Pak. J. Bot., 37(3): 661-665, 2005. EFFECT OF SOIL INCORPORATED HERBICIDES ON WEEDS AND YIELD OF CANOLA (BRASSICA NAPUS L.) A. TANVEER, M. AYUB, A. ALI AND M.A. NADEEM Department of Agronomy, University of Agriculture, Faisalabad, Pakistan Abstract The effect of two soil incorporated herbicides viz., trifluralin @ 0.90,1.20,1.50 kg a.i. ha -1 and acetochlor @ 0.094, 0.124 and 0.312 kg a.i. ha -1 on weeds and yield of canola was evaluated in a field trial at Agronomic Research Area, University of Agriculture, Faisalabad. Trifluralin @ 1.50 kg a.i. ha -1 was very effective in controlling the weeds and reducing their fresh and dry weight. Application of trifluralin @ 1.5 kg a.i. ha -1 reduced the weed density (7.00m -2 ) significantly as compared to control (32.67m -2 ) reducing the weed dry weight from 8.1 gm -2 to 1.7 gm -2 . Trifluralin @ 1.5 kg a.i. ha -1 showed maximum increase (34%) in canola seed yield by increasing the number of pods per plant, seeds per pod and 1000-seed weight. Introduction National average seed yield of canola (1236 kg ha -1 ) is far below than its potential yield of 3689 kg ha -1 (Anony., 2001). Presence of weeds in canola fields is one of the main reasons of its low yield. Weeds affect growth and development of crop plants by competing with crop plants for water, light and nutrients which ultimately leads to low yield. Weed control by manual hoeing is not feasible and has become expensive due to non-availability of labour. Chemical weed control is the best alternative being less laborious, time saving and more effective. According to Munir et al., (1987) fluazilop-butyl (Fusilade) used @ 0.5 kg a.i. ha -1 significantly reduced the weed density and dry weight of Cynodon dactylon and economically appeared to be the best dose. Flusilade @ 0.5 kg a.i. ha -1 also gave significantly higher seed yield of mustard cultivar “Peela Raya” than 0.25 kg a.i. ha -1 . Blackshaw (1989) achieved best control of Brassica kaber + Sinapis arvensis + Thlaspi arvense with 20-30 g ha -1 , Descurainia sophia with 15 g ha -1 and Amaranthus retroflexus with 30 g ha -1 of DPXA 7881 (methyl 2-[(4-ethoxy-6-methylamino-1,3,5-triazin-2-yl) carbamoylsulfanoyl] benzoate) at the 2-leaf stage. Control of weeds increased canola yields without altering oil content and 1000-seed weight. Toll (1989) stated that Butisan S (metazachlor) @ 2 L ha -1 applied at the cotyledon stage of spring rape gave 78-87% control of Atriplex spp, Stellaria media and Matricaria inodora and increased the seed yield by 240 kg ha -1 on clay soil. The same herbicide @ 2.5 L ha -1 gave 58-76% kill of Polygonum tomentosum, Galeopsis spp., and Atriplex spp., and increased seed yield to the extent of 290 kg ha -1 . Weed control also increased oil content. Increase in seed yield of winter rape with different herbicides was 10.2% (Wahmhoff, 1990). Anderson & Bengtsson (1992) reported 6% increase in seed yield of winter oilseed rape in both the narrow (12cm) and hoed wide row (48cm) spacing with Lasso (alachlor) @ 5 L ha -1 , Teridox-500 EC (dimethachlor) @ 4 or 5 L ha -1 by reducing weed number and weight. According to Rana & Angiras (1992), pendimethalin was best