Effect of herbicide application on weed flora under ... - cimmyt

Crop Protection 66 (2014) 1e7

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Effect of herbicide application on weed flora under conservation agriculture in Zimbabwe Tarirai Muoni a, *, Leonard Rusinamhodzi b, Joyful T. Rugare a, Stanford Mabasa a, Eunice Mangosho c, Walter Mupangwa b, Christian Thierfelder b a b c

University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare, Zimbabwe CIMMYT, P.O. Box MP 163, Mount Pleasant, Harare, Zimbabwe Ministry of Agriculture, Weed Research Team, Henderson Research Institute, Private Bag 2004, Mazowe, Zimbabwe

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 December 2013 Received in revised form 5 August 2014 Accepted 12 August 2014 Available online

Increased challenges of weed control in the smallholder farming sector of southern Africa have often resulted in small yields. The objective of this study was to evaluate the effects of different weed control strategies on weed flora and composition under conservation agriculture (CA) systems in Zimbabwe. This study was conducted at three on-station trial sites namely Domboshawa Training Centre (DTC), University of Zimbabwe farm (UZ farm) and Henderson Research Station (HRS) in a maizeesoybean rotation for four seasons from 2009e2010 to 2012e2013 seasons. Hand weeding was done whenever weeds were 10 cm tall or 10 cm in circumference for weeds with a stoloniferous growth habit. Weed identification was done up to the weed species level, and the ShannoneWeiner diversity and evenness index was used to determine the response of weed flora to herbicides. Results showed that there were more weeds in the early years which decreased gradually until the final season. Weed species diversity was not affected by herbicide application and the results indicated that weed species diversity was small in CA systems. Annual weed species constituted a greater proportion of species, and species richness decreased with the duration of the study. Richardia scabra L. and Galinsoga parviflora Cav. were the most common dominant weed species at all sites and in all seasons. Moreover, herbicide application had no effect on the evenness of weeds in the plots but site characteristics had a significant effect on the distribution of weed species (weed species evenness). The results presented in this study suggest that herbicide application facilitates a depletion of weed seed bank/number of weeds over time. Thus, herbicide application in CA has potential to reduce weed density, species richness and species diversity in the long term which may lead to more labour savings and larger yields. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Conservation agriculture Weed species diversity Weed density Herbicide application Weed flora

1. Introduction Weed management challenges in the smallholder farming sector have been reported as one of the major causes of low grain yields in southern Africa. Maize grain yield from smallholder farms averages less than 1 t ha1 and this is often not sufficient to support an average farming family (USAID/Zim-AIED, 2013). Weeds are more efficient in competing with crops for nutrients, water and space, and harbour pest and diseases that all have negative effects on yields obtained at the end of the season (Shrestha et al., 2002). Weed management by smallholder farmers has been practised using the mouldboard plough, for families with access to draft

* Corresponding author. Tel.: þ263 774311136. E-mail address: [email protected] (T. Muoni). http://dx.doi.org/10.1016/j.cropro.2014.08.008 0261-2194/© 2014 Elsevier Ltd. All rights reserved.

power, and through hand hoes by resource poor farmers (Muoni et al., 2013). Most of the smallholders in Zimbabwe use conventional tillage practices for field preparations and for weed control (Vogel, 1994). Although manual weeding using hand hoes is a common practice within smallholder farming, it is labour intensive and is often delayed leading to reduced crop yields (Mashingaidze et al., 2012). Conventional tillage practices often increase soil erosion rates leading to reduced soil quality such as poor soil porosity, nutrient loss and low organic matter content (e.g. Thierfelder and Wall, 2012). Poor soil nutrient statuses in combination with poor weed management practices often contribute to decreased yields. To alleviate this challenge, researchers have suggested a more sustainable method of farming, commonly referred to as Conservation Agriculture. Conservation Agriculture (CA) is defined as a farming system based on three interlinked principles which are (a) maintenance of

