, making the plant appear silvery white. It may reach a hight of 60 centimeters. Each leaf is made up of several pairs of elongated leaflets which measure up to three centimeters in length. The inflorescence is a one-side raceme of several dark reddish purple flowers. Each flower has a densely haired calyx of sepals and a tubular corolla between one and two centimeters in length. The fruit is a flat, hairy legume pod up to 3.5 centimeters long containing multiple seeds.

This plant is used as a cover crop and green manure for the purposes of soil improvement and weed and pest control. In fact, it is one of the best green manuring species for rice fieds in the USA. It is a soil improver and makes a good weed and disease break between cash crops. It is used in crop rotation, for hay and fodder, and as a honey plant, and it has a very high biomass yield.

The seeds and forage of purple vetch have been reported to cause poisoning in humans and in livestock (with a fatalilty rate of up to 69%), so caution is required when feeding them[1]. The purple vetch hides canavanine in its seeds. Canavanine is a toxic analogue of the amino acid arginine. In the purple vetch canavanine is a defensive compound against herbivores and a vital source of nitrogen for the growing embryo.

[1] Harper et al: Vetch toxicosis in cattle grazing Vicia villosa ssp dasycarpa and V benghalensis in Australian Veterinary Journal - 1993