Winter wheat cultivars were studied under moderate drought stress to see how root growth and grain yield were affected. Three cultivars were tested: an old landrace, a modern cultivar, and a newly released one. The new cultivar had the highest grain yield when well-watered or irrigated, but similar to the modern cultivar under drought conditions. The old cultivar had the most topsoil roots but the least subsoil roots, while the new cultivar had the opposite. More subsoil roots after flowering led to higher grain yield in drought conditions. The new cultivar used less water early on but more after flowering, leading to increased grain weight. In summary, having more roots in deeper soil layers helps winter wheat produce more grain when water is limited.