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T. Muoni et al. / Crop Protection 66 (2014) 1e7

a permanent soil cover through crop residues, (b) diverse crop rotations and (c) minimum soil disturbance (FAO, 2010). Conservation agriculture has potential to make more efficient use of natural resources through integrated management of soil, water and biological resources combined with use of external inputs (FAO, 2010). The use of crop residues helps retaining soil moisture which reduces the negative effects of mid-season dry spells common in southern Africa (Thierfelder and Wall, 2010). Residues can suppress weeds during the growing season if applied in sufficient quantity. Minimum soil disturbance and retention of crop residues reduce the rate of soil loss and increase soil biological activities (e.g. Dube et al., 2012). However, the complexity of weed control in CA systems increases due to an increase in perennial weed species (Gan et al., 2008). This has resulted in a general recommendation for increased use of herbicides in the early years of CA adoption (Wall, 2007). Herbicides have been reported to be effective and economically feasible in the smallholder farming sector where CA is being practised (Muoni et al., 2013). Herbicides have the ability to reduce substantially the weeding pressure but there are potential toxic side effects for humans and the environment (Kolpin et al., 1998). Among the recommended herbicides are glyphosate [N-(phosphonomethyl) glycine], atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and metolachlor (2-chloro-N-(2-ethyl-6methylphenyl)-N-2-methoxyl-1-methylethyl) that have different modes of action. Glyphosate is a non-selective systemic herbicide capable to control weeds that have underground rhizomes. Atrazine and metolachlor are selective herbicides that are applied before emergence of weeds and atrazine can also be applied after emergence of weeds and are both effective on broadleaved weeds and some grasses (Croplife, 2006). Rugare and Mabasa (2013) reported that the use of herbicides in CA reduced the variable cost of weed control by at least 21.8% and increased the marginal rate of returns by 306% compared to hand hoe weeding. Although many advantages of using herbicides have been documented, there is little information available on the longer term response of weed species to herbicide weed control strategies in CA systems under Zimbabwean conditions. Increasing the intensity of hand hoe weeding reduces the total weed density and the number of weed species that are observed in the plots (Mashingaidze et al., 2012). Crop rotations also facilitate weed suppression and there may be a different weed species response due to different rotational crops. Several tools can be used to investigate weed species diversity and evenness in a community such as the ShannoneWeiner index (H index for species diversity and E index for species evenness) (Grice et al., 2009). The ShannoneWeiner indices combine species richness (i.e. the number of weed species per area) and species equitability (i.e. how even is the number of species) (Nolan and Callahan, 2006). The hypothesis of this study was that herbicide application in combination with no-till, mulching and crop rotation will decimate the weed species and their density over time. Thus the objective of this study was to evaluate the effects of herbicide strategies on weed flora under conservation agriculture (CA) systems in Zimbabwe. 2. Materials and methods 2.1. Site description The experiments were established at three research locations namely Domboshawa Training Centre (DTC), Henderson Research Station (HRS) and University of Zimbabwe farm (UZ farm). All the three sites are located in natural region II of Zimbabwe and rainfall pattern is unimodal averaging 700e1000 mm per growing season. Rainfall starts in November and ends in April, and mid-summer

