Elsevier

Crop Protection

Volume 88, October 2016, Pages 28-36
Crop Protection

Weed management in maize using crop competition: A review

https://doi.org/10.1016/j.cropro.2016.05.008Get rights and content

Highlights

  • Crop competition includes row spacing, population and competitive cultivars

  • Weed management through crop competition is effective and economic

  • Crop competition results in considerable weed population decreases

  • Responses of weeds to crop competition are variable

Abstract

Weeds are a major constraint to crop production, and are responsible for considerable yield losses in maize production systems throughout the world. Herbicides are widely used for weed control in maize production systems, but can have negative environmental consequences. Researchers have evaluated the use of crop competition and suppression to manage weeds in various crop combinations, including maize-based systems. Crop competition in maize may involve techniques such as reduced row spacing, increased planting density, and the use of competitive cultivars that exhibit weed suppressive potential. In this review, examination of the literature has revealed the considerable value of using crop competition in integrated weed management programs. Research has demonstrated that narrowing row spacing to half the standard distance reduced weed biomass by 39–68%, depending on weed species. Researchers have also demonstrated that increasing maize planting density by up to twice the standard rate achieved a reduction in weed biomass of 26–99%. While little research has been conducted into the use of competitive maize cultivars for weed management, several studies have documented cultivars with potential to suppress weeds. Attributes of weed competitive cultivars include high leaf area index, and other elements of leaf architecture that improve light interception by the crop, so increasing the shading of weeds. Combining crop competition methods with other agronomic practices can increase their effectiveness in controlling weeds. For example, biomass of Setaria italica (L.) Beauv was reduced by 60% when maize planting density was increased by 1.5 times the recommended spacing, and this effect was more pronounced when fertilizer was banded rather than broadcast. In summary, the strategic use of crop competition to control weeds has been a success in many regions, and is an important tool in integrated weed management. The importance of crop competition methods has particular relevance where farmers are unable to afford herbicides, as making use of crop competition is more economical.

Introduction

Maize is one of the world’s major cereal crops, ranking third in importance after wheat and rice (Lashkari et al., 2011). In Southern and Eastern Africa, it is the main source of food and agricultural income for smallholder farmers. However, the amount of maize produced in the continent of Africa is below the world average (Fig. 1), and this is mainly the result of huge yield gaps due to poor weed management practices, coupled with low resource inputs (FAO, 2014, Lobell et al., 2009). Weeds regularly cause devastating maize crop losses (Bajwa et al., 2015). For example, they account on average for 50–90% of crop loss in Africa (Chikoye et al., 2005). For example, the invasion of maize fields in Africa by Striga asiatica (L.) Kuntze has been reported to cause total crop failure in some cases (Khan et al., 2008). Weed management in Africa suffers from low use of herbicides and mineral fertilizers, in addition to lack of available labour for weeding, often resulting in delays that defer weeding past the stage where it is possible to prevent economic damage (Nyamangara et al., 2014, Nyanga et al., 2012). In Africa, weed control is mainly carried out by hand hoeing, but this is only feasible on small areas due to emerging labour constraints in rural districts (Nyamangara et al., 2014). Moreover, declining soil fertility has led to the prevalence of devastating weeds such as Striga, Cynodon dactylon L., Richardia scabra L., which are difficult to control and cause severe crop losses (Reda et al., 2005).

In more developed parts of the world, such as Australia, which are characterised by higher agricultural inputs, farmers rely heavily on the use of herbicides to control weeds (CropLife/Grains Research and Development Corporation, 2008). Herbicides are an efficient tool in the control of weeds, and their proper use can reduce yield losses caused by weeds by up to 13% (Oerke and Steiner, 1996). In the USA, the use of genetically modified glyphosate resistant maize (Round-up® ready maize) accounts for about 10% of the total land under maize production (Gianessi, 2005). Glyphosate resistant maize allows for the use of glyphosate in controlling weeds throughout the season. Glyphosate-resistant maize has been shown to be economical compared to conventional cultivars, but their adoption in Europe and certain other parts of the world is low due to opposition to genetic modification, the availability of wide spectrum of crop alternatives, as well as environmental concerns associated with herbicide use (Gianessi, 2005).

