Introduction
Rice is the second most important cereal in the world after wheat in terms of production, with an average estimate of 520,000 kg being produced in 2022 (FAO 2022; Jones Reference Jones1995). Rice is a major source of dietary energy for many households in Africa (Macauley and Ramadjit Reference Macauley and Ramadjit2015), and Nigeria is the largest producer in West Africa, averaging 3.7 billion kg in 2018 (Soullier et al. Reference Soullier, Demont, Arouna, Lançon and Mendez del Villar2020). Nevertheless, rice consumption has risen steadily by 2.3% per year in Nigeria (Soullier et al. Reference Soullier, Demont, Arouna, Lançon and Mendez del Villar2020). Historically, rice cultivation in Nigeria began in the eighteenth century, when upland varieties were introduced to the high forest zone in western Nigeria (Grist Reference Grist1965). Rainfed agriculture is the main production system (Daramola Reference Daramola2005), and rice cultivation in lowlands is favored within the country (Daramola Reference Daramola2005) due to the regular flooding of fresh water from rivers, streams, and rainfall into swamps all year round (Herdcastle Reference Herdcastle1959; Imeokparia Reference Imeokparia1989). Approximately, 6.7 billion kg of raw paddy were produced from 3.1 million hectares in Nigeria in 2014 (FAOSTAT 2014), out of which 47% was cultivated in the lowlands. Smallholder farmers contribute largely to the total volume of rice cultivated. However, as cultivated lowland areas begin to expand (9.1% and 19.4% per year) in Nigeria and other sub-Saharan countries (Soullier et al. Reference Soullier, Demont, Arouna, Lançon and Mendez del Villar2020), grain yield has remained low, at 1400 kg ha−1 (Arouna et al. Reference Arouna, Devkota, Yergo, Saito, Frimpong, Adegbola, Depieu, Kenyi, Ibro, Fall and Usman2021). Several factors are responsible for such low yield, including the planting of low-yielding cultivars with high susceptibility to pests and diseases, poor agronomic practices (i.e., low plant population, non-uniform and wide spacing, low soil fertility, poor timeliness of operation, etc.), and above all, poor weed management practices (Dossou-Yovo et al. Reference Dossou-Yovo, Vandamme, Dieng, Johnson and Saito2020; Imeokparia Reference Imeokparia1989).
Globally, reductions in rice yield attributed to poor weed management range from 10% to 100%, depending on the type of weed flora and level of infestation (Akobundu Reference Akobundu1987; Chauhan Reference Chauhan2012). This is higher than combined losses from insect pests and diseases of rice in Africa (Akobundu Reference Akobundu1987). Yield reduction due to weed competition varies from country to country by weed species present, and agronomic practices (Arouna et al. Reference Arouna, Devkota, Yergo, Saito, Frimpong, Adegbola, Depieu, Kenyi, Ibro, Fall and Usman2021; Williams et al. Reference Williams, Roberts, Hill, Scardaci and Tibbits1990). Between 80% and 100% reduction in rice yield due to weed competition has been observed in Nigeria (Imeokparia Reference Imeokparia1994; Kehinde Reference Kehinde2002). Moreso, the high infestation of noxious weeds continues to favor severe pests (Rodenburg et al. Reference Rodenburg, Demont, Sow and Dieng2014) and disease infestation in many rice fields.
Apart from yield loss, the occurrence of high weed biomass during harvest indirectly incurs high harvesting costs and reduces rice quality (Akobundu Reference Akobundu1981). Also, labor scarcity can further delay the timely removal of weeds in Nigeria. According to Chikoye et al. (Reference Chikoye, Schulz and Ekeleme2004), a shortage of labor for weed removal is a major production constraint among smallholder farmers in the Northern Guinea Savanna. Another reason for the low yield in the lowlands is improper land leveling, which limits the effectiveness of flooding to suppress weeds under rainfed conditions (Akobundu and Fagade Reference Akobundu, Fagade, Buddenhagen and Barsely1978). Most paddy fields are situated along floodways; using such water resources to improve yield requires technical skills and heavy capital investment that is often unaffordable for farmers.
