Journal of Crop Breeding

Introduction and Objectives:
Iran is recognized as one of the regions of alfalfa origin in the world, possessing a wide genetic diversity related to this crop. In general, alfalfa yield improvement is the main goal of all alfalfa breeding programs. To start a breeding program for one or more traits in a crop such as alfalfa, it is essential to have information about the heritability of traits, especially the narrow-sense heritability (h²_n). By definition, the narrow-sense heritability is the ratio of additive genetic variance to phenotypic variance in a population or set of genotypes. In general, information about the narrow-sense heritability of the interested traits helps the breeder to have a good understanding of the potential of trait improvement through selection. Therefore, one of the important applications of the narrow-sense heritability estimation is to predict the genetic gain from selection among the studied genetic material. One of the most widely used methods in forage crops such as alfalfa to estimate additive genetic variance and narrow-sense heritability is poly-cross test, which is especially used in the production of synthetic cultivars. The aim of this study was to estimate the narrow-sense heritability and predict the genetic yield of selection among Iranian alfalfa cultivar/ecotypes to produce and release one or more synthetic varieties.
Materials and Methods: An experiment was conducted with 10 half-sib families (progenies from poly-cross) derived from poly-crosses between 10 genotypes. These genotypes included two ecotypes of Bami and Yazdi alfalfa, two released cultivars (Ahang and Mandegar), and ecotypes from cold and temperate regions, including KFA3, KFA15, KFA13, KFA11, KFA4, and KFA16. The experiment was executed in a randomized complete block design with three replications in Karaj during the years 2020 to 2022. The experiment was conducted in the research fields of Seed and Plant Improvement Institute, Karaj in September 2020 and the traits were recorded in 2021 and 2022.The studied traits included plant height(cm), number of stems per square meter, regrowth rate(cm), fall dormancy score, and annual yields of dry and fresh forage (t ha⁻¹) over two years. Combined variance analysis of the data related to the measured traits was performed based on a split-plot experiment in time. Genotype and year were considered fixed factors, and replication was taken as a random factor. To calculate additive variance, the genetic variance among half-sib families was first estimated using the expectation of the mean squares. The genetic variance among half-sib families is equal to the covariance within half-sib families and represents additive variance. For autotetraploid plants like alfalfa, the genetic variance among half-sib families equals one-fourth of the additive variance and one-thirty-sixth of dominance variance. The narrow-sense heritability was estimated based on the two-year phenotypic mean of half-sib families, and genetic gain from selection among the families was predicted. Additionally, phenotypic and genotypic coefficients of variation were calculated for various traits.
Results: Statistical analysis results revealed significant differences among half-sib families for regrowth rate (α ≤ 0.01), fall dormancy score (α ≤ 0.01), and dry forage yield (α ≤ 0.05). Two half-sib families, Poly-Yazdi and Poly-KFA3, with 8.7 and 8.6 scores and two half-sib families, Poly-KFA11 and Poly-KFA15, with 5.07 and 5.13 scores, had the maximum and minimum fall dormancy scores, respectively. Poly-KFA3 and Poly-KFA15 exhibited the highest (22.91 t ha⁻¹) and lowest (19.48 t ha⁻¹) dry forage yields, respectively. Narrow-sense heritability estimates for traits such as plant height, number of stems per square meter, regrowth rate, fall dormancy score, fresh forage yield, and dry forage yield were 0.24, 0.25, 0.94, 0.87, 0.42, and 0.61, respectively. The highest genotypic coefficient of variation was observed for fall dormancy score (19.65%), regrowth rate (10.46%), and dry and fresh forage yields (3.80% and 3.02%, respectively). Conversely, the lowest variation was noted for plant height (1.08%) and the number of stems per square meter (1.63%). Regarding the phenotypic coefficient of variation, the highest values were associated with fall dormancy score (21.07%), regrowth rate (10.80%), and dry and fresh forage yields (4.85% and 4.65%, respectively), while the lowest were observed for plant height (2.23%) and the number of stems per square meter (3.29%).
Conclusion: According to the results of this study, the expected genetic gain from selection, using a 50% selection intensity and a parental control factor of 2, was estimated for traits as follows: fresh forage yield (1.52 tons per hectare), dry forage yield (0.62 tons per hectare), plant height (0.4 cm), number of stems per square meter (4.21 stems), regrowth rate 14 days post-harvest (3.68 cm), and fall dormancy score (1.17). Finally, with regard to mean comparison results, narrow-sense heritability estimations and genetic gain from selection, five populations, Bami, Yazdi, KFA3, KFA13, and KFA16, were selected to create new synthetic population after crossing in an isolated field and multi-locations forage yield trial.
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