temperature ranges from 15.5 C to 25.0 C. Domboshawa 0 0 Training Centre (17 37 S, 3110 E and 1560 m above sea level (m.a.s.l)) is located on highly variable soils that are classified as moderately deep Luvisols and Arenosols, and these soils have approximately 5% clay content. Henderson Research Station 0 0 (17 34 S, 30 54 E and 1136 m.a.s.l) soils are classified as Arenosols according to FAO classification originating from granite rocks (Nyamapfene, 1991). The soils at HRS have a high sandy content (>83%) and are generally low in soil organic matter content 0 (Thierfelder and Wall, 2012). University of Zimbabwe farm (17 80 S, 0 31 50 E and 1503 m.a.s.l) is located on clay soils that have high soil organic matter and are classified as Chromic Luvisols under FAO classification (Nyamapfene, 1991). 2.2. Experimental design The experiment commenced in the 2009e2010 cropping season at all sites with maize as the test crop. The experiment was laid in a randomised complete block design (RCBD) with six treatments, replicated three times at all sites. The treatments were; i. Hand hoe weeding only. ii. Paraquat at 0.25 L ha1 a.i (active ingredient) at seeding plus hand hoe weeding. iii. Glyphosate at 1.025 L ha1 a.i at seeding plus hand hoe weeding. iv. Atrazine at 1.80 kg ha1 a.i at seeding plus hand hoe weeding. v. Glyphosate (1.025 L ha1 a.i) þ atrazine (1.80 kg ha1 a.i) at seeding plus hand hoe weeding. vi. Glyphosate (1.025 L ha1 a.i) þ atrazine (1.80 kg ha1 a.i) þ metolachlor (0.96 L ha1 a.i) at seeding plus hand hoe weeding. The recommended application rates for the different herbicides were used in this study and treatments with more than one herbicide where tank-mixed and applied at the same time. Manual hoe weeding was done whenever weeds were 10 cm tall or 10 cm in length for stoloniferous weeds, in circumference. A maizeesoybean rotation was deployed through the trial period. In 2009e2010 and 2011e2012, a uniform maize crop, using the maize variety Pristine 601, was seeded, whereas soybean (variety Safari) was grown in the 2010e2011 and 2012e2013 cropping season after the maize phase. In the maize phase, maize was grown using planting basins at UZ farm and rip lines at HRS and DTC, and maize harvest residues were used as ground cover at approximately 2.5 t ha1 in seasons 1, 2 and 4. In the third season, soybean crop harvest residues were retained and used as ground cover at approximately 1.5 t ha1. During the maize phase weeding was done up to four times at DTC and HRS in 2009e2010 season only whilst in 2011e2012 season and at UZ farm weeding was done only three times throughout the growing season. In the soybean phase weeding was done twice only. Maize was seeded at 0.9 m 0.25 m plant spacing to achieve a target plant population of 44,444 plants ha1. 150 kg ha1 of Compound D (11 kg N: 21 kg P2O5: 11 kg K2O ha1) was applied as a basal dressing at seeding and 150 kg ha1 of ammonium nitrate (52 kg N ha1) was split applied as top dressing at four and seven weeks after emergence. In the soybean phase, inoculated soybean (inoculated with Bradyrhizobia japonicum) was seeded at 0.45 m 0.05 m which translated to a target plant population of 444,444 plants ha1 and no herbicide was applied as initial weed control measure in soybean. A basal application of 150 kg Compound D (11 kg N: 21 kg P2O5: 11 kg K2O ha1) was applied by dribbling 90 g in every 10 m row. No top-dressing was applied to the soybean.


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Fig. 1. Effects of weed control strategies on weed density in four seasons (2009e10, 2010e11, 2011e12 and 2012e13) at Henderson Research Station, Domboshawa Training Centre and University of Zimbabwe. Bars with different letters mean that the weed control strategies were significantly different (P < 0.05) for each season.

The ShannoneWeiner evenness index was calculated using the following formula

2.3. Field measurements In all plots, measuring 6.0 m 6.3 m, weed counts were done prior to each weeding by randomly placing a 0.5 m 0.5 m quadrant four times in each plot at all sites. Weed counts were done by identifying the weeds in each quadrant then their total in the four quadrants was summed up for weed density. Weed identification was done at species level using guidelines outlined by Botha (2001) and, Makanganise and Mabasa (1999). Cyperus esculentus L. and Cyperus rotundas L. were classified as Cyperus spp due to difficulties in identifying them when they were still young. Cynodon nlemfuensis L. was recorded by counting the number of shoots that were observed in the quadrants. 2.4. Calculations and statistical analysis The ShannoneWeiner diversity (H) and evenness (E) indices were used to assess the species composition in each plot. The ShannoneWeiner diversity index was calculated using the following formula:

H ¼ N ln N

X

ðn ln nÞ=N

(1)

where H is the measured species diversity through proportional abundance of species, N is the total population density (m2), n is the population of each weed species (m2).