Despite these challenges, many weed control trials conducted in lowland ecologies across different agroecological zones in Nigeria have estimated a potential yield increase of 3,000 to 5,000 kg ha−1 (Akobundu Reference Akobundu1987; Alhassan et al. Reference Alhassan, Dadari, Babaji and Shebayan2015a; Imeokparia Reference Imeokparia1994; Imeokparia and Okusanya Reference Imeokparia and Okusanya1997; Imeokparia et al. Reference Imeokparia, Lagoke and Olunuga1992; Ismaila et al. Reference Ismaila, Kolo, Odofin and Gana2014; Okafor Reference Okafor1986). Although several attempts have been made to recommend and replicate such yield benefits on farmers’ fields, the outcome has not affected farmers’ yields. It is therefore important to highlight possible weaknesses, and also harness the potential of research recommendations to achieve a sustainable weed management program under changing climate conditions. Hence, the goals of this review are to highlight the strengths and weaknesses of recommended weed management practices for lowland rice in different agroecological zones and propose further research directions for sustainable weed management in Nigerian lowland ecology.
Predominant Weed Flora of Nigerian Lowland Rice Fields
Establishing a rice weed database (Rodenburg et al. Reference Rodenburg, le Bourgeois, Grard, Carara, Irakiza, Makokha, Kabanyoro, Dzomeku, Chiconela, Malombe, Sarra, Ekeleme, Mariko, Andrianaivo and Marnotte2016) is critically necessary to achieve sustainable rice productivity in Nigeria. Weed flora dynamics in lowland ecological systems are influenced by different agronomic practices, which vary from one location to another (Akobundu Reference Akobundu1987). However, a few weed species, such as in the genus Echinochloa and genus Cyperus, are now reported to be global prominent weeds in rice, irrespective of their cultures (Kraehmer et al. Reference Kraehmer, Jabran, Mennan and Chauhan2016). In Nigeria and other sub-Saharan African countries, weeds that exhibited high seeding output, high vigor, and aggressive morphological adaptation are difficult to manage in lowland rice fields with poor water control systems (Akobundu Reference Akobundu1981; MacDonald Reference MacDonald2003; Rodenburg and Johnson Reference Rodenburg, Johnson and Sparks2009).
Most of the reported weeds of Nigerian lowland ecologies are also weeds of global concern in rice (Kraehmer et al. Reference Kraehmer, Jabran, Mennan and Chauhan2016). Table 1 shows some prominent weed species across lowland ecologies in Nigeria. Prevalent grasses include junglerice [Echinochloa colona (L.) Link.], longstamen rice (Oryza longistaminata A.Chev. & Roehr.), Sacciolepis africana C.E. Hubb. & Snowden, and crabgrass (Digitaria horizontalis Wild).; broadleaves include tropical spiderwort (Commelina benghalensis L), swamp morningglory (Ipomoea aquatica Forsk), Ludwigia spp., and African mosquito fern (Azolla africana Desv).; and sedges include smallflower umbrella-sedge (Cyperus difformis L.), yellow nutsedge (Cyperus esculentus L.), purple nutsedge (Cyperus rotundus L.), and fimbry (Fimbristylis littoralis Gaudich). These weeds have resulted in significant yield losses (Rodenburg and Johnson Reference Rodenburg, Johnson and Sparks2009) and have been reported for decades, despite the use of various herbicides. Specifically, the invasion success of grassy weeds is relatively when soil conditions are poor and rice cropping systems are continuous (Alhassan Reference Alhassan, Dadari, Shebayan and Babaji2015b; Imeokparia Reference Imeokparia1989; Ogborn Reference Ogborn and Akobundu1978) due to increased tolerance of weeds to conventional agronomic practices. For instance, a survey of lowland rice weeds in the Sudan Savanna ecological zone of Nigeria shows that D. horizontalis constituted the highest total number of individual weeds and ranked highest in importance value index, followed by E. colona and other weed species (Alhassan et al. Reference Alhassan, Dadari, Shebayan and Babaji2015b). Perennial rhizomatous or tuber-bearing species such as S. africana, Leersia hexandra Sw., wild rice (O. longistaminata), and sedges of the genus Cyperus, are hypothesized to possess a higher tendency to survive adverse environmental conditions (Rodenburg et al. Reference Rodenburg, Meinke and Johnson2011) in African lowland ecosystems.