Herbicides in variable herbicide groups, ranging from pre- to post-emergent, can be used in the efficient management of weeds, thus ensuring an all-season-round weed-free environment for crop production (Mathers and Parker, 2013). Some researchers have recommended the use of herbicides as being economical compared to mechanical weed control (Gianessi, 2014, Muoni et al., 2013). However, the over reliance on herbicides in developed regions has led to increased levels of resistance in certain weed species (Culpepper et al., 2004, Hall et al., 2014, Hull et al., 2014), making the use of herbicides more and more questionable now and in the future. In New Zealand, a spectrum of weeds in maize have been reported to be resistant to the commonly used herbicide atrazine, including Chenopodium album L. and Solanum nigrum L. (Mathers and Parker, 2013). These cases of resistance have led to calls for increased dosages that would contribute to increasing environmental pollution (Koch, 2010, Reganold et al., 2001). This is particularly relevant for the use of atrazine, which remains in the soil for many years (Helling et al., 1988).

Among the other possible means of minimizing resistance to herbicides by weeds is the avoidance of repeated use of herbicides from the same mode-of-action group (Mathers and Parker, 2013). However, this requires correct identification of the weeds, and a proper decision on which chemical to use, to achieve successful control of the weeds (Mathers and Parker, 2013). In developing countries where the choice of herbicides is limited, rotation to other herbicides is restricted. Despite the efforts made by chemical companies to reduce weed resistance, weed evolution towards resistance to chemicals is not at a standstill (Jasieniuk et al., 1996, Vencill et al., 2012, Délye et al., 2013, Matzrafi et al., 2014). Thus, weed management through herbicides is becoming more and more of a challenge due to weed resistance.

It is widely accepted that sustainability is key to increasing agricultural productivity over the long term, while conserving the environment. Crop production in the developing world is changing. For example, minimum tillage and residue retention are advocated by many researchers (Thierfelder and Wall, 2009, Guto et al., 2012a, Guto et al., 2012b, Sissoko et al., 2013). These changes have in turn resulted in shifting weed flora, requiring new strategies to control the emerging spectrum of weeds (Chauhan et al., 2012, Mhlanga et al., 2015). Some researchers have shown that the use of cover crops, and retaining their residues in cropping systems, is very efficient in controlling weeds. However, this can lead to a shift in weed flora, and the value for weed control is dependent on the performance of each specific cover crop (e.g. Mhlanga et al., 2015). Research has also highlighted some of the other challenges encountered with the use of cover crops, such as the preferences of the farmer and the availability of seed. In sub-Saharan Africa (e.g. in Malawi) where the land holding size is small, integration of cover crops may be difficult, as these would replace the main crops without giving a marketable return in the same year, thus limiting adoption of this practice (Mhlanga and Thierfelder, 2015).

Undoubtedly, with these yield losses caused by weeds, challenges faced in weed control, and the need to feed the ever-growing human population, there is need for a shift to more reliable and economic methods of weed control. Some researchers have advocated for ecologically-based weed management tools, as these have the potential to meet the challenges associated with conventional weed management (Chauhan et al., 2012, Chauhan, 2013). In smallholder ecological farming systems, where the use of herbicides is restricted, weeds are viewed as a source of diversification and groundcover. In such farming systems, weed control is effectively achieved through slashing with a sickle at a critical stage. This not only helps to maintain a healthy soil environment, but also increases the needed groundcover to reduce temperature extremes, protect the soil against splash erosion and conserve moisture.

The use of crop competition is a potentially valuable cultural weed control strategy in integrated weed management (IWM) programs (Mohammadi et al., 2012). The use of crop competition involves changes in agronomic practices and plant genetic make-up to reduce the competitive ability of weeds. These changes include (i) seed rate or plant density, (ii) use of competitive cultivars, (iii) row spacing, and (iv) intercropping, amongst others. A study by Mohammadi et al. (2012) demonstrated that decreasing maize row spacing, and increasing maize plant density, diminished weed productivity due to competition for light between the crop and the weeds. A separate study by Forcella et al. (1992) showed that with increased maize densities, it is possible to reduce the use of herbicides, thus reducing production costs and associated risks to the environment. However, the optimum population density will depend on the agro-ecological environment, especially the amount of rainfall received. Williams et al. (2014) concluded that maize planting patterns may not always have a significant impact on weed control, because suppression of weeds is very site and context dependent.

Studies have been conducted to investigate the effects of crop competition on weed populations separately and the results are variable across different environments, depending on the species composition of the targeted weed community and other environmental factors (e.g. Ghafar and Watson, 1983, Marin and Weiner, 2014, Mohammadi et al., 2012, Teasdale, 1998, Tharp and Kells, 2001, Tollenaar et al., 1994). A comprehensive review that integrates the results of these studies, to generate a better understanding of the importance of using crop competition in weed management in maize systems, would be helpful.

