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Weed management using crop competition in Australia
Crop Protection xxx (2016) 1e6
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Weed management using crop competition in Australia Ali Ahsan Bajwa a, b, *, Michael Walsh c, Bhagirath Singh Chauhan b a
School of Agriculture and Food Sciences, The University of Queensland, Gatton, Queensland 4343, Australia The Centre for Plant Science, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, Gatton/Toowoomba, Queensland 4343/4350, Australia c The University of Sydney, Narrabri, NSW 2390, Australia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 April 2016 Received in revised form 17 August 2016 Accepted 21 August 2016 Available online xxx
Adoption of conservation agriculture has brought significant changes in crop management in Australia. Increased reliance on herbicidal weed control is one of the most significant changes that have occurred throughout Australian cropping systems. The singular focus on herbicides for weed management has led to the frequent and widespread evolution of herbicide resistance in several weed species. Herbicide resistance means the loss of herbicide resources and as new herbicides modes of action are unlikely then use of alternative non-chemical cultural weed management options is essential. Crop competition is an approach that can be used to manage weeds for improved crop production. Enhanced crop competition can be achieved through the use of competitive crop species and cultivars, increased seed rates, narrow row spacing, and altered row orientation. These options are already routinely used in Australia and are proven in their ability to reduce weed biomass and fecundity. Although these strategies have been successfully used in Australian cropping systems, the research has frequently been focussed locally and not extrapolated more broadly throughout the Australian grain production region. Crop competition can potentially be a sustainable weed management option in reducing the reliance on herbicides and combating herbicide resistance and therefore, needs to be considered in all integrated weed management plans. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Conservation agriculture Competitive cultivars Crop competition Integrated weed management Row orientation Row spacing
1. Introduction Conservation agriculture has been adopted at a large scale in Australia during the past few decades (Chauhan et al., 2006; Llewellyn et al., 2012). The development of conservation cropping based on stubble retention and reduced tillage has transformed crop production in Australia due to marked improvements in crop available soil moisture and nutrient levels as well as soil structure providing substantial gains in crop productivity (Llewellyn et al., 2004; Bajwa et al., 2015). However, despite these productivity gains, weeds remain a major constraint to productivity in conservation agriculture (Chauhan et al., 2012; Bajwa, 2014). The availability of highly effective herbicides has facilitated the adoption of conservation cropping systems but the subsequent reliance on these herbicides has created the significant problem of herbicide resistance (Walsh and Powles, 2007). Continuous use of herbicides
* Corresponding author. School of Agriculture and Food Sciences, The University of Queensland, Gatton, Queensland 4343, Australia. E-mail address: [email protected] (A.A. Bajwa).
with similar modes of action has caused the evolution of herbicideresistant weed biotypes that now infest crops throughout Australia's production regions (Boutsalis et al., 2012; Broster et al., 2013; Bajwa et al., 2015; Owen et al., 2015). It is estimated that herbicide resistance has, on average, increased the cost of weed control by $55 ha 1 (Llewellyn et al., 2016). Thus, the inclusion of alternative non-chemical management options in weed management programs is essential for sustainable crop production in Australia. The use of crop competition to control weeds is one of the most effective cultural strategies in Australian cropping, where suppression of weed biomass and fecundity leads to crop yield gains (Lemerle et al., 1996, 2004; Scott et al., 2013). Crop competition is an effective approach to weed management that is applicable in large scale conservation production systems (Brennan et al., 2001). The competitiveness of a crop can be enhanced through the use of competitive cultivars, higher plant densities, narrow row spacing, and different row orientation (Lemerle et al., 1996, 2014; Walker et al., 2002; Pathan et al., 2006; Hashem et al., 2010; Borger et al., 2015). Significant progress has been made in Australian cropping
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systems in developing highly competitive and high yielding genotypes of several crops. The traits contributing to competitive ability against weeds include elevated plant height, higher dry matter accumulation, early canopy closure, enhanced leaf area leading to more light interception and shading over understory vegetation, increased nutrient uptake, proliferate root growth, and allelopathic effects (Pavlychenko and Harrington, 1934; Konesky et al., 1989; Balyan et al., 1991; Cousens et al., 1991; Cudney et al., 1991). Competitive cultivars of important crops including, wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), oats (Avena sativa L.), rye (Secale cereale L.), chickpea (Cicer arietinum L.), fababean (Vicia faba L.), lupin (Lupinus angustifolius L.), field pea (Pisum sativum L.), and canola (Brassica napus L.) have been successfully used for weed management in Australia (Lemerle et al., 2014; Gill and Holmes, 1997; Paynter and Hills, 2009; Hashem et al., 2010; Asaduzzaman et al., 2014). Such cultivars not only provided better control of major weed species including, Lolium multiflorum Lam., Lolium rigidum Gaud., and Bromus diandrus Roth. but also yielded more than non-competitive cultivars (Gill et al., 1987; Lemerle et al., 1995, 2001a; Felton et al., 2004). The use of high crop seeding rates to increase crop planting density has consistently been proven to enhance the competitive ability of many winter cereals against weeds. For instance, a twofold increase in the standard seeding rate (55 kg ha 1) of 10 Australian wheat cultivars caused a 43% reduction in L. rigidum biomass (Lemerle et al., 1996). Similarly, an increase in barley seeding rate from 75 to 150 kg ha 1 reduced the weed biomass by up to 61% (Izquierdo et al., 2003). On the other hand, narrow row spacing has proven successful in managing many weeds in Australia (Scott et al., 2013). Likewise, changing the row orientation in different crops has helped to suppress important weeds in Australia (Pathan et al., 2006; Borger et al., 2010). So, these tactics are valuable in modern day agriculture for sustainable crop production. Crop competition is a pragmatic approach to manage problematic weeds, especially under the circumstances of herbicide resistance evolution and limited chemical options (Fig. 1). In this article, four major options (competitive cultivars, seed rate/planting density, row spacing/crop geometry, row orientation) to achieve better crop competition against weeds in Australian cropping systems have been reviewed. This review not only provides an insight into these valuable non-chemical weed control strategies but also highlights the future research directions. 2. Competitive crop species The competitive ability of crops plays an increasingly important role in harnessing economic yield and in effective weed management (Christensen, 1995). Different crops have a varying degree of competitive ability. However, the competitive advantage of crops also depends on overall weed flora in a particular cropping system and even in a specific field (Gill and Holmes, 1997). Felton et al. (2004) compared weed competitive ability of wheat, chickpea, fababean, and canola in a no-till system in Tamworth, New South Wales (NSW), Australia. The wheat was reported as the most competitive crop followed by fababean and canola whereas, chickpea was the least competitive against weeds. However, the results varied in different years of the study. Likewise, different cultivars of a particular crop may also differ in their competitiveness against weeds. In Wagga Wagga, NSW, rye, triticale (X triticosecal), and oats were the most competitive, canola, wheat, and barley were intermediate in competitive ability, and lupin and field peas were the least competitive crops in terms of nutrient uptake and growth suppression of grass weed, L. rigidum (Lemerle et al., 1995). In Western Australia (WA), Hashem et al. (2010) observed
Fig. 1. An illustration of the use of crop competition for weed management (different components of crop competition help to control weeds through resource competition, allelopathic effects, physical smothering, and seedbank limitation which may lead towards high crop yields, economic returns, and environmental safety).
