South African lovegrass (Eragrostis plana Nees) is the most important weed of native pastures in southern Brazil. Management options are limited under water-stress conditions, and glyphosate has been the main tool for control. This study compared four salts of glyphosate applied at three growth stages and determined the glyphosate tolerance level. In addition, the performance of ammonium sulfate (AMS) under two soil moisture conditions (50% and 100% of water-holding capacity) and the effect of AMS on absorption and translocation of radiolabeled [¹⁴C]glyphosate were evaluated. The potassium salt of glyphosate had the fastest activity across growth stages of E. plana, which is more vulnerable to glyphosate at the panicle initiation stage. Isopropylamine salt was the slowest-acting glyphosate formulation. Younger plants were typically more easily controlled than older plants at the full tillering stage. The addition of AMS increased the level of control of drought-stressed E. plana compared with glyphosate alone by increasing translocation out of the treated leaf and consequently increasing the concentration of glyphosate in the primary culm. These data can be used to plan an effective management program for E. plana that takes into account the developmental stage of desired pasture grass species.