This review aims to integrate available global research on cultural weed management strategies in maize. In the studies selected, the same agronomic management practices were employed across all treatments (e.g., herbicide and fertilizer application), and the only difference was the type of cultural weed management method used (i.e., row spacing, planting density, and various competitive cultivars). To avoid repetition of facts, in cases where the same weed and same crop competition aspects appeared in similar environments but in separate studies, we only refer to one of the studies.

As stated in the previous section, weed management using crop competition incorporates the manipulation of certain agronomic practices, including maize row spacing (maize plant density and uniformity within rows), and the competitive ability of maize plants. The use of narrower row spacing, higher maize density, and competitive cultivars, however, depends on the situation and agro-ecology of the farm, especially the distribution of rainfall. The use of each aspect of crop competition has its own advantages, disadvantages, and ease of application. In regions of the world where rainfall distribution is erratic, narrower row spacing and more dense crop populations would result in increased intra-crop competition for limited resources (Hall et al., 1992). In low input areas, limited nutrient availability would also limit the applicability of narrow row spacing and increased planting density, due to the increased intraspecific competition among maize plants that would in turn lead to reduced yields. In such situations, the best approach might be the use of genetically improved competitive cultivars that are more tolerant to stresses presented by both intraspecific crop competition and weed competition (Nissanka et al., 1997). In these regions, increasing plant density may reduce yield significantly (Anderson, 2000).

Combining the aspects of crop competition with other agronomic practices such as precision fertilizer application, has more pronounced effects on weed reduction than each aspect on its own (Di Tomaso, 1995). For example, Anderson (2000) reported that banding fertilizer next to the seed coupled with narrower row spacing and higher maize density resulted in 60% reduction in weeds, compared to broadcasting fertilizer together with wider row spacing and lower maize density.

Crop competitiveness against weeds can be improved by use of higher crop population densities. Use of higher planting densities can accelerate canopy closure, thus promoting interception of radiation by the crop canopy and hence weed suppression (Andrade et al., 2002, Mashingaidze, 2004). As the population density of maize increases, the amount of light that reaches the soil is reduced, altering the microenvironment around the weeds and influencing their emergence, growth, and development (Teasdale, 1995). Stoller et al. (1979) reported that Cyperus esculentus L. can be difficult to control in maize, due to its efficiency in light utilization. In the same study, they reported that infestation by C. esculentus to a level of 100 shoots m−2 could cause up to 8% yield loss in maize. Amongst the possible means to control C. esculentus, increasing plant density of maize has the potential to promote its competitiveness against the weed through improved shading that can reduce weed growth.

Increasing the population density of maize by 300% (i.e., from 33,300 plants ha−1 to 133,300 plants ha−1) in a sandy loam soil heavily infested by C. esculentus (1600 plants m−2) was reported to have a positive effect on the suppression of C. esculentus (Ghafar and Watson, 1983). In two seasons, the density and biomass yield of C. esculentus between maize rows was reduced by 50% and 40% respectively (Table 1). The decrease in density and biomass of C. esculentus was attributed to the reduced amount of light reaching the C. esculentus plants. In a separate study carried out on silt loam soils in the USA, Abutilon theophrasti Medicus populations were reduced by 94% and 96% when maize populations were increased by 1.5 and two times, respectively, compared with the standard of 64,000 plants ha−1 (Teasdale, 1998) (Table 1). These reductions in A. theophrasti populations were attributed mainly to its lower positioning relative to the maize plants, resulting in reduction of light availability to the weed (Teasdale, 1998).

Anderson (2000) found that combining increased maize planting density (from 37,000 to 47,000 plants ha−1) with narrower row spacing (from 76 cm to 38 cm) lead to a decrease in abundance of Setaria italica (L.) Beauv over three years (Table 1). Light that was transmitted to the soil surface in the conventional system was 15–20% greater compared to that in the system with reduced crop rows and increased population density.