that barley and wheat cultivars were more competitive than canola cultivars against L. rigidum. 3. Competitive cultivars The competitive crop cultivars are better able to acquire nutrients, water, light, and space. There are indications that competitive ability is season dependent, strongly influenced by environmental factors (Lemerle et al., 1995). Among cereals, certain wheat crop traits have been identified as conferring a competitive advantage (Challaiah et al., 1986; Wicks et al., 1986). A mid-season wheat cultivar, Heron, was more suppressive of Phalaris tuberosa L., compared with an early maturing dwarf wheat cultivar, Mexico 120 (Hoen and Oram, 1967). Similarly, taller wheat cultivars were found more competitive against L. multiflorum and L. rigidum in the USA and Australia (Appleby et al., 1976; Reeves and Brooke, 1977). Different cultivars of wheat and barley (growing at the same density) differed in their competitive ability against L. rigidum (Lemerle et al., 1995). The yield of competitive cultivars of barley (O'Connor) and wheat (Katunga) was least affected as compared with sensitive cultivars of barley (Skiff) and wheat (Shrike). The
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Katunga cultivar of wheat suffered only a 20% yield reduction due to L. rigidum competition during both the years of the study while it significantly reduced the biomass of L. rigidum (Lemerle et al., 1995). These competitive cultivars of wheat and barley also reduced the nutrient uptake by L. rigidum. Nitrogen (N), phosphorus (P), and potassium (K) uptake in L. rigidum growing with the competitive cultivar of wheat (Katunga) were 11, 2, and 25 kg ha 1, respectively in comparison to N, P, K uptake of 25, 5, and 56 kg ha 1, respectively, by the weed in the presence of sensitive wheat cultivar Shrike (Lemerle et al., 1995). It shows the ability of competitive cultivars to suppress weed growth by limiting the availability of resources for weeds. A comprehensive study comprising about 250 bread wheat and durum wheat (Triticum durum Desf.) genotypes from all over the world, found a large variability in genotypes for their competitive effects on L. rigidum based on origin of genotypes (Lemerle et al., 1996). Overall, bread wheat was more competitive against L. rigidum than durum wheat. Further evaluation of selected Australian cultivars revealed that the older cultivars were more competitive than the current cultivars at that time. This study also revealed that competition from L. rigidum reduced wheat yield up to 50% but that the yield of competitive cultivars was less affected than that of non-competitive cultivars. The competitive advantage of wheat cultivars was associated with vigorous growth, higher and faster biomass accumulation, extensive leaf canopy, and higher tillering capacity. In a study from WA, Gill et al. (1987) observed that the wheat plants were severely affected by B. diandrus at tillering and anthesis stages, which led to a reduction in the number of productive tillers. Competitive ideotypes of canola have been suggested as a tool for weed management (Asaduzzaman et al., 2014). Genotypes of canola differ in their competitive ability against L. rigidum (Lemerle et al., 2011) with hybrids more competitive than open-pollinated genotypes. Lemerle et al. (2014) compared the competitive ability of 16 canola genotypes against L. rigidum and volunteer wheat and observed that cultivars with high competitive ability reduced the weed biomass by 50% and also reduced weed seed production and, thus, seed bank replenishment (Lemerle et al., 2014). Hybrids were more competitive and higher yielding than triazine-tolerant cultivars. Integrated use of competitive cultivars and high planting density also proved effective in suppressing L. rigidum in barley (Paynter and Hills, 2009). Barley cultivars Baudin, Hamelin, and Flagship were more competitive against L. rigidum than Buloke, Gairdner, and Vlamingh (Paynter and Hills, 2009). Cultivars Buloke, Flagship, Hamelin, and Vlamingh have erect growth habits; whereas, Baudin and Gairdner are semi-dwarf cultivars, with a prostrate growth habit. Although genotypic variation in crop-weed competition was observed, no strong relationship between competitive ability and morphological traits was established. Integrated use of competitive barley cultivars and narrow row spacing provided effective control of L. rigidum and B. diandrus (Burke, 2009). Density of early as well as late season L. rigidum was reduced manifolds. In southern Queensland, Wu et al. (2010) evaluated the potential for integrated use of competitive cultivars and high planting density for weed management in sorghum (Sorghum bicolor L. Moench). Sorghum cultivars, MR Goldrush and Bonus MR, were more competitive against Echinochloa esculenta (A. Braun) H. Scholtz, which was used as a weed to mimic Echinochloa weed species (Wu et al., 2010). Superior early growth traits such as elevated plant height, daily growth rate, and shoot biomass of these cultivars suppressed height, biomass, and seed production of E. esculenta. In South Australia, McDonald (2003) compared 21 genotypes of pea for their competitiveness against grass weeds (wheat and L. rigidum) and revealed that taller pea genotypes were
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more competitive against wheat and L. rigidum. Lemerle et al. (2006) also assessed the role of field pea cultivars in L. rigidum suppression. At higher crop densities of field pea, the short stature cultivar had higher yield losses than the tall stature cultivar due to competition with L. rigidum (Lemerle et al., 2006). The competitive advantage of crops over weeds often involves the interaction between genotype and environment rather than just genotypic superiority. Although different genotypes with contrasting growth habits significantly differ in their competitive ability, environmental interactions also affected the overall performance of genotypes. A positive correlation between grain yields of weedy and weed-free plots in competitive wheat cultivars indicated the significant role of local adaptation towards overall crop competitiveness against weeds (Lemerle et al., 2001a). The higher grain yield in cereals like wheat is always the most desired characteristic even while developing a competitive cultivar through breeding. However, the separation of competitiveness and yield potential during a breeding program may provide the true sense of selection (Lemerle et al., 2001b). 4. Seed rate/planting density The use of higher plant densities is a proven cultural weed management tool (Gill and Holmes, 1997; Lemerle et al., 2001b). The seed rate increment has proved beneficial in terms of weed control in several crops (Gill and Holmes, 1997). The principle behind this practice is to increase the competitive ability of the crop against weed species (Lemerle et al., 1995). Usually, the increase in seed rate to increase the total plant population for crop species is done to out-compete weeds by restricting the free space and other resources availability (Carlson and Hill, 1985). The manipulation of this important agronomic practice chiefly depends on the environmental factors, cropping system, and type of existing weed flora (Lemerle et al., 2001b). The agronomic approaches like increased seed rate, are beneficial especially in case of associated weeds which are phenotypically similar to crops (Lemerle et al., 2001b). For instance, L. rigidum has similar morphology and growth habit to wheat crop and can be very hard to manage by conventional means (Cousens, 1996). The thick planting density of wheat achieved through the use of high seed rate (80 kg ha 1) was the practice to obtain higher yields and better weed control when cultivars with high tillering capacity and herbicides for weed control were still not available (Downing, 1921). An increase in planting density of lupin improved the crop yield while decreased the growth and biomass of L. rigidum due to enhanced crop competition (Allen, 1977). Similar kind of growth suppression of L. rigidum by increased plant population of wheat was also reported in NSW (Medd et al., 1985). The increasing wheat plant density had a pronounced inhibitory effect on growth of two important grass weeds, Avena fatua L. and Avena ludoviciana Durieu in a study conducted in the Darling Downs region, Queensland, Australia (Radford et al., 1980). This multi-year trial identified a direct negative correlation between wheat plant density and biomass of A. fatua and A. ludoviciana (Radford et al., 1980). The higher densities (100e400 plants m 2) of wheat in competition with L. multiflorum improved the yield, grain size, and harvest index of wheat while reducing the biomass of L. multiflorum (Hashem et al., 1998). In this study, wheat grown in higher densities was dominant during vegetative stages when; however, at low crop densities, L. multiflorum dominated particularly during crop reproductive stages, resulting in substantial yield losses. Increasing the wheat seed rate from 50 to 200 kg ha 1caused a threefold decrease in the seed production of L. rigidum (Fee and Anderson, 1997). Similarly, higher wheat seed rates of 200 kg ha 1 in WA had a pronounced negative effect on L. rigidum growth and a substantial yield increases in wheat
