Turning the heat up on independent culling in crop breeding

Most grain crops are sensitive to heat stress during anthesis which causes substantial reductions in grain yield, and heat stress tolerance (HST) is therefore an important trait for selection in crop breeding programmes during the 21st century. We stochastically modelled breeding for flowering time, disease resistance, stem strength, and grain yield in a self-pollinating grain crop over the next 60 years, assuming 3-year selection cycles and 1,000 progeny per cycle, with or without priority selection for HST, and with moderate or high selection intensity (10% or 4% selection proportion). HST is measured at 30 °C during anthesis (HST30), and is assumed to be moderately heritable (h2 = 0.3). Genetic progress in a traditional crop breeding programme with independent culling on phenotypic values of individual traits was compared to progress under index selection and optimal contributions selection (OCS) on a BLUP-based economic index. In all three breeding strategies, near-homozygous lines were formed by rapid single seed descent and selection occurred on S5-derived lines. Priority was given to selection on HST30 to match rising ambient temperatures of +4 °C during the experiment. At 60 years, all breeding strategies achieved the HST30 target of +4 units, but economic index was lowest in the traditional breeding programme (2.27-fold), intermediate in index selection (2.57-fold) and
highest in OCS (2.81-fold) under moderate selection intensity. Grain yield rose from 1.50 to 3.38 t ha- 1 in OCS compared to 2.88 t ha- 1 in the traditional strategy. Without selection for HST30, grain yields under all scenarios reached a maximum of 2.30 t ha-1 and began falling around 2060, despite continued investment in breeding for yield. Independent culling on phenotype was the least effective strategy to breed for HST and grain yield during 60 years of global warming.