A previous study, carried out at Elora Research Station, investigated the effect of increased maize density on weed density and biomass under varying weed infestation levels (Tollenaar et al., 1994). Weed infestation levels were determined by varying the weed free period: weed free (no weeds all season), medium infestation (from planting to 5–7 leaf stage of maize), and high infestation (from planting to 3–4 leaf stage of maize). This cultural practice had variable effects on the weeds investigated (shown in Table 1), but increasing planting density ultimately led to a decrease in overall weed density and biomass (Table 1). In this study, increasing maize density from 40,000 plants ha−1 to 100,000 plants ha−1 under medium weed infestation decreased weed density and biomass by up to 25% and 50% respectively, while under high weed infestation, the effect was less pronounced (Tollenaar et al., 1994). These variations in the effect of crop density at different weed infestation levels was attributed to the growth stage of the weeds, and to some extend the weed composition during the period of sampling. Under high weed infestation, increasing maize density resulted in increased abundance of Amaranthus retroflexus L., and this may be due to the fact that A. retroflexus emerges and establishes best under lowered soil temperatures; conditions promoted by increasing crop density (Tollenaar et al., 1994). However, the higher abundance of this species did not result in a significant overall increase in weed density.

Weed control in situations of increased crop density is usually achieved through pronounced shading of weeds. The amount of light intercepted by maize plants will depend on the size of the crop canopy. Canopy size increases with maize growth stage, and plant architecture is determined by both the maize variety and its planting density. To have a better understanding of how maize growth stage and crop density will impact weeds such as A. retroflexus, McLachlan et al. (1993) carried out a study in Ontario. In the study, biomass of A. retroflexus was measured at three maize growth stages, and at three maize planting densities. The biomass of A. retroflexus decreased with increased plant density and with progressing growth stage of maize (Table 1).

Chenopodium album L. biomass was reduced by 43% when maize density was increased by 23% (i.e., from 59,300 to 72,900 plants ha−1) in a sandy soil in the USA (Tharp and Kells, 2001, Table 1). Further increasing the crop population by 29%–83,900 plants ha−1 resulted in a further 58% decrease in the weed biomass (Table 1).

Teasdale (1995) found that, when herbicide was applied at a standard rate, zero weeds emerged when row spacing was halved (from 76 cm to 38 cm) and doubled maize density (from 58,000 to 109,000 plants ha−1), compared with 3% weed cover at standard row spacing and planting density. In the same study, without any herbicide being applied, increased maize population density and reduced row spacing resulted in a 36% reduction in weed cover, including Panicum dichotomiflorum Michx., Eragrostis cilianensis (All.) E. Mosher, Eleusine indica (L.) Gaertn, Solanum ptycanthum Dun., and C. album (Table 1).

In a silty loam soil of low organic matter in the Kermanshah region of Iran, where Mollugo verticillata L., Lamium amplexicaule L., Solanum nigrum L., and Bromus tectorum L. are prevalent, Mohammadi et al. (2012) demonstrated that increasing crop population density and reducing row spacing are effective for weed control. In this study, maize was planted at narrow row spacing (from 75 cm to 25 cm), in combination with three planting densities (66,666 plants ha−1, 83,333 plants ha−1 and 99,999 plants ha−1). Progressive increase in plant density by 25% and 50% resulted in the decrease of total weed biomass by 26% and 37%, respectively (Table 1). Maize in narrower rows and denser populations have earlier canopy closure, thus presenting greater competition to emerging weeds. Light penetration is reduced, altering weed growth patterns and development (Knezevic et al., 1999). While research has demonstrated that increased seeding rate can be useful in weed management under various environmental conditions, careful planning is necessary to avoid excessive intraspecific competition, resulting in reduced yield. Future research would benefit from closer collaboration between breeders, agronomists, crop physiologists and weed scientists, to produce cultivars that are more tolerant to high population densities, while possessing physical traits that have a high suppressive effect on weeds. However, these cultivars may have negative effects on companion crops in cases where they are intercropped.

Row spacing influences canopy cover, which in turn determines the amount of light that can penetrate to the ground (Bradley, 2006). For example, at narrower row spacing, the amount of light that reaches the soil surface is reduced, thus altering the microenvironment of weed seeds and ultimately the number of weeds that will emerge (Bradley, 2006). Because maize has larger seedlings compared to the weeds, narrow row spacing results in a more uniform spatial distribution of the maize plants, so that the crop intercepts light more efficiently (Flenet et al., 1996). As reported in the previous section, the combination of 50% narrower row spacing (from 76 cm to 38 cm) and 27% higher maize planting density (from 37,000 to 47,000 plants ha−1) is an effective strategy in the control of weeds such as S. italica (Anderson, 2000, Table 2). The concept of utilizing spatial uniformity to control weeds is further explored in a study by Marin and Weiner (2014), in which maize plants were arranged in conventional rows and in grid patterns. Brachiaria brizantha Hochst. Ex A. Rich. biomass was up to 75% lower in the plots planted in a grid pattern, compared with conventional planting (Fig. 2). A study by Tharp and Kells (2001) showed that decreasing maize row spacing from 76 cm to 56 cm resulted in a 23% decrease in the biomass of C. album, while halving the spacing to 38 cm resulted in 27% decrease in weed biomass (Table 2).