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(Peltzer, 1999). In another study from WA, increased barley plant density reduced biomass and tiller number of L. rigidum (Paynter and Hills, 2009). Although the genotypic variation had a significant impact on crop competitiveness against L. rigidum, the planting density variation played a dominant role in weed suppression. Increasing seed rate to increase crop density is more helpful for weed management in low input or organic production systems (Lemerle et al., 2004). Moreover, herbicide-resistant weeds can be suppressed efficiently through this practice. In southern Australia, while the recommended wheat crop density was 100e150 plants m 2, wheat densities above 200 plants m 2 suppressed L. rigidum present at densities ranging from 50 to 450 plants m 2, depending on environmental conditions and wheat cultivar (Lemerle et al., 2004). Increasing wheat density from 100 to 200 plants m 2 decreased L. rigidum biomass from 100 to 50 g m 2 (Lemerle et al., 2004). In general, higher crop densities increased grain yield and reduced L. rigidum biomass while any reductions in grain size were negligible. It was also observed that the impact of cultivar and climate on yield was negligible as compared with that of crop density. The general recommendation from these multi-location trials was that high wheat plant densities suppressed L. rigidum and, increased crop yield. After a series of experiments in South Australia, Sloane et al. (2004) concluded that early vigour (leaf area and dry matter production) of wheat was improved by increasing plant density; however, this positive effect on early growth did not translate to similar yield improvements. It can be inferred from these findings that increased seed rates may help to obtain early crop vigour which is helpful to suppress weeds early in the season and, thus, enable the crop to dominate over weeds during later growth stages. Increasing crop density to suppress weeds through competition was also found effective to reduce herbicide rates (Walker et al., 2002). Increasing wheat density up to 150 plants m 2 provided maximum grain yield and reduced the seed production of two important weeds, A. ludoviciana and Phalaris paradoxa L. whereas, the herbicide rate was reduced by 50% of the recommended rate (Walker et al., 2002). Seed rate also affected the herbicide efficacy in wide row conservation systems. For instance, increased planting density (up to 700 plants m 2) of wheat under wide row spacing (36 cm) improved the efficacy of diclofop-methyl (Lemerle et al., 2013). In a study from southern Queensland, Wu et al. (2010) compared six sorghum cultivars at three different planting densities (4.5, 6.0, and 7.5 plants m 2) for weed management and crop yield. Although some cultivars were more competitive against E. esculenta, increased sorghum densities had a pronounced effect on E. esculenta growth. Sorghum planted at the density of 7.5 plants m 2 reduced the density, biomass, and seed production of E. esculenta by 22, 27, and 38%, respectively, as compared with sorghum density of 4.0 plants m 2 (Wu et al., 2010). Similar to cereal species, a planting density increments in legume pasture and crop species provided weed suppression. Five legume forage species, Trifolium subterraneum L., Trifolium michelianum Savi., Trifolium alexandrinum L., Medicago murex Wild, and Vicia benghalensis L., had a variable influence on L. rigidum density and biomass not only due to different growth habits but also due to variable seed rates and seed sizes (Dear et al., 2006). Small-seeded T. michelianum and T. alexandrinum were more suppressive against L. rigidum when their seed rate was increased; whereas, boldseeded M. murex and V. benghalensis reduced light penetration to L. rigidum by 50% due to better canopy closure over a wide range of seed rates (Dear et al., 2006). It was suggested that increased plant densities of small-seeded legume forages may help to suppress L. rigidum due to higher relative growth rates. In a study from Minlaton (South Australia) and Horsham (Victoria), increase in
lentil (Lens culinaris L.) density from 100 to 200 plants m 2 decreased the density and biomass of canola (simulated brassica weed species) (McDonald et al., 2007). Similarly, higher plant densities provided better control of volunteer wheat and L. rigidum in canola cultivars with relatively weak competitive ability (Lemerle et al., 2011). The adoption of increasing seed rates for weed management has been prominent among Australian grain growers. It has become an essential part of IWM plans for grain growers, especially in areas where herbicide-resistant weeds are dominant. Llewellyn et al. (2004) conducted a comprehensive survey in WA involving over 130 grain growers in order to characterize the IWM strategies for the region with special focus on L. rigidum. Increasing crop plant densities was identified as a major practice routinely used to manage weeds particularly L. rigidum. In the same survey, about 61% of surveyed growers were using higher wheat plant densities as part of IWM programs. This was a marked increase from 1996 when an estimated 24% growers were using this practice. This result demonstrates the recognition of higher crop plant densities as an effective weed management practice. Increasing plant density is an effective way to suppress weeds; however, it is not a pragmatic option in some regions particularly in Australia's low rainfall crop production region (Gill and Holmes, 1997). A decrease of 10e15% in grain size was observed when the wheat population was increased from 75 to 200 plants m 2 in NSW (Medd et al., 1985). Similarly, increasing wheat population from 90 to 280 plants m 2 in WA reduced the average wheat grain size by 5% (Gill and Holmes, 1997). Reductions in grain size can also result in increased grain losses during harvest and problems in marketing. 5. Row spacing/planting geometry Altering the row spacing in grain crops has proved significantly beneficial in terms of yield improvement and weed management in many parts of Australia (Lemerle et al., 2001b). Wheat is one of the major crops in which this practice has shown remarkable effects and grain growers have considered it as an efficient weed management tool. It has been reported that wheat competitive ability might be reduced by increasing row spacing (Lemerle et al., 2001b). It might be due to the fact that increased inter-row space allows more weeds to grow and flourish with less shading and competition by crop plants. For instance, varying the row spacing in wheat from 18 to 36 cm while keeping the seed rate unchanged (50 kg ha 1) reduced the competitive ability of wheat against L. rigidum (Peltzer et al., 2009). But the effect of increasing row spacing was unclear in case of increased seed rate (150 kg ha 1), which suggests that increased seed rate may compensate for the negative impact caused by wider row spacing which is sometimes inevitable especially in conservation tillage systems of Australia where previous crop residues are retained (Lemerle et al., 2001b). Narrow row spacing has been effective in weed management in many arable crops in Australia. Narrowing the row spacing from 36 to 18 cm in field peas improved the L. rigidum control substantially (Lemerle et al., 2006). Felton (1976) reported that row spacing and plant density within a row had an interactive effect on weed density and soybean [Glycine max (L.) Merr] yield. It was revealed that weeds had no negative impact on yield at narrow row spacing (25 cm) but 20, 26, and 37% reductions in yield were observed at 50, 75, and 100 cm row spacing, respectively. High yield losses at wider row spacing were attributed to high weed density and severe weed-crop competition (Felton, 1976). In a study from northern NSW, narrow row spacing reduced weed pressure in rainfed grain sorghum (Holland and McNamara, 1982). In southern and central Queensland, multiple experiments were conducted to evaluate the potential of row spacing manipulation for weed control in sorghum