E ¼ H=ln N

(2)

where E is evenness of weed species and H and N are as explained earlier. The ShannoneWeiner diversity index value of zero indicates that there is only one species available (Nolan and Callahan, 2006). The larger the value, the higher the diversity of species within the area. Species evenness ranges from 0 to 1 and values close to 1 show that the species are uniformly distributed in the plots. All weed density, species diversity, richness and evenness data was subjected to normality and homogeneity of variance test and the data was normally distributed. The data was then subjected to the analysis of variance using Genstat 6th edition to assess the treatment effects on weed density, species richness, species diversity and species evenness at each site in all seasons. To assess treatment, crop, sites and season interactions a linear mixed model in Genstat 6th edition was used. In the model, treatments and sites were treated as fixed factors and season was treated as a random effect. Season was treated as a random factor because the quality of the season could not be determined experimentally. Mean separation was done using the least significant difference (LSD) test at P 0.05 on all significant data.

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3. Results 3.1. Effect of different weed control strategies on weed density Weed control strategies and site characteristics had a strong influence (P < 0.001) on the weed density. A strong crop treatment interaction (P ¼ 0.008) was observed in the linear mixed model analysis. At DTC, weed density was high in the first season and it decreased over the time (Fig. 1). Weed control strategies had a significant effect (P ¼ 0.0001) on weed density in 2009e2010 season. More weeds were observed in 2009e2010 season and manual weeding only had the largest weed density in that season (706 weeds m2). In the paraquat plus manual weeding treatment, most weeds were found in treatments with herbicides but the weed density in 2012e2013 decreased by at least 77% of the weed density recorded in the 2009e10 season. In the 2010e2011 season, weed control strategies had no significant effect on weed density under a soybean crop. In 2011e12 season, weed control strategies had a significant effect (P ¼ 0.0002) on weed density. The treatments atrazine plus manual weeding, and atrazine þ glyphosate and metolachlor plus manual weeding both had low weed density (60 and 63 weeds m2 respectively). Comparably more weeds were recorded in paraquat plus manual weeding and the atrazine þ glyphosate treatment (Fig. 1). In 2012e2013 season weed control strategies had no significant differences on weed density but weed density was very low in all treatments compared to other seasons. Furthermore, manual weeding, paraquat plus manual weeding, and glyphosate plus manual weeding had now similar weed densities. At HRS, weed control strategies had a significant effect (P < 0.05) on weed density in 2009e2010 season only and manual weeding only had the highest weed density (280 weeds m2) recorded and the lowest weed density was 69 weeds m2 in glyphosate þ atrazine plus manual weeding treatment (Fig. 1). In the 2010e2011, 2011e2012 and 2012e2013 season, weed control strategies had no significant effect (P > 0.05) on weed density at this site. It was noted that the 2012e2013 season had generally the lowest weed counts compared to other seasons. At UZ farm, weed control strategies exerted significant differences (P ¼ 0.0003) on weed density in 2009e10 season only and the largest weed density was recorded in manual weeding only (Fig. 1). Glyphosate plus manual weeding had 189 weeds m2 and the smallest weed density was recorded in glyphosate þ atrazine þ metolachlor plus manual weeding (78 weeds m2) (Fig. 1). The data for the 2010e2011, 2011e2012 and 2012e2013 seasons closely followed the data obtained at HRS for the similar seasons. 3.2. Effects of different treatments, crops and sites on weed species diversity and evenness under CA The results showed that site, crop grown and their interactions had a significant effect (P < 0.001, P ¼ 0.015 and P < 0.001 respectively) on weed species diversity in CA plots. Weed species diversity was low at all sites and in all seasons and the treatments had no significant differences on weed species diversity at all sites and in all seasons. At DTC the species diversity index was higher in the maize phase than in soybean phase. The same trend was observed at HRS. At UZ farm the maize phase had higher weed species diversity index than the soybean phase, as observed at all other sites. Soybean phases at UZ farm had higher weed species diversity indices than those recorded at DTC and HRS in all seasons. The results from the combined linear mixed model indicated that site had a significant effect on species evenness (P < 0.001). However, treatments and different crops grown (maize and soybean)

