Yield and composition of lucerne stands in Central Otago after different winter grazing and weed control treatments

The effect of grazing date and time of spraying with a glyphosate/atrazine herbicide combination on a three year-old lucerne stand was studied on-farm in Central Otago. Total annual dry matter (DM) yield was highest in the weedy unsprayed control (14.8 t DM/ha) and lowest in the crop sprayed on 18 September (10.1 t DM/ ha). However, lucerne DM yield was highest from the 3 July and 22 August spray treatments (11.4 ± 0.39 t DM/ ha) and lowest in the unsprayed control at 7.7 t DM/ha. Phytotoxicity symptoms of glyphosate on the lucerne tended to be limited to crops sprayed on 18 September. To maximise yield, a winter clean-up grazing (June) followed by a winter herbicide application (July/August) when lucerne was approximately 3 cm high and with <100 kg DM/ha was required. After this, grazing earlier than October should be avoided as a September grazing resulted in a lucerne yield of about 1.0 t DM/ha less than grazing in October and November. Spring lucerne production was delayed by spraying in September. None of the spray treatments killed the crop, but they all reduced weed content from about 40% to