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and sunflower (Helianthus annuus L.) (Osten et al., 2006). It was revealed that wide row spacing (1.5 m) increased weed pressure in both crops and weed seed production was also increased; however, yield penalty due to direct weed competition was reduced as compared with the recommended row spacing (1 m). Furthermore, reducing row spacing at high weed pressure reduced weed biomass and weed seed production in sorghum and sunflower. 6. Row orientation The orientation of crop rows is an important factor affecting crop growth and development and crop competitiveness against weeds. Variable responses of crops and weeds to light direction and light penetration are important attributes in the management of weeds through changing row orientation (Holt, 1991, 1995). An east-west row direction creating a near right angle to the direction of sunlight causes crop shading of weeds growing between and inside crop rows and therefore, suppresses weed growth and development (Holt, 1995). Pathan et al. (2006) conducted a series of experiments on the effect of row orientation on crop growth and weed control at Merredin in the WA wheat belt and observed a remarkable 42 and 43% increase in wheat and barley grain yields, respectively, in an east-west row orientation as compared with a north-south orientation. Additionally, the change from north-south to east-west row orientation resulted in 81 and 87% reductions in weed biomass in wheat and barley, respectively. Borger et al. (2010) evaluated the potential of east-west row orientation of five crops, wheat, barley, canola, field peas, and lupins at two locations (Beverley and Merredin) in WA to suppress weed growth and observed substantial reductions in biomass of Arctotheca calendula (L.) Levyns. and Emex australis Steinheil and increases in crop yields for east-west orientation due to improved light interception by crop canopies and shading effect on inter-row weeds. Changing row orientation from north-south to east-west in wheat and barley reduced the growth and fecundity of L. rigidum (Borger et al., 2014, 2015). About 47% less light was available to L. rigidum plants growing between east-west planted crop rows compared with those between north-south rows. The reduced light availability resulted in reduced seed production of L. rigidum by about 48% (Borger et al., 2015). Higher seed rates integrated with east-west orientation further reduced the growth and fecundity of L. rigidum in wheat and barley. Thus, east-west row orientation is a pragmatic option for weed management in row crops although the potential of this strategy has not been well explored yet. 7. Conclusions and future research directions In the wake of herbicide resistance evolution and changing weed flora in response to management practices, crop competition is a valuable weed management option for inclusion in Australian crop production systems. Competition tactics not only provide short-term in-crop weed control but also reduce the long-term weed seed-bank replenishment by reducing weed seed inputs. Crop husbandry practices are highly mechanized and therefore, well suited for implementing crop competition strategies such as, competitive cultivars, high plant densities, narrow row spacing, and east-west row orientation. The options discussed here have proved successful in many Australian winter crops and cropping systems. However, research on the value of these strategies is limited for summer crops. Additionally, the weed management potential of crop competition has been extensively evaluated in WA and parts of SA and NSW but needs to be explored more extensively in northern region cropping systems. Legume crops including, chickpea, pea, lentil, and fababean require more attention in regards to development of
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competitive genotypes and planting density studies. Several studies have been conducted to manage established and more common weed species like L. rigidum, L. multiflorm, Raphanus raphanistrum L., A. fatua, B. diandrus, and Echinochloa spp. through crop competition. However, little emphasis has been given to emerging weed species such as Chloris truncata R. Br., Chloris virgata Sw., Chloris gayana Kunth, and Conyza bonariensis (L.) Cronq. More attention should be given to integrated use of competitive cultivars and planting density, narrow row spacing, and east-west row orientation or combinations of these strategies on weed growth and development as well as grain yield. The inclusion of enhanced crop competition as a routine weed control approach in an integrated management program will undoubtedly lead to more effective control and a reduction in herbicide resistance evolution. References Allen, J.M., 1977. Weeds in grain lupins.1. The effect of weeds on grain lupin yields. Aust. J. 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993e1001. Llewellyn, R.S., Ronning, D., Ouzman, J., Walker, S., Mayfield, A., Clark, M., 2016. Impact of Weeds on Australian Grain Production: the Cost of Weeds to Australian Grain Growers and the Adoption of Weed Management and Tillage Practices. Report for GRDC. CSIRO, Australia. Available online: https://grdc.com. au/bookshop. McDonald, G.K., 2003. Competitiveness against grass weeds in field pea genotypes. Weed Res. 43, 48e58. McDonald, G.K., Hollaway, K.L., McMurray, L., 2007. Increasing plant density improves weed competition in lentil (Lens culinaris). Anim. Prod. Sci. 47, 48e56. Medd, R.W., Auld, B.A., Kemp, D.R., Murison, R.D., 1985. The influence of wheat density and spatial arrangement on annual ryegrass, Lolium rigidum Gaudin, competition. Crop Past. Sci. 36, 361e371. Osten, V., Wu, H., Walker, S., Wright, G., Shields, A., 2006. Weeds and summer crop row spacing studies in Queensland. In: Preston, C., Watts, J.H., Crossman, N.D. (Eds.), Proceedings of the 15th Australian Weeds Conference. 24-28 Sep 2006. Adelaide, South Australia, pp. 347e350. Owen, M.J., Goggin, D.E., Powles, S.B., 2015. Intensive cropping systems select for greater seed dormancy and increased herbicide resistance levels in Lolium rigidum (annual ryegrass). Pest Manage. Sci. 71, 966e971. Pathan, S., Hashem, A., Borger, C., 2006. Crop row orientation induced photosensory effect on the competitive interactions of crops and weeds. In: Preston, C., Watts, J.H., Crossman, N.D. (Eds.), Proceedings of the 15th Australian Weeds Conference, Adelaide, South Australia. 24-28 Sep 2006, pp. 351e354. Pavlychenko, T.K., Harrington, J.B., 1934. Competitive efficiency of weeds and cereal crops. Can. J. Res. 10, 77e94. Paynter, B.H., Hills, A.L., 2009. Barley and rigid ryegrass (Lolium rigidum) competition is influenced by crop cultivar and density. Weed Technol. 23, 40e48. Peltzer, S., 1999. Increased crop density reduces weed seed production without increasing screenings. In: Bishop, A.C., Boersma, M., Barnes, C.D. (Eds.), Proceedings of the 12th Australian Weeds Conference. Tasmania, Hobart, 12-16 Sep 1999, pp. 510e512. Peltzer, S.C., Hashem, A., Osten, V.A., Gupta, M.L., Diggle, A.J., Riethmuller, G.P., Douglas, A., Moore, J.A., Koetz, E.A., 2009. Weed management in wide-row cropping systems: a review of current practices and risks for Australian farming systems. Crop Past. Sci. 60, 395e406. Radford, B.J., Wilson, B.J., Cartledge, O., Watkins, F.B., 1980. Effect of wheat seeding rate on wild oat competition. Aust. J. Exp. Agric. Anim. Husb. 20, 77e81. Reeves, T.G., Brooke, H.D., 1977. The effect of genotype and phenotype on the competition between wheat and annual ryegrass. In: Proceedings of the 6th Asian Pacific Weed Science Conference, Jakarta, Indonesia, p. 166. Scott, B.J., Martin, P., Riethmuller, G.P., 2013. Graham Centre Monograph No. 3: Row Spacing of Winter Crops in Broad Scale Agriculture in Southern Australia. NSW Department of Primary Industries, Orange. Available at: www.grahamcentre. net. Sloane, D.H.G., Gill, G.S., McDonald, G.K., 2004. The impact of agronomic manipulation of early vigour in wheat on growth and yield in South Australia. Crop Past. Sci. 55, 645e654. Walker, S.R., Medd, R.W., Robinson, G.R., Cullis, B.R., 2002. Improved management of Avena ludoviciana and Phalaris paradoxa with more densely sown wheat and less herbicide. Weed Res. 42, 257e270. Walsh, M.J., Powles, S.B., 2007. Management strategies for herbicide-resistant weed populations in Australian dryland crop production systems. Weed Technol. 21, 332e338. Wicks, G.A., Ramsel, R.E., Nordquist, P.T., Schmidt, J.W., 1986. Impact of wheat cultivars on establishment and suppression of summer annual weeds. Agron. J. 78, 59e62. Wu, H., Walker, S.R., Osten, V.A., Robinson, G., 2010. Competition of sorghum cultivars and densities with Japanese millet (Echinochloa esculenta). Weed Biol. Manage 10, 185e193.
