| Summary: | Abstract
In 2005 and 2006, 30 and 15 cowpea genotypes were respectively evaluated for plant growth and symbiotic performance at
Manga in Northern Ghana, in order to identify Nz-fixing potential of these cowpea genotypes as source of N for cropping
systems. The results showed differences in biomass production by the 30 or 15 cowpea genotypes. In 2005, cultivars Fahari,
Mchanganyiko, IT97K-499-39, IT93K-2045-29 and IT84S-2246 produced the most shoot biomass, while Apagbaala, Brown
Eye, ITH98-46, Vita 7 and Iron Grey produced the least. Of the 15 genotypes tested in 2006, cv. TVu1l424 produced the
largest amount of biomass, and ITH98-46, the least. Isotopic analysis of 15N in plant parts also revealed significant
differences in 1)15N of the cowpea genotypes studied. As a result, the percent N derived from fixation (% Ndfa) also differed
among the cowpea genotypes tested in 2005, with only 5 out of the 30 cultivars obtaining over 50% of their N from
symbiotic fixation. Whether expressed as mg Nplant' or kg N.ha-I
, the levels of Nz fixation by the cowpea genotypes
varied considerably during 2005 and 2006, with values ofN contribution ranging from 14.1 kg N.ha-1 by cv. TVul509 to
157.0 kg N.ha-1by IT84S-2246 in 2005. The amounts ofN-fixed in 2006 ranged from 16.7 kg N.ha-1 by cv. ITH98-46 to
171.2 kg N.ha-1 by TVu11424, clearly indicating genotypic differences in symbiotic N yield. Re-evaluating 15 out of the 30
cowpea genotypes for Nz fixation in 2006, revealed higher % Ndfa values (>50%) in all (15 cowpea genotypes) relative to
those tested in 2005, indicating greater dependence on Nz fixation for their N nutrition even though, the actual amounts of
fixed-N were lower in 2006. This was due, in part, to reduced plant biomass as a result of very late sampling in 2006, close
to physiological maturity (72 DAP in 2006 vs. 46 DAP in 2005) when considerable leaf matter was lost. The amount ofNfixed
in 2006 can therefore be considered as being under-estimated.
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