had no effect on the evenness of weeds in the plots. A significant site crop interaction was observed. The results indicated that species were not uniformly distributed in the plots at all sites and in all seasons. 3.3. Effects of different treatments and sites on weed species richness at all sites Weed species richness was greatly affected by site and treatments in the combined linear mixed model analysis (P < 0.001 and P ¼ 0.001 respectively). However, within sites and seasons, the analysis showed no treatment effects on species richness in all years except in 2011e2012 season at DTC. More weed species were observed in the early years of the study and the weed species decreased with the duration of the experiment. In the 2011e2012 season, atrazine plus manual weeding, glyphosate þ atrazine plus manual weeding and glyphosate þ atrazine þ metolachlor plus manual weeding had similar species richness. In the 2009e2010 season, the dominant weed species observed were Amaranthus hybridus L., Leucas martinicensis (Jacq.) R. Br, Cyperus spp, Conyza albida Spreng., Galinsoga parviflora Cav., Eleusine indica L., Richardia scabra L. and Commelina benghalensis L. In 2010e2011 season species richness and the number of dominant weed species decreased. In this season only five weed species were observed in large numbers and these were: A. hybridus L., G. parviflora Cav., E. indica L., and R. scabra L. In the 2011e2012 season, manual weeding, paraquat plus manual weeding and glyphosate plus manual weeding had similar number of weed species as recorded in 2010e2011 season. A further decrease in dominant species was also observed in 2011e2012 season where E. Indica L., G. parviflora Cav., R. scabra L. and C. benghalensis L. were the common weed species. In the 2012e2013 season the species richness in manual weeding, paraquat plus manual weeding and glyphosate plus manual weeding decreased by more than 50% and atrazine plus manual weeding had the highest number of species. Only two dominant species (G. parviflora Cav. and R. scabra L.) were recorded under manual weeding. At HRS weed control strategies had no significant differences on species richness in all the four seasons. Manual weeding, paraquat plus manual weeding and atrazine plus manual weeding had almost the same number of weed species (8 weed species). The herbicide combinations (glyphosate þ atrazine plus manual weeding and glyphosate þ atrazine þ metolachlor plus manual weeding) had the same number of weed species (6 weed species). Dominant weed species in this season were Cyperus spp, Digiteria sanguinalis L., E. Indica L., Bulbostylis hispidula Vahl and R. scabra L. The species richness decreased in 2010e2011 season and the smallest number of weed species (4 weed species) was recorded in glyphosate þ atrazine þ metolachlor plus manual weeding. Glyphosate plus manual weeding had higher species richness than all the other treatments. The dominant weed species in 2010e2011 season were D. sanguinalis L. and R. scabra L. In 2011e2012 season there was an increase in species richness and more weed species were observed in paraquat plus manual weeding (9 weed species). Also the number of dominant weed species also increased which included Dactylocterium aegyptium L., D. sanguinalis L., B. hispidula Vahl and R. scabra L. In 2012e2013 species richness decreased and only two species were dominant (D. aegyptium L. and R. scabra L.). Manual weeding only had the lowest species richness and atrazine plus manual weeding had more weed species. At UZ farm, weed control strategies had no significant effect on species richness in all seasons. In 2009e2010 season the number of dominant weed species was five which included A. hybridus L., G. parviflora Cav, Foeniculum vulgare Mill, L. martinicensis and R. scabra. The dominant species recorded in this season were lower


