To select new check rice varieties for evaluating cold tolerance at the booting stage higher than ‘Very-tolerant’, precisely, cold tolerance was investigated using a deep-water irrigation system at 7 breeding stations for 5 years from 2004 to 2008.
Judging from the percentage sterility, new check varieties from pre-existing ‘Very-tolerant’ to ‘Very-tolerant 9’ in the very-early heading group (‘Yumekogane’ class) and from ‘Very-tolerant’ to ‘Very-tolerant 11’ in the very-early (‘Kakehashi’ class), early, early-to-moderate and moderate heading groups were selected. Using the new check varieties, new breeding lines with high cold tolerance and good agronomic traits were developed.