a References: Adeyemi et al. Reference Adeyemi, Adigun, Hosu, Fanawopo, Daramola and Osipitan2017; Ajala et al. Reference Ajala, Muhammad, Yakubu, Adamu and Busari2019a, 2019b; Akobundu and Ahissou Reference Akobundu and Ahissou1985; Alagbo and Akinyemiju Reference Alagbo and Akinyemiju2018b; Alhassan Reference Alhassan, Dadari, Shebayan and Babaji2015b; Bakare et al. Reference Bakare, Ndarubu, Ukwungwu and Ochigbo2008a, Reference Bakare, Ukwungwu and Aliyu2008b, Reference Bakare, Ukwungwu and Abdulrazak2008c; Daramola et al. Reference Daramola, Adigun and Olorunmaiye2020; Enyinnia Reference Enyinnia1993; Evbuomwan Reference Evbuomwan1992; Gbanguba et al. Reference Gbanguba, Ismaila, Kolo and Umar2011; Imeokparia Reference Imeokparia1994, Reference Imeokparia1989; Imeokparia and Okusanya Reference Imeokparia and Okusanya1997; Imeokparia et al. Reference Imeokparia, Lagoke and Olunuga1992; Ismaila et al. Reference Ismaila, Kolo, Odofin and Gana2015; Jiya et al. Reference Jiya, Daniya and Matthew Kolo2019; Kamai et al. Reference Kamai, Omoigui, Kamara and Ekeleme2020; MacDonald Reference MacDonald2003; Okafor Reference Okafor1986.
b Abbreviations: B, Broadleaf; G, Grass; S, Sedge; P, Perennial; A, Annual; √, Available weeds.
Understanding the growth pattern, ecology, and herbicide tolerance/resistance status of important weed species (Figure 1) in Nigeria is necessary to develop an appropriate weed management strategy within a changing climate. Only a few studies have identified the biology of common weed species in Nigerian lowland ecology (Imeokparia Reference Imeokparia1989; Yakubu and Ayeni Reference Yakubu and Ayeni2001). Furthermore, a good knowledge of the weed flora dynamics in specific lowland areas could be achieved by taking a periodic survey of the weed flora, weather conditions, and prevalent agronomic practices as they may affect weed distribution.
Figure 1. Nigerian map showing various agroecologies.