than the species recorded in 2009e2010 season, namely L. martinicensis (Jacq.) R. Br, F. vulgare Mill, G. parviflora Cav and R. scabra L. Although the species richness decreased in 2010e2011 season, it increased in the succeeding season (2011e2012) in all treatments except in paraquat plus manual weeding. Bidens pilosa L. was an additional dominant weed species that was observed. Species richness decreased in 2012e2013 season by more than 50% when compared to all the other seasons in all treatments and only L. martinicensis (Jacq.) R. Br, G. parviflora Cav and R. scabra L. were dominant weed species. R. scabra L. was a common weed species that was observed in all treatments and at all sites in all seasons. At DTC, G. parviflora Cav additionally was common and was observed in all seasons and in all treatments. At UZ, L. martinicensis (Jacq.) R. Br and G. parviflora Cav were the most common weed species besides R. scabra L. and all were observed throughout the study period. 4. Discussion 4.1. Effect of different weed control strategies on weed density at all sites and in all seasons The results showed that different weed control strategies (manual weeding and herbicides) reduced the weed density over time no matter which strategy was used. In manual weeding the decrease in weed density may be due to the suppressing effect of crop residues during the growing season. Crop residues help reduce weed seeds germination by impeding the light to reach the seeds and also facilitate suppression of the emerged weeds (Chhokar et al., 2007). Also weed seeds at the soil surface degrade faster under CA due to increased soil biological activities. Weed seeds at the soil surface are too exposed to predation hence, their chances of emerging are reduced thus reducing the weed density (Mwale, 2009). The decline in weed density over time can also be attributed to continuous weeding before weed plants set seed. This may have reduced the weed seed bank, which may have otherwise germinated under CA systems. The results concurred with Mashingaidze et al. (2012) who reported that increasing hand hoe weeding intensity significantly reduces weed density in the fields over time. The maizeesoybean rotation additionally contributed to the decline in weed density at all sites. Soybean has a high plant population per hectare and provides more ground cover and shading, which gives it a competitive advantage over weeds enabling two weeding to be sufficient for the whole season. Hence, rotational systems that include a legume with narrow row spacing have a greater potential of suppressing weeds in the long run as reported by Wall (2007) even in the absence of herbicides use. Residual herbicides such as atrazine provide longer weed control that help better reduction of broadleaf and annual grasses weed density within the seasons. Paraquat and glyphosate have no residual control effect hence their suppressive effect control is only effective if they are continuously applied during the season e.g. by using specialised equipment such as weed wipes or sprayers with shields. Controlling small weeds contributed to the decrease in weed density at all sites over time as weeds did not set seed, which may have substantially depleted the weed seed bank at the soil surface. This was previously observed by Mwale (2009). As no other seeds from the sub-soil are ploughed up under CA, there is hope that the use of chemical products like herbicides may only be temporarily necessary. Thus providing continuous year round weed control under CA is recommended to avoid weeds setting seed (ZCATF, 2012). A decrease in weed density also suggests that yield losses due to crop-weed competition is reduced and also less time is spent on weeding (Muoni et al., 2013).

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4.2. Effects of different treatments, crops and sites on weed species diversity and evenness under CA The results showed that site and crop as well as their interactions had significant effects on the weed species diversity index. This may be due to differences of soil types and the different ground cover between maize and soybean. Under soybean, fewer weeds were observed hence, the weed species diversity decreased. However, under the maize phase with reduced ground cover, weed species diversity increased. Herbicide treatments had no significant differences on weed species diversity because herbicides such as paraquat, glyphosate and metolachlor are non-selective and will control all present weeds. Although atrazine is selective against broadleaved weeds and some annual grasses, most controlled species that were recorded in this study were annual weed species. The presence of crop residues combined with effective hand weeding also suppressed weeds leading to low weed species diversity (Jones et al., 1999). Reduced weed species diversity may also be caused by minimum soil disturbance practised under CA, which reduces the chances of developing more complicated perennial weed species. This is due to reduced cultivation that favours weeds with protracted germination pattern (Chivinge, 1988). The results also indicated that sites had a significant effect on the evenness of weed species which may be due to different management practices that were practised before the establishment of the trials. The weeds were growing freely and allowed to set seed before the trial was conducted. The reduced weed species diversity means there is reduced crop-weed competition for water, space, nutrients and harbouring of pests that may lower the farmers' yields. A further decrease in weed species diversity may also suggest that the seed bank is getting continuously smaller with improved weed management, thus reduced herbicide dosages may be used to control weeds. The results also indicated that sites had a significant effect on the evenness of weed species which may be due to different management practices that were practised before the establishment of the trials. The weeds were growing freely and allowed to set seed before the trial was conducted. The unevenness of weeds in the plots enables spot weeding to be done in the fields. This also reduces the labour requirements and the herbicide quantity that may be needed in situations where weeds are evenly distributed. 4.3. Effects of different treatments and sites on weed species richness at all sites There was a decrease in species richness at all sites as the number of the seasons increased. This is due to the decrease in weed density through improved weed management practices that were done in the plots. The decrease can also be attributed to decrease of weed seeds in the soil thus some species were effectively controlled. The dominant weed species were annual and perennial species. All species that were recorded at all sites where small seeded and shallow germinators, and some species such as Cyperus spp reproduce by both seed and tubers (Botha, 2001). Weeds which produce small seeds increase in CA systems because they can germinate even when covered by crop residues (Makanganise and Mabasa, 1999). However, these weeds are also easily controlled when they are young because they have a shallow root system. Also the accumulation of the weed seeds at the soil surface increases their chance to germinate in one season and they are exposed to insect predation, fungal and bacterial attack thus depletion of the weed seed bank is high (Wagner and Mitschunas, 2008). In the soybean phase, species richness decreased because some weeds emerged later in the season or even failed to

