The effect of chemical weed control and weed competition on the growth and development of chickpeas (Cicer arietinum L.): A dissertation submitted in partial fulfilment of the requirements for the degree Bachelor of Agricultural Science with Honours

Abstract

Kabuli chickpeas ( Cicer arietinum L.) were sown on 10 December 1993 and subjected to a range of treatments for weed control. There were seven pre-emergence treatments, ten post-emergence treatments and three controls, unweeded, hand weeded and hand hoed at 42 days after sowing. The herbicides tested were bentazone, clethodim, cyanazine, haloxyfop, MCPB, metribuzin and terbuthylazine. These were applied at the recommended dose rate, twice the recommended dose rate and/or in combination at the recommended rate. Weed population and dry matter production was suppressed by pre-emergence application of cyanazine, metribuzin, terbuthylazine and cyanazine plus metribuzin in combination. These herbicides had no effect on chickpea population, biomass accumulation or seed yield. Post-emergence applications of the broadleaf herbicides, cyanazine, bentazone and combinations containing either of these two herbicides caused severe phytotoxicity and reduced seed yield. In addition they provided poor suppression of weeds. The grass herbicides, clethodim and haloxyfop provided poor suppression of broadleaf weeds. As a result, chickpea biomass accumulation and seed yield was reduced, but mean seed weight was increased. Hoeing at 42 days after sowing provided adequate weed control initially but not during the later stages of crop growth. The highest seed yield of 375 g/m² was obtain from plants sprayed with cyanazine at 1.0 kg a.i./ha pre-emergence. The presence of weeds reduced seed yield from 360 g/m² to 249 g/m². Chickpea nodulation was affected by the presence of weeds and the control measure employed. The hand weeded control produced an average of 4.44 nodules with a nodule dry weight of 1.03 g/plant. Average nodule dry weight was reduced to 0.52 g/plant in the weedy control. All post-emergence herbicide treatments, metribuzin and terbuthylazine at 0.75 kg a.i./ha had a marked effect on mean nodule dry weight. Nodules on treated plants ranged from 1.48 nodules/plant with at 0.50 kg a.i./ha to 4.02 nodules/plant with cyanazine at 1.0 kg a.i./ha pre-emergent. The effect of chemicals on nodule mass was severe. All treatments except terbuthylazine at 1.0 kg a.i./ha, clethodim at 0.12 kg a.i./ha and hoeing reduced the number of nodules/plant. Mean nodule dry weight ranged from 0.06 g/plant with bentazone plus MCPB to 0. 73 g/plant with clethodim at 0.12 kg a.i./ha. Positive yield gains were obtained from plots treated with all pre-emergence herbicides, clethodim at 0.12 kg a.i./ha, haloxyfop at both rates and from plots that had been kept weed free and hoed at 42 days after sowing. The remaining treatments all gave yield losses. The maximum economic return of $5622/ha was obtained from pre-emergence application of cyanazine at 1.0 kg a.i./ha. Conversely the lowest return of - $173/ha was obtained from post-emergence application of cyanazine at 2.0 kg a.i./ha. The economic return from hand weeding was $1750/ha and from hoeing at 42 days after sowing was $4479/ha. The economic return from no weed control was $3448/ha. The breakeven yield loss for all herbicide treatments ranged from 0.56% to 2.38%. The breakeven yield loss for hand weeding was 57.35% and for hoeing was 11.95%.