Role of Variety and Fertilizer Practices on Cowpeas (Vigna unguiculata) Yield and Field Incidence of the Parasitic Weed Alectra vogelii (Benth) in Central Malawi

Grain legumes are an important component of the food systems in Malawi. The parasitic legume witchweed species Alectra vogelii (Benth) is among the problem pests with serious infestations in groundnuts (Arachis hypogea), soybeans (Glycine max), cowpeas (Vigna unguiculata) and other legumes. A study was conducted in 2013/14 and ‘14/15 seasons to evaluate the effects of three cowpea varieties (IT82E-16, Sudan 1 and Alectra-resistant Mkanakaufi) and fertilizer practices (no fertilizer applied, 5 t ha-1 cattle manure and 100 kg ha-1 of inorganic 23-21-0+4S on cowpea grain yield, yield components and Alectra emergence at three sites in central Malawi. Cowpea grain yields ranged 400-2400 kg ha-1. There were significant (P < 0.05) variety effects on yield in 4 of 6 site-years, with variety IT82-16 consistently giving the highest yields (range 1200-2400 kg ha-1). There were significant variety effects on A. vogelii emergence with Mkanakaufiti having no Alectra throughout. Application of cattle manure strongly suppressed A. vogelii in 6 site-years all at 60 days after planting, while inorganic fertilizer suppressed Alectra in 3 of the 6 site-years. Cattle significantly but marginally (about 250 kg ha-1) increased yield in 2 of the 6 site-years. The results show potential to suppress A. vogelii with cattle manure application. However, further studies are required to understand the causes of the limited yield response under manure or fertilizer application to make the practices attractive to farmers. More variety improvement studies to produce a range of varieties with better local adaptability and response to fertility amendments are recommended.