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germinate when the ground cover was high. Hence, few weed species were recorded under the soybean phases. It is also possible that soybean could have suppressed weeds via allelopathy (Wang et al., 2010). Paraquat plus manual weeding and glyphosate plus manual weeding had similar species richness to manual weeding because paraquat and glyphosate have no residual control, hence they controlled weeds at planting only (Muoni et al., 2013). In treatments with atrazine, the species richness was small because of the longer residual control characteristic of the herbicides (Williams et al., 2010). This reduced the germination of most weed species (especially annual species). The dominant weed species at all sites and in all seasons was R. scabra L. which is a shallow germinator and has small seeds (Botha, 2001). Thus it can emerge even when covered by residues or with little soil cover, contributing to high weed density during the cropping season. Also R. scabra L. has protracted germination pattern that makes it germinate throughout the season especially after cultivation (Chivinge, 1988). Its first flush in the season is the highest to be experienced in the growing season and it only reproduced by seed only. Its appearance in all seasons could be due to its large number of seed in the soil that was depleted gradually during the course of the study. The weed is controlled by herbicides such as glyphosate, atrazine and metolachlor but hand hoe weeding is essential to control that has escaped the herbicide spray. Succeeding hand hoe weeding is necessary to control all emerging weeds before they set seed. Avoiding this weed from setting seed creates better chances of depleting it in the soil seed bank. R. scabra L. grow better in exhausted soils hence, with gradually increasing soil fertility under CA there are chances that the weed may not be observed in large numbers (Mavunganidze et al., 2009). 5. Conclusion The use of herbicides in combination with manual hoe weeding and applying all the CA principles facilitates decrease in weed density over time. Continuous weeding under CA promotes weed control before the weed species set seed, thus reducing of the number of weed seeds in the soil seed bank. Herbicide use together with crop residues reduces weed species diversity under CA systems. Rotating maize with soybean also reduces weed species diversity through suppressing weeds during the growing weeds. Weed species were not evenly distributed at all sites indicating dominance of some weed species under different soil types and environments. Improved weed management practices reduced the species richness at all sites. This suggests that herbicides, crop residues and hand hoe weeding played a key role in reducing the weed species that were observed at the onset of the research. Reduction in weed density, species diversity and evenness suggests that the use of herbicides may be reduced or discontinued after some time, which will be a great benefit for smallholder farmers. However, these findings require further assessments at on-farm level under farmer management practices. From this study it can be concluded that weed control involving herbicides are an important aspect in the control of weeds in smallholder farmer systems of southern Africa. However, the reduction in weed density and species composition in all weed control strategies under CA shows that with good and effective weed control, the weed seed bank can be depleted, which will reduce the labour requirements for smallholder farmers and costs for herbicides over time. Acknowledgements Gratitude is due to the International Maize and Wheat Improvement Centre (CIMMYT) under the CGIAR Research

Program (CRP) MAIZE for logistically supporting and to the International Fund for Agriculture Development (IFAD) for funding the research. Special thanks go to Mr Sign Phiri and Mr Herbert Chipara for technical support to make the research a success. Field staffs at Henderson Research Station and Domboshawa Training Centre are greatly acknowledged for their limitless effort during the course of the research. We would like to extend our gratitude to the anonymous reviewers for their constructive criticism. Special thanks go to Blessing Mhlanga, Regina Hlatywayo, Wadzanai Mvundura, Connie Madembo, Givemore Makonya, Siyabusa Mkuhlani and Jephias Mataruse for helping on the study. To Deniah Fadzai Nyereyegona and Muoni family, you are all special.

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