In a study carried out on a clay soil in Minnesota (Johnson and Hoverstad, 2002), row spacing was coupled with various herbicide applications, to investigate their effect on populations of Setaria faberi. Herbicide treatments were either a) 3.0 kg active ingredient (ai) ha−1 acetochlor plus 1.1 kg ai ha−1 atrazine, applied pre-emergence; or b) a mixture of 42 g ae ha−1 imazethapyr and 14 g ae ha−1 imazapyr tank-mixed with 420 g ai ha−1 bromoxynil, applied at 5-, 10-, 20-, or 30-cm Setaria faberi Herrm. plant height. At narrower spacing and acetochlor and atrazine herbicide treatments, density of S. faberi was 98% higher than at wider row spacing (Table 2). This observation was attributed to greater interception of herbicide by the canopy at narrower row spacing, which resulted less of the herbicide reaching the target weed (Johnson and Hoverstad, 2002).

As highlighted in the previous section, the study by Mohammadi et al. (2012) showed that combining reduced row spacing (from 75 cm to 50 cm) with increased planting density has the potential to reduce weed biomass by 24% (Table 2). Murphy et al. (1996) documented a 16% reduction in biomass (from 377 g m−2 to 315 g m−2) of late emerging weeds (A. retroflexus, C. album, and Setaria viridis (L.) Beauv), by reducing row width from 75 cm to 50 cm (Table 2). This decrease in weed biomass was attributed to increased leaf area index in narrower rows. In narrow rows, photosynthetic photon flux density (PPFD) transmittance to weeds is reduced, resulting in decreased photosynthetic activity and hence lower weed biomass (McLachlan et al., 1993). Photosynthetic photon flux density is the amount of photosynthetically active light that falls onto the surface of plants.

Similarly, greater PPFD interception by the crop occurred in maize planted at 35 cm row spacing (i.e. 50% lower than the standard 70 cm spacing), resulting in lower photosynthesis by weeds, thus reducing weed biomass of weeds in a community consisting mainly of Setaria verticillata (L.) P. Beauv, Cyperus rotundus L., Cynodon dactylon (L.) Pers, Amaranthus quitensis Kunth, C. album, Datura ferox L. and Convolvulus arvensis L. (Acciaresi and Zuluaga, 2006). Without the use of herbicides, over the two seasons this study was carried out, total weed biomass was reduced by 68% under narrow row spacing conditions, compared to conventional wider rows (Table 2).

Dalley et al. (2004) investigated the effects of glyphosate application timing coupled with reduced row spacing in glyphosate-resistant maize. The results obtained by Dalley et al. (2004) were similar to those obtained by Acciaresi and Zuluaga (2006), with greater light interception by the crop being observed in the narrower compared to the wider rows. When glyphosate was applied sequentially at five different weed growth stages over four seasons, the greater light interception by the maize canopy resulted in 60% less weed biomass compared to wider rows (Table 2). In the same study, a single application of glyphosate at the beginning of the season, coupled with halved row spacing, reduced weed biomass by 39% over four seasons (Table 2).

Experiments on clay and sandy clay loam soils in Zimbabwe showed that narrowing maize rows has a positive effect on weed control (Mashingaidze et al., 2009). Maize was planted at the standard 90 cm spacing, and at two narrower row spacings (75 cm and 60 cm). The level of weed reduction was variable across the different row widths, but narrower rows resulted in reduced weed density and biomass (Table 2). Thus, this practice has a valuable role as part of an IWM program. Despite the weed control potential of narrowed row spacing published by other authors, Bradley (2006) reported that the suppressive effects of narrow row spacing on weeds are not always apparent, and tend to diminish over the course of the season. Again, there is need for future research focused on planting patterns that lead to high weed suppression while sustaining maize productivity.

Competitive cultivars are thought to confer some level of suppression on certain levels of weed infestations (Andrew et al., 2015). Traits of competitive cultivars include high light-intercepting leaf architecture, greater speed of development, enhanced partitioning of assimilates, improved plant height, and an ability to produce allelochemicals that inhibit weed growth and development (Christiansen, 1995, Wu et al., 1999). A lot of work has been done with other cereals such as wheat and barley, but limited work has been done in maize. For example, Lemerle et al. (1996) screened for wheat varieties that showed potential for suppression of Lolium rigidum Gaud. biotypes from across the world, and concluded that some wheat varieties (released earlier) had higher suppression potential. Ford and Pleasant (1994) identified certain maize cultivars that exhibited weed suppressive potential under high levels of weed infestation, and this was attributed to particular above ground traits, such as large leaf area, vertical leaf orientation, and greater plant height. These traits lead to improved light interception and thus shading off weeds, leading to a reduction in weeds.

