Rht-B1p as an agronomically equivalent alternative to Rht-B1b in a spring bread wheat population

The dwarfing allele Rht-B1p has been characterised only against the wild-type Rht-B1a . Breeders need the contrast with Rht-B1b , the allele of most semi-dwarf wheats. We tested the two for agronomic equivalence within one biparental spring wheat population segregating for Rht-B1p , Rht-B1b and Ppd-D1 (3,147 plants, 119 lines, two seasons), with margins of ±5 cm for height, ±2 days for phenology, and 5% and 10% of the trait mean, using linear mixed models on plot means. No difference was demonstrated for any of the 14 traits. After Holm adjustment, the two haplotypes were equivalent at the 5% margin for plant height and internode number, and at the 10% margin for six traits, and spike density, which was treated as exploratory. The three phenological traits were inconclusive. Rht-B1p reduces height by 21–25 cm relative to the wild type. Between the haplotypes, the difference was +0.18 cm (90% CI −1.55 to +1.91). The design was sensitive: within the Rht-B1p class, Ppd-D1b delayed heading by 4.74 and flowering by 4.15 days. No Rht-B1 × Ppd-D1 interaction was found. In the F3 families of 2018, the Rht-B1p class was 6.0 cm shorter; an allelic difference may appear in some backgrounds. In this population, at one location over two seasons, the Rht-B1p haplotype was agronomically equivalent to the Rht-B1b haplotype in plant height. Plot yield, tillering, lodging and establishment remain to be tested.