Grain yield and contribution of symbiotically fixed nitrogen to seed in cowpea varieties using the 15N natural abundance technique

Abstract

Identification of cowpea varieties with superior grain yield and high nitrogen fixing potential is crucial to increasing productivity of the crop among small-scale farmers in the Northern Province. A field experiment was established during the 1997/98 growing season to evaluate six cowpea varieties for their grain yield, protein content, yield components and proportion of seed N derived from symbiotic fixation (%Ndfa) through the natural abundance technique. The varieties evaluated were PAN311, Glenda, Bechauana white, Chappy, Encore and NWK. PAN311 produced the highest grain yield of 1635 kg ha−1 which was 90% higher than the average across varieties. The lowest yielding variety was Glenda with an average of 368 kg ha−1. The high grain yield of PAN311 could be attributed to its high number of pods per plant, seed mass as well as harvest index. Seed protein concentration ranged from 237–275 g kg−1 but did not differ among the varieties. Estimates of %Ndfa in seed ranged from 23.02–28.80% with Bechuana white showing the highest dependance and PAN311 the least. PAN311 is clearly the highest grain yielding variety, though several of the other varieties have the potential of improving farmers yield which is estimated to be about 400 kg ha−1. © 2000 Taylor & Francis Group, LLC.