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Contents lists available at ScienceDirect
Crop Protection journal homepage: www.elsevier.com/locate/cropro
Weed management using crop competition in Australia Ali Ahsan Bajwa a, b, *, Michael Walsh c, Bhagirath Singh Chauhan b a
School of Agriculture and Food Sciences, The University of Queensland, Gatton, Queensland 4343, Australia The Centre for Plant Science, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, Gatton/Toowoomba, Queensland 4343/4350, Australia c The University of Sydney, Narrabri, NSW 2390, Australia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 April 2016 Received in revised form 17 August 2016 Accepted 21 August 2016 Available online xxx
Adoption of conservation agriculture has brought significant changes in crop management in Australia. Increased reliance on herbicidal weed control is one of the most significant changes that have occurred throughout Australian cropping systems. The singular focus on herbicides for weed management has led to the frequent and widespread evolution of herbicide resistance in several weed species. Herbicide resistance means the loss of herbicide resources and as new herbicides modes of action are unlikely then use of alternative non-chemical cultural weed management options is essential. Crop competition is an approach that can be used to manage weeds for improved crop production. Enhanced crop competition can be achieved through the use of competitive crop species and cultivars, increased seed rates, narrow row spacing, and altered row orientation. These options are already routinely used in Australia and are proven in their ability to reduce weed biomass and fecundity. Although these strategies have been successfully used in Australian cropping systems, the research has frequently been focussed locally and not extrapolated more broadly throughout the Australian grain production region. Crop competition can potentially be a sustainable weed management option in reducing the reliance on herbicides and combating herbicide resistance and therefore, needs to be considered in all integrated weed management plans. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Conservation agriculture Competitive cultivars Crop competition Integrated weed management Row orientation Row spacing
1. Introduction Conservation agriculture has been adopted at a large scale in Australia during the past few decades (Chauhan et al., 2006; Llewellyn et al., 2012). The development of conservation cropping based on stubble retention and reduced tillage has transformed crop production in Australia due to marked improvements in crop available soil moisture and nutrient levels as well as soil structure providing substantial gains in crop productivity (Llewellyn et al., 2004; Bajwa et al., 2015). However, despite these productivity gains, weeds remain a major constraint to productivity in conservation agriculture (Chauhan et al., 2012; Bajwa, 2014). The availability of highly effective herbicides has facilitated the adoption of conservation cropping systems but the subsequent reliance on these herbicides has created the significant problem of herbicide resistance (Walsh and Powles, 2007). Continuous use of herbicides
* Corresponding author. School of Agriculture and Food Sciences, The University of Queensland, Gatton, Queensland 4343, Australia. E-mail address: [email protected] (A.A. Bajwa).
with similar modes of action has caused the evolution of herbicideresistant weed biotypes that now infest crops throughout Australia's production regions (Boutsalis et al., 2012; Broster et al., 2013; Bajwa et al., 2015; Owen et al., 2015). It is estimated that herbicide resistance has, on average, increased the cost of weed control by $55 ha 1 (Llewellyn et al., 2016). Thus, the inclusion of alternative non-chemical management options in weed management programs is essential for sustainable crop production in Australia. The use of crop competition to control weeds is one of the most effective cultural strategies in Australian cropping, where suppression of weed biomass and fecundity leads to crop yield gains (Lemerle et al., 1996, 2004; Scott et al., 2013). Crop competition is an effective approach to weed management that is applicable in large scale conservation production systems (Brennan et al., 2001). The competitiveness of a crop can be enhanced through the use of competitive cultivars, higher plant densities, narrow row spacing, and different row orientation (Lemerle et al., 1996, 2014; Walker et al., 2002; Pathan et al., 2006; Hashem et al., 2010; Borger et al., 2015). Significant progress has been made in Australian cropping
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systems in developing highly competitive and high yielding genotypes of several crops. The traits contributing to competitive ability against weeds include elevated plant height, higher dry matter accumulation, early canopy closure, enhanced leaf area leading to more light interception and shading over understory vegetation, increased nutrient uptake, proliferate root growth, and allelopathic effects (Pavlychenko and Harrington, 1934; Konesky et al., 1989; Balyan et al., 1991; Cousens et al., 1991; Cudney et al., 1991). Competitive cultivars of important crops including, wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), oats (Avena sativa L.), rye (Secale cereale L.), chickpea (Cicer arietinum L.), fababean (Vicia faba L.), lupin (Lupinus angustifolius L.), field pea (Pisum sativum L.), and canola (Brassica napus L.) have been successfully used for weed management in Australia (Lemerle et al., 2014; Gill and Holmes, 1997; Paynter and Hills, 2009; Hashem et al., 2010; Asaduzzaman et al., 2014). Such cultivars not only provided better control of major weed species including, Lolium multiflorum Lam., Lolium rigidum Gaud., and Bromus diandrus Roth. but also yielded more than non-competitive cultivars (Gill et al., 1987; Lemerle et al., 1995, 2001a; Felton et al., 2004). The use of high crop seeding rates to increase crop planting density has consistently been proven to enhance the competitive ability of many winter cereals against weeds. For instance, a twofold increase in the standard seeding rate (55 kg ha 1) of 10 Australian wheat cultivars caused a 43% reduction in L. rigidum biomass (Lemerle et al., 1996). Similarly, an increase in barley seeding rate from 75 to 150 kg ha 1 reduced the weed biomass by up to 61% (Izquierdo et al., 2003). On the other hand, narrow row spacing has proven successful in managing many weeds in Australia (Scott et al., 2013). Likewise, changing the row orientation in different crops has helped to suppress important weeds in Australia (Pathan et al., 2006; Borger et al., 2010). So, these tactics are valuable in modern day agriculture for sustainable crop production. Crop competition is a pragmatic approach to manage problematic weeds, especially under the circumstances of herbicide resistance evolution and limited chemical options (Fig. 1). In this article, four major options (competitive cultivars, seed rate/planting density, row spacing/crop geometry, row orientation) to achieve better crop competition against weeds in Australian cropping systems have been reviewed. This review not only provides an insight into these valuable non-chemical weed control strategies but also highlights the future research directions. 2. Competitive crop species The competitive ability of crops plays an increasingly important role in harnessing economic yield and in effective weed management (Christensen, 1995). Different crops have a varying degree of competitive ability. However, the competitive advantage of crops also depends on overall weed flora in a particular cropping system and even in a specific field (Gill and Holmes, 1997). Felton et al. (2004) compared weed competitive ability of wheat, chickpea, fababean, and canola in a no-till system in Tamworth, New South Wales (NSW), Australia. The wheat was reported as the most competitive crop followed by fababean and canola whereas, chickpea was the least competitive against weeds. However, the results varied in different years of the study. Likewise, different cultivars of a particular crop may also differ in their competitiveness against weeds. In Wagga Wagga, NSW, rye, triticale (X triticosecal), and oats were the most competitive, canola, wheat, and barley were intermediate in competitive ability, and lupin and field peas were the least competitive crops in terms of nutrient uptake and growth suppression of grass weed, L. rigidum (Lemerle et al., 1995). In Western Australia (WA), Hashem et al. (2010) observed
Fig. 1. An illustration of the use of crop competition for weed management (different components of crop competition help to control weeds through resource competition, allelopathic effects, physical smothering, and seedbank limitation which may lead towards high crop yields, economic returns, and environmental safety).
