Weed science research achievements on maize in Ethiopia: a review

Major weeds of maize

In maize cropping systems, previous different farm management practices such as crop rotation, fertilizer application, herbicide usage and ploughing frequency are major factors affecting weed species types, distribution and infestation. On the other hand, weed flora are influenced by the current crop population density, sowing geometry, growth duration, growth rhythm, moisture period and fertility status (Ali et al., 2016; Muluadam and Demisew, 2021). The moist and/or hot climate of the maize-growing belts of Ethiopia encourage rapid and abundant growth of weeds, consequently the crop fields are usually heavily infested with the weed plants. Various weed species considerably impede maize production, but their importance varies across maize producing agro-ecologies in the country.

Around 88 major weed floras under 23 plant families have been reported in maize across various localities (Table 1). Aboveground weed flora of 62 species in Abobo district of Gambella (Dinberu et al., 2011), 21 species in Bako (Megersa and Fufa, 2017), 15 species in Jimma (JARC, 2021) and six dominant weed species in north western Ethiopia (Takele et al., 2022) were recorded. According to the findings, majority of the encountered maize associated weed species were annual (71) as compared to perennial (17). Similarly, broad leaved weeds (66) outnumbered grass (18), sedge (2) and parasitic (2) weeds in the crop fields.

Impact of weeds on maize

Weeds infestation immensely affects maize production and productivity in Ethiopia. Weeds force farmers to spend their time on managing them; otherwise, they cause a considerable loss and this sadly is a common feature observed in sorghum and maize growing areas of the country (Muluadam and Demisew, 2021).

Previous studies showed that weeds reduced maize yields by 35–87.5% depending on the crop cultivar types, density and sowing geometry; weed types, density and duration of competition; moisture period, fertility status, and tillage practice (Ali et al., 2016; Muluadam and Demisew, 2021). Weeds interference on maize production system could be in different forms such as competition, allelopathy and/or parasitism.

Competition

Ethiopia, as one of the tropical arid and semi-arid countries confronts constraints due to a high temperature, erratic rainfall pattern, high rate of evapotranspiration, and fast and vigorous growth of weeds in crops (Lulseged et al., 2015; Muluadam and Demisew, 2021). The rapid and vigorously growing weeds aggressively compete with crop plants for scarce soil resources like moisture and organic matter, and sunlight. In maize, weeds compete for growth factors and the crop yield losses due to competition have been estimated at different localities. Keeping maize weed-free throughout the cropping season is preferred to avoid the possible impacts, but considering the management cost it needs to determine the critical period of weed competition. A study by Muluadam and Demisew (2021) showed that maize and sorghum crops are highly sensitive to weed competition, especially during their early growth stage.

Weed rating inversely impacted yield (b = −0.5; p < 0.001) where fields with the lowest weed rating had the highest yield (4.6 t·ha^-1) than those with the highest rating (2.3 t·ha^-1) (Lulseged et al., 2015). Accordingly, critical weed-free periods of 25–30 days after sowing (DAS) and optimum weed control periods of 30–50 days after emergence (DAE) have been developed and recommended for major weeds of maize, respectively. The critical timing of parthenium removal to avoid 5% and 10% maize grain yield loss was 8 and 17, and 13 and 23 DAE during year 2012 and 2013, respectively (Safdar et al., 2016). The relatively shorter critical period of parthenium competition in maize crop suggested its high allelopathic effect in addition to its competitive behavior.