As hypothesized by Weiner et al. (2010), weed suppression at increased weed densities may be higher under reduced maize phenotypic plasticity, spatially uniform conditions and smaller initial weed size. To extend this hypothesis, Marin and Weiner (2014) carried out a study in which the suppressive effect of three different maize cultivars (one traditional cultivar- Novillero, and two newer cultivars- Amarillo ICA V-305 and Híbrido HR Oro-Amarillo) on B. brizantha were investigated under grid pattern planting and conventional rows. The traditional cultivar, Novillero, suppressed weeds more than the two newer cultivars at various maize population densities and spatial arrangements (Fig. 2). The high suppressive ability of the traditional cultivar was attributed mainly to lower phenotypic plasticity, with lower variation in the angle of insertion of the oldest living leaf at harvest thus supporting the hypothesis.

In a study by Lindquist and Mortensen (1998), the suppressive effect of two newer maize cultivars on Abutilon theophrasti planted at different densities was compared with that of two traditional cultivars. The new cultivars had greater leaf area index (LAI) and PPFD, resulting in increased light interception. Thus, less light reached the weeds, reducing their productivity. All the studies support the idea that a genotype by environment (G × E) interaction has a more pronounced suppressive effect on weeds, thus there is need to shift cropping practices even when planting improved varieties. Breeding programs need to focus more on the phenotypic attributes of maize cultivars that maximize their suppressive effects on weeds, which can produce more economic weed management options.

Besides the demonstrated reduction of weed populations, improved cultural weed control methods have a positive effect on maize yields (e.g., Acciaresi and Zuluaga, 2006, Acciaresi and Zuluaga, 2006, Lindquist and Mortensen, 1998, Murphy et al., 1996).

Combined with other agronomic practices such as fertilizer placement, cultural weed control measures may be effective in reducing yield losses in maize. It has been shown that banding of fertilizer in a maize system with increased plant population density and reduced row spacing, had lower yield loss (13% versus 43%) compared with conventional row spacing and planting density, where fertilizer was broadcast (Anderson, 2000, Fig. 3). Increasing planting density can result in increased grain yields (Tharp and Kells, 2001) at a range of weed infestation levels (Tollenaar et al., 1994). However, a peak in production will occur where any further increase in crop planting density will lead to yield losses, as intraspecific competition becomes too strong.

Additionally, Teasdale (1998) showed seasonal differences in maize yields at different maize population densities (Fig. 4). Increasing maize planting density by 50% resulted in increased maize grain yield in one season only (1994), but resulted in decreased yields beyond that. In a drought season, yield decreased with increased crop density. The effects were less pronounced in the final season, with no differences between the density treatments (Fig. 4). This variation in maize yield between seasons revealed the potential for detrimental effects of higher planting density under resource-limiting conditions.

The suppressive effects of the more competitive maize canopy can contribute to reduced herbicide requirements, while at the same time controlling weeds effectively (Forcella et al., 1992). In a study by Teasdale (1995), the use of narrower rows and increased plant population with only 25% of the recommended herbicide rate showed no significant differences in weed control with where recommended herbicide rates were applied but under conventional row spacing and maize population.

The increase in grain yield when maize population density is increased, or when row width is reduced, is attributed mainly to the greater number of plants per unit area, resulting in more efficient exploitation of nutrients (Tollenaar et al., 1994). At higher planting density, maize is more evenly distributed in space, thus suppressing weeds, which gives maize a competitive advantage over the weeds leading to greater yields.

Section snippets

Conclusions

This review suggests that the use of crop competition as an integrated weed management tool can be effective. Conventional plant spacings depend on the recommendations of the region, but most researchers prefer reduced row spacing, which had significant suppressive effects on different dominant weeds. The level of reduction was variable depending on the specific weeds found in the communities, environmental factors and other management practices, but reduced crop rows usually resulted in lower

Acknowledgements

The authors would like to thank the International Maize & Wheat Improvement Centre (CIMMYT) and The University of Queensland (UQ) for providing a conducive environment in which to conduct this work. We gratefully acknowledge financial support provided by the CGIAR Research Program MAIZE.

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