Irrigation was applied at different rates and frequencies during five consecutive periods of vegetative growth of the forage turnip Brassica rapa var. rapa cv. Barkant, grown in the field in north-west Tasmania, Australia, during the spring and summer of 1999–2000 (Season 1) and 2000–01 (Season 2). Irrigation applied before root expansion did not increase the dry matter (DM) of turnips (leaf plus root) in either season. At the following four harvests in each season, DM increased linearly in proportion to the cumulative amount of irrigation applied before the harvests. Irrigation water use efficiency, as measured by the slopes of the linear regressions, ranged from 5.7 to 17.2 kg DM ha^–1 mm^–1 in Season 1 and from 19.2 to 26.0 kg DM ha^–1 mm^–1 in Season 2. The effective use of water (EUW; yield increase/evapotranspiration within a period) was calculated for each of the five periods in Season 2 to identify the vegetative growth periods when the response ( kg DM ha^–1 mm^–1) was greatest and limited irrigation water could be applied most effectively. EUW of irrigated turnip increased from 16.8 kg DM ha^–1 mm^–1 at the onset of root expansion to 53.5 kg DM ha^–1 mm^–1 when root growth rate was a maximum, but declined thereafter. Scarce irrigation should be applied between the onset of root expansion and approximately 8 weeks later, when the response to irrigation ( kg DM ha^–1 mm^–1) was greatest.