A study of drought stress was carried out on three guinea grass (Panicum maximum Jacq.) populations having three different levels of genetic variability; the most variable reproduces sexually. Three preconditioning treatments were used. In the wet treatment, plants were watered daily; in the medium, when soil water potential (ψ_s) reached about —1.0 bar (18 drying cycles); and in the dry when ψ_s reached about — 15 bars (eight drying cycles). For all three populations, drought‐preconditioned plants had significantly lower leaf water potentials (ψ_L) and relative water contents (RWC) than daily‐watered plants when turgor pressure reached zero during a drying cycle. Additionally, ψ_L values for stomatal closure (threshold ψ_L) were lower for the dry and medium treatments than for the wet one. Adaptability to drought was measured in terms of stomatal closure and forage production under stress conditions. Populations with intermediate to high levels of genetic variability performed best in relation to drought adaptability and forage production. Threshold leaf water potential appeared to be useful in selecting new germplasms for drought resistance. The variable populations might therefore allow improvement of forage production by breeding under water stess conditions.