The influence of three years of zero tillage on weed populations and crop response was studied at three locations in Southern Manitoba, The crops grown were wheat (Triticum aestivum L., cv. Manitou), barley (Hordeum vulgare L., cv. Conquest and cv. Paragon), flax (Linum usitatissimum L., cv. Noralta), and rapeseed (Brassica napus L. var. annua Koch cv. Target and cv. Turret). The crops were sown using a triple disc drill system. The weed growth at the time of seeding was controlled with 1,1'-dimethyl-4,4'-bipyridinium ion (paraquat) plus 6,7- dihydrodipyrido (1,2-a:2',1'-c) pyrazinediium ion (diquat) at 0.84 + 0.28 kg/ha, with paraquat plus (2,4-dichlorophenoxy) acetic acid (2,4-D) ester at 0.84 + 1.12 kg/ha, and with paraquat plus 3,5-dibromo-4-hydroxybenzonitrile (bronoxynil) and ((4-chloro-o-tolyl) oxy) acetic acid (MCPA) at 0.28 + 0.56 kg/ha. Annual weed populations were lower under zero tillage than under conventional tillage. The species which were reduced in number were Setaria viridis (L.) Beauv. (green foxtail),Avena fatua, L. (wild oats), Polygonum scabrum Moench (green smartweed), and Polygonum convolvulus L. (wild buckwheat). The perennial weeds, Agropyron repens (L.) Beauv. (quack grass) and Cirsium arvense (L.) Scop. (Canada thistle), presented problems under zero tillage. Other herbicide treatments which resulted in good weed control were paraquat at 0.28 kg/ha applied in a spray volume of 69 l/ha, 2-sec-butyl-4,6-dinitrophenol (dinoseb) at 4.48 and 8.97 kg/ha, and N-(phosphonomethyl) glycine (glyphosate) at 0.56 kg/ha. Paraquat at 0.28 kg/ha was more effective when applied in 69 l/ha spray volume than when applied in 138, 183, or 243 l/ha spray volume, and was more effective when applied in the evening than when applied in the morning. Dinoseb resulted in excellent weed control at the time of spraying and gave residual control of broad-leaved weeds, green foxtail, and wild oats, Wheat (cv. Neepawa), barley (cv. Conquest), and Avena sativa L. cv. Harmon (oats ) were tolerant to dinoseb at 8.97 kg/ha. Glyphosate at 0.56 kg/ha resulted in weed control equivalent to control from paraquat plus diquat at 0.84 + 0.28 kg/ha. More crop plants emerged under zero...