Carrier water quality is an essential consideration for enhancing herbicide performance. Water hardness can negatively affect some herbicides. Two separate dose-response experiments were conducted to investigate the effect of carrier water hardness and ammonium sulfate (AMS) and urea ammonium nitrate (UAN) as adjuvants on the performance of 2,4-D plus MCPA and mesosulfuron plus iodosulfuron plus mefenpyr-diethyl. The experimental factors included herbicide rates at five levels (63.3, 126.6, 253.1, 506.3 and 1012.5 g a. i. ha−1 for 2,4-D plus MCPA and 1.125, 2.25, 4.5, 9 and 18 g a. i. ha−1 for mesosulfuron plus iodosulfuron plus mefenpyr-diethyl), carrier water hardness based on concentrations of CaCO3 at five levels (0, 250, 500, 750, and 1000 mg L−1), and tank-mix of ammonium sulfate (AMS) and urea ammonium nitrate (UAN) as adjuvants at three levels (0, 1%, and 2% [w/v]). The results indicated that increased carrier water hardness up to 500 mg L−1 did not affect the performance of 2,4-D plus MCPA for wild mustard control. However, water hardness higher than 500 mg L−1 led to a reduction in the herbicide performance. The application of AMS and UAN was effective on the performance of 2,4-D plus MCPA herbicide in controlling wild mustard and overcame the negative effects of carrier water hardness. The performance of mesosulfuron plus iodosulfuron plus mefenpyr-diethyl herbicide in controlling wild mustard was not affected by carrier water hardness. In the case of herbicides such as mesosulfuron plus iodosulfuron plus mefenpyr-diethyl, which are not sensitive to carrier water hardness, the use of 2% AMS or UAN may also increase herbicide performance.

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