In the greenhouse, soil applications of primisulfuron (40 g ai ha–1) reduced growth of emerged rhizome johnsongrass plants more than nicosulfuron (40 g ai ha–1). Both herbicides reduced growth more when applied to foliage only; a further decrease in growth did not occur for applications to both soil and foliage. Primisulfuron did not completely prevent regrowth of johnsongrass with any application method. Nicosulfuron prevented regrowth when applied to the foliage and to both soil and foliage. In single-year field studies in corn at four dryland sites and two irrigated sites, 50:50 split applications of primisulfuron (40 g ai ha–1) and nicosulfuron (35 g ai ha–1) approximately 2 wk apart provided the most consistent rhizome johnsongrass control compared with early or late single applications when visually rated 8 wk after the first application. Nicosulfuron treatments were more effective than primisulfuron treatments at both dryland sites the first year and at one of two dryland sites the second year. Primisulfuron and nicosulfuron at the irrigated site the first year were equivalent in efficacy. Nicosulfuron was more effective than primisulfuron at the irrigated site the second year. Primisulfuron or nicosulfuron treatments more than doubled corn yields at the dryland sites both years regardless of application timing. Split applications of primisulfuron or nicosulfuron provided the highest yields (approximately 80% increase over untreated plots) at the irrigated site the first year. All treatments provided equivalent yield increases (approximately 50%) the second year.