Seasonal Performance of White Clover in Mixed-Sward Grazing Pasture Highlights Genotype by Environment Interaction

White clover is an important forage crop because of its nutritional value, ability to provide plantavailable nitrogen via symbiosis with Rhizobium soil bacteria, and year-round availability of dry matter (DM) yield. However, its performance in mixed sward-based pastures is characterised by seasonal variability and declining DM yield over time. The identification of white clover genotypes adapted for across seasonal performance is an important goal in white clover breeding. In this study, we evaluated the seasonal performance of 200 white clover half-sib families using visual growth scores and calibrated dry matter yield based on growth scores measured for three years in two locations. Results showed significant variation for growth scores across years, seasons and locations. Significant G×E was observed in the form of year, location and season interactions. Calibrated DM yield was highest in the second-year summer with clover content declining in the third year. Spring and winter were identified as potential vulnerable periods for white clover growth in pastures

Data_Sheet_1_Quantitative genetic analysis reveals potential to breed for improved white clover growth in symbiosis with nitrogen-fixing Rhizobium bacteria.docx

White clover (Trifolium repens) is integral to mixed pastures in New Zealand and temperate agriculture globally. It provides quality feed and a sustainable source of plant-available nitrogen (N) via N-fixation through symbiosis with soil-dwelling Rhizobium bacteria. Improvement of N-fixation in white clover is a route to enhancing sustainability of temperate pasture production. Focussing on seedling growth critical for crop establishment and performance, a population of 120 half-sibling white clover families was assessed with either N-supplementation or N-fixation via inoculation with a commercial Rhizobium strain (TA1). Quantitative genetic analysis identified significant (p < 0.05) family additive genetic variance for Shoot and Root Dry Matter (DM) and Symbiotic Potential (SP), and Root to Shoot ratio. Estimated narrow-sense heritabilities for above-ground symbiotic traits were moderate (0.24–0.33), and the strong (r ≥ 0.97) genetic correlation between Shoot and Root DM indicated strong pleiotropy or close linkage. The moderate (r = 0.47) phenotypic correlation between Shoot DM under symbiosis vs. under N-supplementation suggested plant growth with mineral-N was not a strong predictor of symbiotic performance. At 5% among-family selection pressure, predicted genetic gains per selection cycle of 19 and 17% for symbiotic traits Shoot DM and Shoot SP, respectively, highlighted opportunities for improved early seedling establishment and growth under symbiosis. Single and multi-trait selection methods, including a Smith-Hazel index focussing on an ideotype of high Shoot DM and Shoot SP, showed commonality of top-ranked families among traits. This study provides a platform for proof-of-concept crosses to breed for enhanced seedling growth under Rhizobium symbiosis and is informative for other legume crops.</p