Effective Preemergence Herbicides for Rigid Ryegrass (Lolium rigidum Gaud.) Control in Irrigated Bread Wheat (Triticum aestivum L.)

Three on-farm weed control experiments were conducted in irrigated bread wheat in the Doukkala perimeter, Morocco, in 2015-16 and 2016-17 in order to study the efficacy of 4 pre-emergence herbicide treatments for controlling rigid ryegrass that is resistant to 13 post-emergence herbicides. Results showed that 3 pre-emergence herbicides [i) chlorotoluron, 2000 g/ha + isoxaben, 74.8 g/ha; ii) prosulfocarb, 4000 g/ha; iii) prosulfocarb, 2000 g/ha + s-metolachlor, 300 g/ha] reduced rigid ryegrass shoot biomass by > 90% 1 to 3 months after treatments (MAT). Pendimethalin (1320 g ha-1) achieved 83-99% rigid ryegrass control 1 to 3 MAT. The four herbicide treatments were safe on wheat in one experiment, but reduced wheat density in 2 other experiments due to heavy rain (about 100 mm) after herbicide treatments and before crop emergence. Grain yields in sprayed plots ranged from 6.6 to 9.8 t ha-1, 4.4 to 7.4 t ha-1, 7.3 to 8.9 t ha-1 in experiments 1 to 3, respectively. Straw yields were 11.4 to 15.4, 9.6 to 15.8, and 10.1 to 14.5 t ha-1 in the 3 experiments, respectively. These preemergence herbicides need to be used by wheat growers as part of an integrated weed management program. Further research is needed to explore ways to avoid wheat injury, that could be occasionally caused by heavy rain or irrigation, after preemergence herbicide application and before crop emergence.