Abstract:
There has been no work conducted in Ethiopia to evaluate and document the progress made in improving the genetic yield potential and associated traits of soybean varieties by grouping in to early, medium and late maturity classes. Hence, soybean varieties released/registered in Ethiopia from 1974 to 2015, were grouped in to early (7), medium (7) and late (11) and evaluated at Bako to determine the genetic gain in grain yield potential and associated traits in the last 38, 31 and 12 years of breeding period respectively. The varieties were grown in a randomized complete block design with three replications in 2015 main cropping season under rainfed condition. Yield increment calculation was conducted by using the mean of oldest varieties as a reference. In early set, grain yield was found to decrease from 1418.73 kg ha-1 to 997.6 kg ha-1 , medium set increased from 2314.5 kg ha-1 to 3075.5 kg ha-1 and for late set increased from 2558.7 kg ha-1 to 3253.9 kg ha-1 over the last 38, 31 and 12 years of soybean breeding period respectively. As estimated from coefficient of linear regression of variety means for each trait on year of release in early, medium and late set soybean varieties, the average annual increase in grain yield was 4.7, 21.7 and 91.55 kg ha-1, 6.4, 39.6 and 124.1 kg ha-1 for biomass yield, 0.048, 0.14 and 0.53% for harvest index respectively. Grain yield was significantly and positively correlated with days to flower, plant height, biomass yield, seed growth rate, grain yield day-1, biomass production rate and harvest index for early set. In medium set grain yield was significantly and positively correlated with days to maturity, plant height, biomass yield, seed growth rate, grain yield day-1, biomass production rate, harvest index, number of pods plant-1, number of seeds pod-1 , number of seeds plant-1.In case of late set maturity groups, grain yield was significantly and positively correlated with number of pods plant-1, number of seeds pod-1, number of seeds plant-1, seed weight, harvest index, biomass production rate, biomass yield, seed growth rate and grain yield day-1 respectively. Stepwise regression in early set maturity group indicated that most of the variation in grain yield was caused by grain yield day-1, 100 seed weight and number of seeds pod-1, in medium set, due to grain yield day-1 and biomass yield while in late set, yield variation was caused due to biomass production rate, harvest index and days to maturity respectively. Generally, grain yield showed consistent increment over years in medium and late maturity classes, while in early set maturity group grain yield over years was inconsistent since Nova variety was released for special purpose (oil content).Therefore; further improvement in soybean yield in early, medium, and late maturity classes is possible.