Allelopathy

Allelochemicals such as phenolics, alkaloids, fatty acids, indoles, terpens and others liberate from weeds through leaf leachates, decomposition of plant residues, volatilization and root exudates that affect crop productivity (Ali et al., 2016). Many studies have revealed the allelopathic effect of different weed species on maize seed germination, stand establishment, growth, yield and the physiology of the crop plants (Table 2). Allelopathic water extracts of Plantago lanceolata, Anagallis arvensis, Medicago polymorpha, Phragmites australis, Ammi visnaga, Silybum marianum, Malcolmia africana, Emex spinosa, Calendula arvensis, Fumaria indica, Convolvulus arvensis and Rumex crispus showed toxicity against maize, sunflower and wheat germination (Khan et al., 2012). Aqueous extracts of all plant parts of Parthenium hysterophorus (shoot, leaf, inflorescence, root and whole plant part) inhibited the germination and growth of maize, demonstrating the allelopathic potential of the weed plant in the crop field (Anteneh, 2015). Residues of leaves, stems, roots and whole plants of Polygonum hydropiper, Amaranthus spinosus, Cyperus esculentus, Cyperus rotundus and Imperata cylindrical, and their mixtures imposed an inhibitory effect on the emergence of maize seedlings (Ali et al., 2016).

When added to soil at 5% and 10% concentrations, the inhibition of Lantana camara, Ageratum conyzoides and Eupatorium adenophorum residues on the emergence and shoot growth of wheat, rice and maize were reported by Ali et al. (2016). Eucalyptus (Eucalyptus camaldulensis) leaf extracts (20–100% of a 50 g/L solution) in laboratory and powders (0.5–3% w/w base) in greenhouse were found to have allelopathic effects on seed germination and seedling growth of the tested poaceous crops including maize (Awadallah and Eman, 2017). Similarly, pigweed (Amaranthus viridis) above-ground parts have allelopathic effects on seed germination and seedling growth of the tested poaceous crops including maize (Awadallah et al., 2017). According to Abiyu and Nagappan (2015); and Wakshum et al. (2018), Lantana camara leaf powder significantly inhibited seed germination, speed of germination, shoot and root length, stem thickness and biomass of wheat and maize. Moreover, Calotropis procera (sodom apple) leaves have allelopathic effects on seed germination and seedling growth of the tested poaceous crops including maize (Awadallah and Salwa, 2019).

Allelopathic potential has been also found in many crops like maize, rape seed, niger seed, sunflower, alfalfa, sorghum and wheat, and extracts of these crops contain allelochemicals that release from dead and decaying parts of the plant body (MuahmmadWaheed et al., 2013). If wisely planned, the application of allelopathy in cropping systems is quite effective in managing agricultural pests and improving the productivity of agricultural systems. The extract of maize root residues has been found effective as bio-herbicide in the control of Bidens pilosa (Modupe and Joshua, 2013). Several research findings have found that maize plant produces a number of secondary metabolites that inhibit weed plant growth at high concentrations. Having an allelopathic effect, the incorporation of E. globulus residues as green manure into soil could be a feasible practice to reduce the reliance on synthetic herbicides in maize-based cropping systems (Carolina et al., 2013).

Parasitism

Noxious root parasitic weeds like Striga spp. (witch-weeds) are widely spread in semi-arid regions of Africa including Ethiopia. The Striga hermonthica and S. aspera in northwestern and S. asiatica in the highlands of the eastern and southern parts of the country are threatening maize production. They are resulting in a complete crop loss under the worst conditions. According to Nagassa and Belay (2021), Striga spp. cause growth inhibition and yield losses of 20–100% in maize, sorghum, rice, pearl millet, finger millet and sugar cane. Prodigious seed production, prolonged viability of the seeds in soil and the subterranean nature of the early stages of parasitism in the host crops field make the control of the parasite difficult by conventional methods (Berhane, 2016).

In general, the occurrence of different weed species types and their infestation level vary from field to field mainly based on past farming history on a given field. The heavy infestation of weeds undoubtedly has considerable impacts on the germination, growth, production and productivity of crop plant. The different forms of weed interference on the crop plant indicate that weed management is a critical factor for maize production. Hoeing, hand weeding and pre-emergence herbicides application are the most frequently used weed control practices to minimize effect of weeds in maize fields of the country. However, so far none of them has been found effective and sustainable in controlling weeds in maize fields.