that barley and wheat cultivars were more competitive than canola cultivars against L. rigidum. 3. Competitive cultivars The competitive crop cultivars are better able to acquire nutrients, water, light, and space. There are indications that competitive ability is season dependent, strongly influenced by environmental factors (Lemerle et al., 1995). Among cereals, certain wheat crop traits have been identified as conferring a competitive advantage (Challaiah et al., 1986; Wicks et al., 1986). A mid-season wheat cultivar, Heron, was more suppressive of Phalaris tuberosa L., compared with an early maturing dwarf wheat cultivar, Mexico 120 (Hoen and Oram, 1967). Similarly, taller wheat cultivars were found more competitive against L. multiflorum and L. rigidum in the USA and Australia (Appleby et al., 1976; Reeves and Brooke, 1977). Different cultivars of wheat and barley (growing at the same density) differed in their competitive ability against L. rigidum (Lemerle et al., 1995). The yield of competitive cultivars of barley (O'Connor) and wheat (Katunga) was least affected as compared with sensitive cultivars of barley (Skiff) and wheat (Shrike). The
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Katunga cultivar of wheat suffered only a 20% yield reduction due to L. rigidum competition during both the years of the study while it significantly reduced the biomass of L. rigidum (Lemerle et al., 1995). These competitive cultivars of wheat and barley also reduced the nutrient uptake by L. rigidum. Nitrogen (N), phosphorus (P), and potassium (K) uptake in L. rigidum growing with the competitive cultivar of wheat (Katunga) were 11, 2, and 25 kg ha 1, respectively in comparison to N, P, K uptake of 25, 5, and 56 kg ha 1, respectively, by the weed in the presence of sensitive wheat cultivar Shrike (Lemerle et al., 1995). It shows the ability of competitive cultivars to suppress weed growth by limiting the availability of resources for weeds. A comprehensive study comprising about 250 bread wheat and durum wheat (Triticum durum Desf.) genotypes from all over the world, found a large variability in genotypes for their competitive effects on L. rigidum based on origin of genotypes (Lemerle et al., 1996). Overall, bread wheat was more competitive against L. rigidum than durum wheat. Further evaluation of selected Australian cultivars revealed that the older cultivars were more competitive than the current cultivars at that time. This study also revealed that competition from L. rigidum reduced wheat yield up to 50% but that the yield of competitive cultivars was less affected than that of non-competitive cultivars. The competitive advantage of wheat cultivars was associated with vigorous growth, higher and faster biomass accumulation, extensive leaf canopy, and higher tillering capacity. In a study from WA, Gill et al. (1987) observed that the wheat plants were severely affected by B. diandrus at tillering and anthesis stages, which led to a reduction in the number of productive tillers. Competitive ideotypes of canola have been suggested as a tool for weed management (Asaduzzaman et al., 2014). Genotypes of canola differ in their competitive ability against L. rigidum (Lemerle et al., 2011) with hybrids more competitive than open-pollinated genotypes. Lemerle et al. (2014) compared the competitive ability of 16 canola genotypes against L. rigidum and volunteer wheat and observed that cultivars with high competitive ability reduced the weed biomass by 50% and also reduced weed seed production and, thus, seed bank replenishment (Lemerle et al., 2014). Hybrids were more competitive and higher yielding than triazine-tolerant cultivars. Integrated use of competitive cultivars and high planting density also proved effective in suppressing L. rigidum in barley (Paynter and Hills, 2009). Barley cultivars Baudin, Hamelin, and Flagship were more competitive against L. rigidum than Buloke, Gairdner, and Vlamingh (Paynter and Hills, 2009). Cultivars Buloke, Flagship, Hamelin, and Vlamingh have erect growth habits; whereas, Baudin and Gairdner are semi-dwarf cultivars, with a prostrate growth habit. Although genotypic variation in crop-weed competition was observed, no strong relationship between competitive ability and morphological traits was established. Integrated use of competitive barley cultivars and narrow row spacing provided effective control of L. rigidum and B. diandrus (Burke, 2009). Density of early as well as late season L. rigidum was reduced manifolds. In southern Queensland, Wu et al. (2010) evaluated the potential for integrated use of competitive cultivars and high planting density for weed management in sorghum (Sorghum bicolor L. Moench). Sorghum cultivars, MR Goldrush and Bonus MR, were more competitive against Echinochloa esculenta (A. Braun) H. Scholtz, which was used as a weed to mimic Echinochloa weed species (Wu et al., 2010). Superior early growth traits such as elevated plant height, daily growth rate, and shoot biomass of these cultivars suppressed height, biomass, and seed production of E. esculenta. In South Australia, McDonald (2003) compared 21 genotypes of pea for their competitiveness against grass weeds (wheat and L. rigidum) and revealed that taller pea genotypes were
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more competitive against wheat and L. rigidum. Lemerle et al. (2006) also assessed the role of field pea cultivars in L. rigidum suppression. At higher crop densities of field pea, the short stature cultivar had higher yield losses than the tall stature cultivar due to competition with L. rigidum (Lemerle et al., 2006). The competitive advantage of crops over weeds often involves the interaction between genotype and environment rather than just genotypic superiority. Although different genotypes with contrasting growth habits significantly differ in their competitive ability, environmental interactions also affected the overall performance of genotypes. A positive correlation between grain yields of weedy and weed-free plots in competitive wheat cultivars indicated the significant role of local adaptation towards overall crop competitiveness against weeds (Lemerle et al., 2001a). The higher grain yield in cereals like wheat is always the most desired characteristic even while developing a competitive cultivar through breeding. However, the separation of competitiveness and yield potential during a breeding program may provide the true sense of selection (Lemerle et al., 2001b). 4. Seed rate/planting density The use of higher plant densities is a proven cultural weed management tool (Gill and Holmes, 1997; Lemerle et al., 2001b). The seed rate increment has proved beneficial in terms of weed control in several crops (Gill and Holmes, 1997). The principle behind this practice is to increase the competitive ability of the crop against weed species (Lemerle et al., 1995). Usually, the increase in seed rate to increase the total plant population for crop species is done to out-compete weeds by restricting the free space and other resources availability (Carlson and Hill, 1985). The manipulation of this important agronomic practice chiefly depends on the environmental factors, cropping system, and type of existing weed flora (Lemerle et al., 2001b). The agronomic approaches like increased seed rate, are beneficial especially in case of associated weeds which are phenotypically similar to crops (Lemerle et al., 2001b). For instance, L. rigidum has similar morphology and growth habit to wheat crop and can be very hard to manage by conventional means (Cousens, 1996). The thick planting density of wheat achieved through the use of high seed rate (80 kg ha 1) was the practice to obtain higher yields and better weed control