Eric Koetz

Incremental crop tolerance to weeds: A measure for selecting competitive ability in Australian wheats

Total reliance on herbicides for weed control is unsustainable with the spread of herbicide resistance and the environmental need to reduce pesticide use. Strongly competitive wheat crops that have high tolerance to weed pressure and therefore maintain high yields in the presence of weeds are a low-cost option for reducing dependence on herbicides. We examined the feasibility of selecting for wheat tolerance to weeds by crossing varieties differing for traits associated with competitiveness. Competitive ability and yield potential must be treated as separate traits for selection. Current measures of crop tolerance to weed competition do not separate the two traits so that selection based on these measures is often synonymous with selection for yield potential rather than pure tolerance. We propose a new measure, termed Incremental Crop Tolerance (ICT) that reflects the incremental yield difference between genotypes associated with tolerance, over and above differences in underlying yield potential.

Competitive ability of Australian canola (Brassica napus) genotypes for weed management

Abstract. Canola (Brassica napus L.) is an important break crop in Australian cropping systems but weeds are a major cost to production and herbicide-resistant weeds are spreading. The potential competitive ability of canola genotypes to both suppress weed growth and maintain grain yield and quality in the presence of weeds has not been determined in Australia. Two experiments examined the range in competitive ability of 16 B. napus genotypes against annual ryegrass (Lolium rigidum Gaud.) and volunteer wheat (Triticum aestivum L.) over two contrasting seasons. Weed biomass at flowering was generally reduced 50% more in the presence of the strongly competitive genotypes than the least competitive, and this has significant benefits for lower weed seed production and reduced seedbank replenishment. Suppression of weed growth was negatively correlated with crop biomass. Significant differences in grain yield of canola were recorded between weedy and weed-free plots, depending on crop genotype, presence of weeds and season. Crop yield tolerance (where 0% = no tolerance and 100% = complete tolerance) to ryegrass competition ranged from 0% (e.g. with CB-Argyle) to 30–40% (e.g. with the hybrids 46Y78 and Hyola-50) in the dry season of 2009. Yield tolerance was higher (50–100%) with the lower densities of volunteer wheat and in the 2010 season. The range between genotypes was similar for both conditions. The hybrids and AV-Garnet were higher yielding and more competitive than the triazine-tolerant cultivars. The ranking of genotypes for competitiveness was strongly influenced by seasonal conditions; some genotypes were consistently more competitive than others. Competitive crops are a low-cost tactic for integrated weed management to reduce dependence on herbicides and retard the spread of herbicide-resistant weeds.

Herbicide efficacy for control of annual ryegrass (Lolium rigidum Gaud.) is influenced more by wheat seeding rate than row spacing

Abstract. Conservation cropping systems with no-till and stubble retention improve soil condition and water conservation. However, tillage is replaced by herbicides for weed control in these systems, increasing the threat of herbicide resistance. In the medium to high rainfall zones of the southern wheatbelt of Australia and under irrigation, wider row spacing is used to enable seeding into heavy stubble loads and to avoid stubble burning. Some evidence suggests that wider rows lead to reduced crop competitive ability and crop yields, greater herbicide dependence, and increased spread of resistance. Our aim was to test the hypothesis that increasing seeding rate compensated for reduced competitive ability at wider row spacings, especially when herbicide performance was suboptimal. We examined the impact of two wheat row spacings (18 and 36 cm) and five seeding rates (resulting in a range of densities of ∼80–700 plants/m2) on control of Lolium rigidum with five rates of post-emergence application of diclofop-methyl (Hoegrass®), ranging from label rate to lower rates, over two growing seasons. In the presence of L. rigidum, wheat grain yield was unaffected by row spacing but was significantly reduced at low seeding rates, especially at lower herbicide rates. Lolium rigidum was suppressed at higher crop densities but was also unaffected by row spacing. Grain yield was maximised when post-emergence herbicide was applied at 60–100% of the recommended dose at wheat densities >∼300 plants/m2. Significant levels of the weed remained in the crop at anthesis in all treatments. Weed dry matter ranged from 525 g/m2 at low crop densities and with no herbicide to 150 g/m2 with the recommended rate of herbicide and high wheat densities. The implications of manipulating crop competitive ability to improve weed control are discussed, especially in conditions where herbicide performance is unreliable due to weeds developing herbicide resistance or adverse environmental conditions.

Seeding rate and cultivar effects on canola (Brassica napus) competition with volunteer wheat (Triticum aestivum)

Abstract. Canola (Brassica napus L.) is an important rotational crop in the temperate cropping zone of southern Australia. Herbicide-resistant weeds are rapidly spreading and reducing canola grain yield and quality. Crop competition is a useful tool for reducing weed costs and dependence on herbicides, and retarding the spread of herbicide resistance. The potential interaction of canola seeding rate and cultivar for weed management has not been quantified in Australia. A field experiment was conducted in three environments to examine the impact of two contrasting canola cultivars (a low vigour type and a high vigour hybrid) at four seeding rates (10–100 plants/m2) on volunteer wheat (∼50 plants/m2). Significant but variable effects of crop seeding rate, cultivar and weed were detected on canola density and grain yield, and on the suppression of volunteer wheat. The canola hybrids suppressed volunteer wheat more than the less vigorous cultivars in all the experiments. There was no benefit of increasing canola seeding rate above the normally recommended rate of 40 plants/m2 for weed suppression. The seed production of volunteer wheat on average doubled when canola density dropped from 40 to 10 plants/m2. Treatment effects on canola grain yield losses from weeds were less than those on weed suppression. The grain yield of both cultivars was reduced between 30% and 40% with weeds at a canola density of 40 plants/m2 and plateaued above this density in weedy conditions. Maintaining canola plant establishment and using competitive cultivars is critical to avoiding weed seedbank replenishment, and reducing canola yield losses from weed competition.