Weed management research results

Weeds are always present in the farming system and often the most significant pest of maize in Ethiopia. The level of weed pressure is important to the profitability of the crop production. The carryover effects of proper management practices might be a means of reducing the early weed load and helping to reduce the crop production costs. Adequate weed management knowledge and technology availability are crucial for proper reduction of the impacts of weeds on the crop production. Maize cropping practices such as rotation, proper land preparation, growing of improved varieties, intercropping and soil fertility management are determinant for the effectiveness of the crop production. According to Musa et al. (2014), the impact of technical inefficiency of maize farmers is as high as 66%. It needs to design research and extension packages based on the model farmers’ best practices.

Tesfaye et al. (2014) reported that proper weeds management in maize can increase the crop yield by up to 96%. Timely weeding makes the management effort effective than late weeding. It is labor constraint due to overlapping of activities that prevents timely weeding. When weeding delayed, costs of weeding increase and grain yield decline. Research is currently ongoing in the lowland tropics to determine the effect of reducing weed seed production by late weeding, zero tillage, and off-season management on weed pressure. Trials on the effect of farm practices such as crop rotations, intercropping, growing resistant or tolerant varieties, and maize-weed management are also being conducted (WSRS, 2015).

Cropping system: crop rotation and/or intercropping

Maize–haricot bean intercropping suppressed weeds, and was more productive and cost-effective than sole crop production in southern Ethiopia (Workayehu and Wortmann, 2011). Use of conservation agriculture-based maize–legume cropping systems was increased soil organic matter, total nitrogen, available phosphorus, and soil water infiltration rate by 37, 54, 12.3 and 327%, respectively, when compared to conventional tillage with maize mono-cropping over three years (Yayeh et al., 2022). By the conservation agriculture-based maize–haricot bean intercropping systems, weed infestation was reduced over the years and maize grain yield increased by 54% over the conventional tillage. Similarly, intercropping maize with forage legumes such as cowpea, silver-leaf and velvet bean has the potential to restore soil fertility and control weeds without reducing the crop grain yield (Marilyn et al., 2022).

Growing resistant and/or tolerant variety

Sowing an early emerging, fast-growing and competitive maize variety shall be made while cultivating the crop on weed-infested fields. Crop competition in maize may involve the use of competitive varieties that exhibit weed suppression potential. Weed-competitive varieties have a high leaf area index, and other leaf architecture elements that improve light interception by the crop, increasing the weeds shading capacity (Blessing et al., 2016). Another area that has been largely overlooked is the problem of parasitic weeds. Striga spp. inflict considerable losses on maize production, and no resistance has been found within the maize genome unlike for sorghum. Before releasing maize varieties in striga endemic areas, its genotypes must be screening for resistance and/or tolerance to the weed species.

Mechanical control; pre-plant tillage and/or inter-row cultivation

Farmer practices such as proper ploughing and inter-row cultivation are not given strong attention for weed management. The primary objectives of tillage are to prepare a smooth seedbed, alleviate soil compaction and reduce weeds. The clean seedbed is essential to reduce the weed seed bank in soil and reducing post-planting weed pressure. Lack of oxen energy for tillage, force farmers to practice usually minimum tillage that results in high grass weed infestation, thus allowing low labor for land preparation results with the high weed pressure. According to Bakhtiar et al. (2011), the harvested maize grain yield was considerably higher by conventional tillage (2429 kg·ha^-1) than zero-tillage (2271 kg·ha^-1). The authors also indicated that wheel-hoe weeder is found more effective than manual and oxen labor particularly on properly tilled light soils that the implement can control 85% of the maize weed species.

Cultural control; hoeing, hand-pulling or mulching

Unlike many other pests, weeds can be controlled through hand weeding before they damage the crop if sufficient labor is available. Because of the limited resource base, the subsistence farming community relies on hand weeding for the control of weeds. As a result of the overlap of farm operations during the crop season; however, farmers either leave their farms un-weeded or perform weeding late in the season. According to Bakhtiar et al. (2011), a considerably higher grain yield of 2863 kg·ha^-1 was recorded by practicing hand weeding compared to plastic or live plants mulching (2145 kg·ha^-1) or weedy check (1422 kg·ha^-1). However, besides weed pressure reduction, water conservation by mulching makes maize more resilient to the high rainfall variability occurring in some parts such as the Central Rift Valley of Ethiopia (Sime et al., 2015).