when cultivars with high tillering capacity and herbicides for weed control were still not available (Downing, 1921). An increase in planting density of lupin improved the crop yield while decreased the growth and biomass of L. rigidum due to enhanced crop competition (Allen, 1977). Similar kind of growth suppression of L. rigidum by increased plant population of wheat was also reported in NSW (Medd et al., 1985). The increasing wheat plant density had a pronounced inhibitory effect on growth of two important grass weeds, Avena fatua L. and Avena ludoviciana Durieu in a study conducted in the Darling Downs region, Queensland, Australia (Radford et al., 1980). This multi-year trial identified a direct negative correlation between wheat plant density and biomass of A. fatua and A. ludoviciana (Radford et al., 1980). The higher densities (100e400 plants m 2) of wheat in competition with L. multiflorum improved the yield, grain size, and harvest index of wheat while reducing the biomass of L. multiflorum (Hashem et al., 1998). In this study, wheat grown in higher densities was dominant during vegetative stages when; however, at low crop densities, L. multiflorum dominated particularly during crop reproductive stages, resulting in substantial yield losses. Increasing the wheat seed rate from 50 to 200 kg ha 1caused a threefold decrease in the seed production of L. rigidum (Fee and Anderson, 1997). Similarly, higher wheat seed rates of 200 kg ha 1 in WA had a pronounced negative effect on L. rigidum growth and a substantial yield increases in wheat
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(Peltzer, 1999). In another study from WA, increased barley plant density reduced biomass and tiller number of L. rigidum (Paynter and Hills, 2009). Although the genotypic variation had a significant impact on crop competitiveness against L. rigidum, the planting density variation played a dominant role in weed suppression. Increasing seed rate to increase crop density is more helpful for weed management in low input or organic production systems (Lemerle et al., 2004). Moreover, herbicide-resistant weeds can be suppressed efficiently through this practice. In southern Australia, while the recommended wheat crop density was 100e150 plants m 2, wheat densities above 200 plants m 2 suppressed L. rigidum present at densities ranging from 50 to 450 plants m 2, depending on environmental conditions and wheat cultivar (Lemerle et al., 2004). Increasing wheat density from 100 to 200 plants m 2 decreased L. rigidum biomass from 100 to 50 g m 2 (Lemerle et al., 2004). In general, higher crop densities increased grain yield and reduced L. rigidum biomass while any reductions in grain size were negligible. It was also observed that the impact of cultivar and climate on yield was negligible as compared with that of crop density. The general recommendation from these multi-location trials was that high wheat plant densities suppressed L. rigidum and, increased crop yield. After a series of experiments in South Australia, Sloane et al. (2004) concluded that early vigour (leaf area and dry matter production) of wheat was improved by increasing plant density; however, this positive effect on early growth did not translate to similar yield improvements. It can be inferred from these findings that increased seed rates may help to obtain early crop vigour which is helpful to suppress weeds early in the season and, thus, enable the crop to dominate over weeds during later growth stages. Increasing crop density to suppress weeds through competition was also found effective to reduce herbicide rates (Walker et al., 2002). Increasing wheat density up to 150 plants m 2 provided maximum grain yield and reduced the seed production of two important weeds, A. ludoviciana and Phalaris paradoxa L. whereas, the herbicide rate was reduced by 50% of the recommended rate (Walker et al., 2002). Seed rate also affected the herbicide efficacy in wide row conservation systems. For instance, increased planting density (up to 700 plants m 2) of wheat under wide row spacing (36 cm) improved the efficacy of diclofop-methyl (Lemerle et al., 2013). In a study from southern Queensland, Wu et al. (2010) compared six sorghum cultivars at three different planting densities (4.5, 6.0, and 7.5 plants m 2) for weed management and crop yield. Although some cultivars were more competitive against E. esculenta, increased sorghum densities had a pronounced effect on E. esculenta growth. Sorghum planted at the density of 7.5 plants m 2 reduced the density, biomass, and seed production of E. esculenta by 22, 27, and 38%, respectively, as compared with sorghum density of 4.0 plants m 2 (Wu et al., 2010). Similar to cereal species, a planting density increments in legume pasture and crop species provided weed suppression. Five legume forage species, Trifolium subterraneum L., Trifolium michelianum Savi., Trifolium alexandrinum L., Medicago murex Wild, and Vicia benghalensis L., had a variable influence on L. rigidum density and biomass not only due to different growth habits but also due to variable seed rates and seed sizes (Dear et al., 2006). Small-seeded T. michelianum and T. alexandrinum were more suppressive against L. rigidum when their seed rate was increased; whereas, boldseeded M. murex and V. benghalensis reduced light penetration to L. rigidum by 50% due to better canopy closure over a wide range of seed rates (Dear et al., 2006). It was suggested that increased plant densities of small-seeded legume forages may help to suppress L. rigidum due to higher relative growth rates. In a study from Minlaton (South Australia) and Horsham (Victoria), increase in
lentil (Lens culinaris L.) density from 100 to 200 plants m 2 decreased the density and biomass of canola (simulated brassica weed species) (McDonald et al., 2007). Similarly, higher plant densities provided better control of volunteer wheat and L. rigidum in canola cultivars with relatively weak competitive ability (Lemerle et al., 2011). The adoption of increasing seed rates for weed management has been prominent among Australian grain growers. It has become an essential part of IWM plans for grain growers, especially in areas where herbicide-resistant weeds are dominant. Llewellyn et al. (2004) conducted a comprehensive survey in WA involving over 130 grain growers in order to characterize the IWM strategies for the region with special focus on L. rigidum. Increasing crop plant densities was identified as a major practice routinely used to manage weeds particularly L. rigidum. In the same survey, about 61% of surveyed growers were using higher wheat plant densities as part of IWM programs. This was a marked increase from 1996 when an estimated 24% growers were using this practice. This result demonstrates the recognition of higher crop plant densities as an effective weed management practice. Increasing plant density is an effective way to suppress weeds; however, it is not a pragmatic option in some regions particularly in Australia's low rainfall crop production region (Gill and Holmes, 1997). A decrease of 10e15% in grain size was observed when the wheat population was increased from 75 to 200 plants m 2 in NSW (Medd et al., 1985). Similarly, increasing wheat population from 90 to 280 plants m 2 in WA reduced the average wheat grain size by 5% (Gill and Holmes, 1997). Reductions in grain size can also result in increased grain losses during harvest and problems in marketing. 