<p>Grain number primarily determines grain yield under optimal conditions in bread wheat, influenced by the partitioning of above-ground dry matter to the spike during stem elongation and floret survival. Floret survival depends on the supply of assimilates to the spike during the rapid spike growth phase from early booting to anthesis. Therefore, understanding the mechanisms underlying spike dry matter partitioning and the efficiency with which spike biomass is converted into grains and their genetic basis in high biomass backgrounds is critical. The present study evaluated a panel of 150 genetically diverse elite spring wheat lines over two field cycles for grain-partitioning traits at anthesis + 7&#xa0;days (GS65 + 7d), grain yield and harvest index (grain dry matter (DM) / above-ground DM). A genome-wide analysis identified 37 MTAs (Marker Trait Associations) linked to grains m⁻², grain yield, fruiting efficiency (grains per g spike DM at GS65 + 7d), spike partitioning index (spike DM / above-ground DM at GS65 + 7d), spike DM per unit area, stem-internode lengths, and plant organ partitioning, explaining 6–14% of phenotypic variation. Key MTAs were located on chromosomes 1A, 2B, 6B and 7A, with candidate genes linked to assimilate transport (<i>AKT2</i>), stem elongation (<i>ABCC15</i>) and reproductive development (<i>PAP24</i>, <i>NAP1;1</i>). Our findings showed early reproductive traits like spike DM partitioning and stem-internode lengths determined grain sink strength and grains m⁻² in high-biomass wheat. Building on previous research focusing on grains m⁻² and harvest index at harvest, these findings identify MTAs specifically linked to partitioning traits at anthesis + 7d underlying grain number, providing an extended understanding of the genetic basis of grains m⁻², harvest index and grain yield. The new MTAs related to grain partitioning traits at anthesis + 7d may help breeders combine alleles to improve the harvest index in high-biomass germplasm.</p>