Evaluation location: , United States
The parents of the population, BTx623_ is a combine type elite line that germinates poorly during early cool season of the year and Niu Sheng Zui (PI 568016) is a Chinese landrace that exhibit >70% germination under cool conditions in temperate regions of the US sorghum belt (USDA hardiness zones 4b to 6b). The BTx623NSZ_Recombinant Inbred Mapping Population (RIMP) was developed by hand emasculation of the female parent BTx623 and pollination with pollen from. The resulting F1 hybrid was intermediate in height, with brown seed color. A total of 300 F2 plants were planted, but only 292 lines were advanced single seed descent breeding technique in Lubbock, Texas from F3 to F4 generations. From F5 to F7, seeds were produced alternately between Lubbock, Texas and Puerto Rico. At the F6:7 stage of development ten representative uniform plants were tagged and seeds were bulked from the 10 tagged plants to compose each line. A total of 292 RILs were generated and are used to represent the BTx623NSZ_RIMP.
In spring of 2015 through 2017, the BTx623NSZ_RIMP including the two parents and 8 commercial checks were planted in replicated plots in three locations representing the US sorghum belt temperate region, USDA zones 4b to 6b between April 1 and 18 (differing due to latitudes of location and precipitation) to evaluate variation for early season field traits for cold tolerance based on field emergence and seedling vigor, biomass. The locations used for field testing were; Lubbock, Texas (33.6°N -101.88°W, 2381 ft. elevation and Manhattan, Kansas (39.21°N -96.51°W, 1053 ft. elevation). The mean ambient and soil temperatures for all three locations at the time of planting were: Lubbock, TX- 18.3; 17.7 °C; Manhattan, KS- 12.2; 14.4°C; Wall, SD- 12.6; 12.5°C. Each entry was planted in 10 m long plots with 100 cm spacing between plots and equal number of seeds were planted per plot. Field emergence was evaluated on a weekly basis and at 30 days after planting as % field emergence. Seedling vigor was rated on a 1-5 scale, with rating of 1 as robust vigor and 5 as poor vigor. Subsequently, five seedlings in the inner section of each plot were harvested for above ground biomass to determine dry weight.