5. Row spacing/planting geometry Altering the row spacing in grain crops has proved significantly beneficial in terms of yield improvement and weed management in many parts of Australia (Lemerle et al., 2001b). Wheat is one of the major crops in which this practice has shown remarkable effects and grain growers have considered it as an efficient weed management tool. It has been reported that wheat competitive ability might be reduced by increasing row spacing (Lemerle et al., 2001b). It might be due to the fact that increased inter-row space allows more weeds to grow and flourish with less shading and competition by crop plants. For instance, varying the row spacing in wheat from 18 to 36 cm while keeping the seed rate unchanged (50 kg ha 1) reduced the competitive ability of wheat against L. rigidum (Peltzer et al., 2009). But the effect of increasing row spacing was unclear in case of increased seed rate (150 kg ha 1), which suggests that increased seed rate may compensate for the negative impact caused by wider row spacing which is sometimes inevitable especially in conservation tillage systems of Australia where previous crop residues are retained (Lemerle et al., 2001b). Narrow row spacing has been effective in weed management in many arable crops in Australia. Narrowing the row spacing from 36 to 18 cm in field peas improved the L. rigidum control substantially (Lemerle et al., 2006). Felton (1976) reported that row spacing and plant density within a row had an interactive effect on weed density and soybean [Glycine max (L.) Merr] yield. It was revealed that weeds had no negative impact on yield at narrow row spacing (25 cm) but 20, 26, and 37% reductions in yield were observed at 50, 75, and 100 cm row spacing, respectively. High yield losses at wider row spacing were attributed to high weed density and severe weed-crop competition (Felton, 1976). In a study from northern NSW, narrow row spacing reduced weed pressure in rainfed grain sorghum (Holland and McNamara, 1982). In southern and central Queensland, multiple experiments were conducted to evaluate the potential of row spacing manipulation for weed control in sorghum
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and sunflower (Helianthus annuus L.) (Osten et al., 2006). It was revealed that wide row spacing (1.5 m) increased weed pressure in both crops and weed seed production was also increased; however, yield penalty due to direct weed competition was reduced as compared with the recommended row spacing (1 m). Furthermore, reducing row spacing at high weed pressure reduced weed biomass and weed seed production in sorghum and sunflower. 6. Row orientation The orientation of crop rows is an important factor affecting crop growth and development and crop competitiveness against weeds. Variable responses of crops and weeds to light direction and light penetration are important attributes in the management of weeds through changing row orientation (Holt, 1991, 1995). An east-west row direction creating a near right angle to the direction of sunlight causes crop shading of weeds growing between and inside crop rows and therefore, suppresses weed growth and development (Holt, 1995). Pathan et al. (2006) conducted a series of experiments on the effect of row orientation on crop growth and weed control at Merredin in the WA wheat belt and observed a remarkable 42 and 43% increase in wheat and barley grain yields, respectively, in an east-west row orientation as compared with a north-south orientation. Additionally, the change from north-south to east-west row orientation resulted in 81 and 87% reductions in weed biomass in wheat and barley, respectively. Borger et al. (2010) evaluated the potential of east-west row orientation of five crops, wheat, barley, canola, field peas, and lupins at two locations (Beverley and Merredin) in WA to suppress weed growth and observed substantial reductions in biomass of Arctotheca calendula (L.) Levyns. and Emex australis Steinheil and increases in crop yields for east-west orientation due to improved light interception by crop canopies and shading effect on inter-row weeds. Changing row orientation from north-south to east-west in wheat and barley reduced the growth and fecundity of L. rigidum (Borger et al., 2014, 2015). About 47% less light was available to L. rigidum plants growing between east-west planted crop rows compared with those between north-south rows. The reduced light availability resulted in reduced seed production of L. rigidum by about 48% (Borger et al., 2015). Higher seed rates integrated with east-west orientation further reduced the growth and fecundity of L. rigidum in wheat and barley. Thus, east-west row orientation is a pragmatic option for weed management in row crops although the potential of this strategy has not been well explored yet. 7. Conclusions and future research directions In the wake of herbicide resistance evolution and changing weed flora in response to management practices, crop competition is a valuable weed management option for inclusion in Australian crop production systems. Competition tactics not only provide short-term in-crop weed control but also reduce the long-term weed seed-bank replenishment by reducing weed seed inputs. Crop husbandry practices are highly mechanized and therefore, well suited for implementing crop competition strategies such as, competitive cultivars, high plant densities, narrow row spacing, and east-west row orientation. The options discussed here have proved successful in many Australian winter crops and cropping systems. However, research on the value of these strategies is limited for summer crops. Additionally, the weed management potential of crop competition has been extensively evaluated in WA and parts of SA and NSW but needs to be explored more extensively in northern region cropping systems. Legume crops including, chickpea, pea, lentil, and fababean require more attention in regards to development of
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competitive genotypes and planting density studies. Several studies have been conducted to manage established and more common weed species like L. rigidum, L. multiflorm, Raphanus raphanistrum L., A. fatua, B. diandrus, and Echinochloa spp. through crop competition. However, little emphasis has been given to emerging weed species such as Chloris truncata R. Br., Chloris virgata Sw., Chloris gayana Kunth, and Conyza bonariensis (L.) Cronq. More attention should be given to integrated use of competitive cultivars and planting density, narrow row spacing, and east-west row orientation or combinations of these strategies on weed growth and development as well as grain yield. The inclusion of enhanced crop competition as a routine weed control approach in an integrated management program will undoubtedly lead to more effective control and a reduction in herbicide resistance evolution. References Allen, J.M., 1977. Weeds in grain lupins.1. The effect of weeds on grain lupin yields. Aust. J. 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(Ed.), Proceedings of the 19th Australasian Weeds Conference, Hobart, Tasmania, 1-4 Sep 2014, pp. 278e281. Borger, C.P., Hashem, A., Powles, S.B., 2015. Manipulating crop row orientation and crop density to suppress Lolium rigidum. Weed Res. http://dx.doi.org/10.1111/ wre.12180. Boutsalis, P., Gill, G.S., Preston, C., 2012. Incidence of herbicide resistance in rigid ryegrass (Lolium rigidum) across south-eastern Australia. Weed Technol. 26, 391e398. Brennan, J.P., Lemerle, D., Martin, P., 2001. Economics of increasing wheat competitiveness as a weed control weapon. In: Proceedings of the 45th Annual Conference of the Australian Agricultural and Resource Economics Society, Adelaide, pp. 1e15. Broster, J.C., Koetz, E.A., Wu, H., 2013. Herbicide resistance levels in annual ryegrass (Lolium rigidum Gaud.) and wild oat (Avena spp.) in southwestern New South Wales. Plant Prot. Q. 28, 126e132. Burke, K., 2009. 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