Chemical Weed Control
To cultivate rice on a large scale in Nigeria, chemical weed control represents a practical and economical alternative to hand weeding (Akobundu Reference Akobundu1981; Alhassan et al. Reference Alhassan, Dadari, Babaji and Shebayan2015a; Bakare et al. Reference Bakare, Ndarubu, Ukwungwu and Ochigbo2008a, Reference Bakare, Ukwungwu and Aliyu2008b, Reference Bakare, Ukwungwu and Abdulrazak2008c; Imeokparia and Okunsanya Reference Imeokparia and Okusanya1997; Jiya et al. Reference Jiya, Daniya and Matthew Kolo2019; Omovbude et al. Reference Omovbude, Kayii, Ukoji, Udensi and Nengi-Benwari2020). Selective pre-emergence herbicides such as pretilachlor plus dimethametryn, oxadiazon, and butachlor, and postemergence herbicides such as piperophos plus propanil, thiobencarb, and propanil have been recommended for weed control in lowland rice at varying rates in different agroecologies (Akobundu Reference Akobundu1987; Bakare et al Reference Bakare, Ndarubu, Ukwungwu and Ochigbo2008a, Reference Bakare, Ukwungwu and Aliyu2008b, Reference Bakare, Ukwungwu and Abdulrazak2008c; Enyinnia and Nwosu Reference Enyinnia and Nwosu1992; Jiya et al. Reference Jiya, Daniya and Matthew Kolo2019; Okafor Reference Okafor1981; Omovbude et al. Reference Omovbude, Kayii, Ukoji, Udensi and Nengi-Benwari2020). The timely application of these herbicides has reduced weed infestation more effectively compared to manual weeding. However, achieving season-long weed control with these herbicides remains a challenge (Adigun et al. Reference Adigun, Lagoke and Adekpe2005, Reference Adigun, Adeyemi, Lagoke, Olorunmaiye, Daramola and Babatunde2016). Weeds such as O. longistaminata, S. africana, Echinochloa spp., and Cyperus spp. have shown tolerance to recommended rice herbicides to date (Bakare et al Reference Bakare, Ndarubu, Ukwungwu and Ochigbo2008a, Reference Bakare, Ukwungwu and Aliyu2008b, Reference Bakare, Ukwungwu and Abdulrazak2008c; Imeokparia Reference Imeokparia1989; Jiya et al. Reference Jiya, Daniya and Matthew Kolo2019). Achieving season-long weed control with herbicides may further necessitate strategic integration of other weed management practices.
Along with the rising living standard in Nigeria, consumers’ demand for rice is shifting from quantity to quality. However, herbicide application has not been consistent with the recommended rate of herbicides. Farmers sometimes apply herbicides with little or no reference to the recommended rate (Alagbo and Akinyemiju Reference Alagbo and Akinyemiju2018b; Rodenburg et al. Reference Rodenburg, Johnson, Dieng, Senthilkumar, Vandamme, Akakpo, Allarangaye, Baggie, Bakare, Bam, Bassoro, Abera, Madiama, Dogbe, Gbakatchétché, Jaiteh, Kajiro, Kalisa, Kamissoko, Sékou, Kokou, Mapiemfu-Lamare, Lunze, Mghase, Maïga, Nanfumba, Niang, Rabeson, Segda, Sillo, Tanaka and Saito2019). For instance, glyphosate and a mixture of 2,4-D amine plus propanil (i.e., 47% and 65% respectively) were applied below recommended rates, whereas paraquat and 2,4-D (i.e., 34% and 200% respectively) were overused in lowland fields (Alagbo and Akinyemiju Reference Alagbo and Akinyemiju2018b). The latter suggests some levels of risk to soil (Kaur et al. Reference Kaur, Singh and Phutela2014), water, and aquatic life in Nigerian lowland ecologies. The environmental impacts of these conventional herbicides are rarely understood. Hence, there is a need to investigate farmers’ resource use efficiency in Nigerian lowland ecologies.
The state of farmers’ knowledge of available herbicides for rice production is unknown. Most farmers in sub-Saharan Africa lack expert advice on herbicide application (Rodenburg et al. Reference Rodenburg, Johnson, Dieng, Senthilkumar, Vandamme, Akakpo, Allarangaye, Baggie, Bakare, Bam, Bassoro, Abera, Madiama, Dogbe, Gbakatchétché, Jaiteh, Kajiro, Kalisa, Kamissoko, Sékou, Kokou, Mapiemfu-Lamare, Lunze, Mghase, Maïga, Nanfumba, Niang, Rabeson, Segda, Sillo, Tanaka and Saito2019). Tables 2 and 3 show some selected recommended herbicides for different agroecological zones in Nigeria, although it is still not clear whether these herbicides are available in the Nigerian market (Rodenburg et al. Reference Rodenburg, Johnson, Dieng, Senthilkumar, Vandamme, Akakpo, Allarangaye, Baggie, Bakare, Bam, Bassoro, Abera, Madiama, Dogbe, Gbakatchétché, Jaiteh, Kajiro, Kalisa, Kamissoko, Sékou, Kokou, Mapiemfu-Lamare, Lunze, Mghase, Maïga, Nanfumba, Niang, Rabeson, Segda, Sillo, Tanaka and Saito2019). To increase rice yield sustainably, consistent evaluation of new rice herbicides against prevalent weeds is necessary, and such technology could be extended to farmers.