Good crop husbandry

The planting time, seeding pattern, rate and spacing; and fertilizer application played a significant role on maize yield (Lulseged et al., 2015). Lately planted maize experiences greater weed problems, nitrogen deficiency and water deficits usually result in lower grain yields. Then the maize yield reduces markedly and progressively as the time of planting delayed after the onset of the rain. The early-sown crop plants benefit by receiving full-season rainfall, suffering less weed competition and a high soil nitrate level at the beginning of the rains which subsequently declines due to leaching and a slower rate of humus decomposition. The actual utilization of nitrogen by the crop is highly affected by the weedy conditions of the field under which urea is applied.

Competition in maize may involve techniques such as reduced row spacing and increased planting density that exhibit weed suppressive potential. The row-sowing of maize saves seeds and fertilizer, makes weeding and other agronomic practices easier, and then provides high crop yields. Moreover, increasing the maize plant population from 30,000 to 60,000 plants per hectare was increased the crop yield from 2055 to 2412 kg·ha^-1 (Bakhtiar et al., 2011). The row planting of maize preconditions band-application of fertilizers that favors the crop growth more than inter-row weeds and also enhances productivity.

In Ethiopia, fertilizer application is a good input in improving the maize crop husbandry and farmers was mainly relied on commercial fertilizers to sustain livelihood security. Application of 150 kg·ha^-1 NPS (blended nitrogen, phosphorus and sulfur fertilizers) at sowing and 250 kg·ha^-1 urea splinted into two; half at knee height and the remained half before flowering stages are recommended for efficient weed control and maize growth improvement (Takele et al., 2022). Farmyard manure applications at the rate of 4 to 5 t·ha^-1 also suggested to favor the crop plants more than weeds. Presently, the unaffordable price of inorganic fertilizers has become a burning issue in the fertilizer-use habituated crop production system of the country, indicating the importance of farmyard manure utilization.

Chemical control

Herbicides are preferred over manual weeding when they alleviate labor tension and/or increase yield by controlling weeds better. The effectiveness of various combinations and rates of post and pre-emergence herbicides were compared for their effect on weeds and productivity of maize in Ethiopia. Weed control in maize with herbicides have been suggested by many researchers and research centers of the country (Firehywot et al., 2013; Tesfaye et al., 2014; Megersa and Fufa, 2017; AmARC, 2020; JARC, 2021). The herbicide evaluation woks were compared and confirmed the reliability and promising potential of some products in effectively controlling weeds and enhancing yield of maize (Table 3).

Firehywot et al. (2013) found that twice hand weeding, atrazine or s-metolachlor application at 2 L·ha^-1 resulted in higher weed control efficiency (> 94%) than other treatments in maize fields (Table 4). The highest maize grain yield (4664 kg·ha^-1) was obtained with twice hand weeding and hoeing followed by atrazine 2 kg·ha^-1 (4534 kg·ha^-1) and s-metolachlor 2 kg·ha^-1 (4490 kg·ha^-1) application which were increased by 56, 55 and 54% over uninterrupted weed growth throughout the crop season. In addition, Megersa and Fufa (2017) reported that application of pre-emergence herbicides (Lunar 537.5 EC at 3 L·ha^-1, Venus 500 SE at 6 L·ha^-1 and s-metolachlor + atrazine at 3 L·ha^-1 with 200 L·ha^-1 water solution) considerably reduced weed infestation compared to weedy check plot there by boosted maize grain yield. JARC (2021) reported that pre-emergence herbicide Gesparim combi at a rate of 3.5 kg·ha^-1 and Lasso + atrazine at 5 L·ha^-1 supplemented with one hand weeding at 50–55 DAE kept the maize field weed-free throughout the season and gave the highest maize grain yield (5,593 kg·ha^-1). In addition, the report indicated that s-metolachlor+ atrazine at 4 L·ha^-1 and alazine 35/20 SE at 5 L·ha^-1 were found effective in managing weeds in maize fields of the area.

