Improvement in grain and fodder yields of early-maturing promiscuous soybean varieties in the guinea savanna of Nigeria
Thirteen early-maturing and promiscuously nodulating soybean [Glycine max (L.) Merr.] varieties that belonged to maturity group I developed at the International Institute of Tropical Agriculture (IITA) from 1980 to 1996 were evaluated at two locations in the Guinea savanna of Nigeria in 2001 and 2002 to determine genetic gain in grain yield and associated agronomic traits. The varieties were grown in a randomized complete block design with four replications. Data on grain yield and seven other traits were recorded. The study showed that grain yield potential of the soybean varieties increased from 1117 to 1710 kg ha–1 during two decades of breeding in IITA. The average rate of increase per year per release period (1980–1996) was 24.2 kg ha–1, which is 2.2%. Fodder yield also showed an annual increase of 22.81 kg ha–1 yr–1. Recent varieties had better lodging resistance scores of 1.25 to 1.37 in a 1 to 5 scoring scale (1 being resistant and 5 susceptible), shattering resistance scores of 1.6 to 2.7, and improved natural nodulation scores of 3.1 to 3.9. Old varieties had 50% or more pod shattering while the corresponding figure for new varieties was around 25%. Old varieties produced few and small nodules, while new varieties had many big nodules although size and number of nodules alone may not necessarily indicate nodule efficiency in N2 fixation. Grain yield showed a positive and highly significant (P 0.01) correlation coefficient (r = 0.75) with fodder yield, implying that high biomass could serve as an indicator of good yielding ability.