Table 2. Herbicides recommended for lowland rice in Nigeria. a
a Abbreviations: RF, Rain Forest; SGS, Southern Guinea Savanna; SS, Sudan Savanna; POST, Postemergence; PRE, Preemergence.
a References: Akobundu Reference Akobundu1981; Alhassan et al. 2015; Enyinnia Reference Enyinnia1993; Evbuomwan Reference Evbuomwan1992; Imeokparia Reference Imeokparia1990, Reference Imeokparia1994; Imeokparia et al. Reference Imeokparia, Lagoke and Olunuga1992; Okafor Reference Okafor1986; WSSA 2010.
b Abbreviations: AHAS, acetohydroxyacid synthase; ALS, acetolactate synthase; EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase; PPO, protoporphyrinogen oxidase.
Cultural Weed Control
Water Requirement for Weed Management
Maintaining optimum water levels in a uniform, leveled field (where such is achievable) has been widely adopted to suppress weeds under anaerobic field conditions. Good water management is essential to minimize weed infestation in lowland rice because it significantly reduces the germination of weed seeds (Akobundu Reference Akobundu1987; Ismaila et al. Reference Ismaila, Kolo, Odofin and Gana2015). Timing, duration, and flooding depths will determine the degree of weed suppression under anaerobic conditions (Ismaila et al. Reference Ismaila, Kolo, Odofin and Gana2015). In Nigeria, chemical weed control has proven to be effective under irrigated field conditions in the past (Imeokparia Reference Imeokparia1990) when integrated with good land preparation and proper water management (IITA 1974). Most lowland fields where rice is cultivated in Nigeria are rainfed, situated along coastal plains, and characterized by undulating land terrains without bunds or good drainage systems. Under these conditions, the use of water as a control strategy is difficult due to poor flooding control (Akobundu Reference Akobundu1981; Akobundu and Fagade Reference Akobundu, Fagade, Buddenhagen and Barsely1978). For instance, in southeastern Nigeria, the adoption of flooding as a weed management option under hydromorphic soil conditions is unrealistic due to impeded drainage (Enyinnia Reference Enyinnia1993; Pons Reference Pons1982). Such uncontrolled drainage tends to favor the high proliferation of problem weeds (Enyinnia Reference Enyinnia1993) such that the cost of land clearing is high, particularly for subsistence-level farmers, and troublesome weed seeds are carried over to the next cropping season.
Weed control trials in Nigeria must factor in the poor drainage systems in major lowland ecologies. Effective weed management can be achieved through timely weeding interventions (other than flooding) at an early growth stage until rice is fully established to suppress weed growth at later stages (Akobundu Reference Akobundu1981, Reference Akobundu1987; Ismaila et al. Reference Ismaila, Kolo, Odofin and Gana2014).