Integrated weed management strategy

In maize fields, integrated management strategy aimed at combining the economically feasible, environmentally safe and socially acceptable weed control options (Takele et al., 2022). This could be the incorporation of two or more weed control methods such as prevention, cultural, biological and/or chemical control to reduce their interference while maintaining acceptable yields of the crop. If various weed management components are integrated in a systematic manner, significant advances in weed management technology can be achieved.

Gebisa and Gressel (2007) reported that a combination that included resistant varieties, fertilizer and tied-ridge gave significantly higher sorghum yield followed by one that combined local varieties with fertilizer and tied-ridge in north Wollo. In semiarid areas of the Central Rift Valley, conservation agriculture by covering soil with crop residues reduced soil water evaporation, erosion as well as degradation; water run-off and weed problems, and then improved soil fertility, water infiltration, soil biological activity, soil water availability and maize-bean productivity (Liben et al., 2017).

Similarly, glyphosates such as herbazed, round-up 36 Sl and weedknock at 2 L·ha^-1 rate mixed in 200 L·ha^-1 water on 15–18 days before planting application supplemented by hand weeding on 40 DAS provided efficient weed control and then considerably boosted maize grain yield at Bako (Megersa et al., 2018). Twice-hand weeding and a plant density of 53,333 plants per hectare were found to be suitable practices for attaining optimum grain yield for the hybrid maize BH-546 in Assosa district (Getahun et al., 2018).

Furthermore, application of s-metolachlor + atrazine at 2 L·ha^-1 in combination with 65 cm inter-rows spacing (IRS) found as the best option for effective weed management in maize (Megersa et al., 2020). The integration of narrower IRS with hand pooling plus hoeing improved weed control efficiency there by enhanced maize grain yield (Table 5).

The increasing incidence of striga has been attributed to poor soil fertility, low soil moisture and the intensification of land use through continuous cereal cultivation. Thus, potentially successful approaches developed to control this weed include rotation with trap crops or legumes, applying pre-emergence herbicides, growing resistant or tolerant varieties, sowing clean seeds, intercropping with legumes, soil amendments with fertilizer or manure, water conservation, use of push-pull technology and effective microbes (Fusarium oxysporum and arbuscular mycorrhizal fungi) as biological control agent (Nagassa and Belay, 2021). Hence, to successfully reduce infestation pressure and the impact of the parasitic weed, an integrated management strategy needs to be developed considering the ecological and socio-economic conditions of the major maize growing areas of the country.

Research gaps and future directions

Weed species distribution and interference is dynamic causing consistent maize yield reduction in Ethiopia. However, there is no strong weed science research activity, and then latest information, knowledge and technology that can assist to adequately manage maize weeds in the country. Parasitic and allelopathic effect of weeds are often overlooked, yet they are likely important as potential stress on maize plant physiological functions and productivity. So far there are few individual weed control methods that ineffectively adopted by maize growers, and there is luck of dependable integrated weed management strategy.

The dynamic nature of weed species’ demands intensive weed science research endeavor in Ethiopia. In the absence of the latest information, knowledge and technology on appropriate weed management measures, it is hardly possible to maximize the grain yield of maize. It could be good to frame future avenues of research that could help to move studies of competition, parasitism and allelopathy into the brooder ecological context. In general, the weed management research at representative major maize growing locations shall be strengthened within the context of developing an integrated weed management approach that considers the social, economic and agro-ecological conditions of a specific locality. Moreover, it needs to upgrade the knowledge and capacity of smallholder farmers by promoting improved weed management technologies.