Use of Weed Competitive Cultivars
Crop competitiveness, being one of the most important features in achieving long-term weed control (Ferrell et al. Reference Ferrell, MacDonald and Brecke2006; Saito et al. Reference Saito, Sokei and Wopereis2012), is a function of a cultivar’s weed-competitive ability to maintain a high yield despite severe weed pressure (Jannink et al. Reference Jannink, Orf, Jordan and Shaw2000; Saito et al. Reference Saito, Sokei and Wopereis2012). Rice cultivars differ in weed competitiveness (Haefele et al. Reference Haefele, Johnson, M’Bodj, Wopereis and Miezan2004) in different ecologies (Saito et al. Reference Saito, Sokei and Wopereis2012). To reduce a farmer’s reliance on herbicides for weed control in water-limited lowland environments, the use of weed-suppressive cultivars has been suggested (Binang et al. Reference Binang, Ekeleme and Ntia2011; Gibson et al. Reference Gibson, Hill, Foin, Caton and Fischer2001; Saito et al. Reference Saito, Sokei and Wopereis2012) in combination with other favorable agronomic practices such as timely weed removal and good tillage operation. Rice cultivars that compete well against weeds are often tall with rapid vegetative growth, droopy leaves, and high leaf area (Akobundu and Ahiosu 1985; Binang et al. Reference Binang, Ekeleme and Ntia2011; Haefele et al. Reference Haefele, Johnson, M’Bodj, Wopereis and Miezan2004; Saito et al. Reference Saito, Sokei and Wopereis2012). For instance, the drooping leaves and higher tillering ability of the ‘NERICA 1’ cultivar resulted in good canopy formation, which contributed to its suppression of weed growth and higher grain yield compared to other NERICA cultivars (Kolo and Umaru Reference Kolo and Umaru2012).
Unfortunately, studies on weed-competitive lowland rice cultivars in Nigeria are limited. ‘FARO 55’ (Usman et al. Reference Usman, Kolo and Oladiran2013) and ‘FARO 59’ are the only known cultivars with weed suppressive traits used in Nigeria (Oluwaseyi et al. Reference Oluwaseyi, Nehemmiah and Zuluqurineen2016). Alternatively, cultivar mixtures have proven effective in suppressing weeds in lowland rice crops in Nigeria (Binang et al. Reference Binang, Ekeleme and Ntia2011). For example, cultivar mixtures of ‘Muduga’ and ‘FARO 15’ at ratios of 1:4 and 3:2 produced significantly strong rice yields due to intraspecific competition among cultivars for light and growth resources and thus capable of reducing weed density (Binang Reference Binang, Okpara, Shiyam and Ntia2010). The competitive potentials of such cultivar mixtures against weeds now depend on the choice of cultivars and transplanting intervals coupled with other agronomic practices (Binang Reference Binang, Okpara, Shiyam and Ntia2010). To address the fast-growing problem of herbicide tolerance in rainfed lowland ecology, the role of rice cultivars in their ability to suppress weed growth in various agroecologies (Figure 1) should be studied further, and promising cultivars exploited for sustainable weed management in water-limited lowlands.
Cropping Pattern
The potential of good cultural practices to effectively suppress weeds, when used in combination with other weed control methods, cannot be underestimated in terms of reduced labor costs, weed aggressiveness, pest and disease control, and seeding input (Oyewole et al. Reference Oyewole, Ajayi and Ojuekaiye2010). Crop competitiveness with weeds can be improved by varying seed rates or seeding methods of rice (Hay and Walker Reference Hay and Walker1989; Zimdahl Reference Zimdahl2004). For instance, low seeding rates of rice using productive cultivars ensures the establishment of vigorous, competitive, and highly productive tillers that are capable of suppressing weeds at reduced seeding costs (Chauhan Reference Chauhan2012; Gopal et al. Reference Gopal, Jat, Malik, Kumar, Alam, Jat, Mazid, Saharawat and McDonald Andrew2010; Hay and Walker Reference Hay and Walker1989). Crops are also known to gain more competitiveness over weeds when established in high densities (Danmaigoro et al. Reference Danmaigoro, Ishaya and Ibrahim2016). Studies have also shown that narrow interrow spacing (15 to 20 cm) effectively suppresses weeds (Ahmed and Moody Reference Ahmed and Moody1980; Akobundu Reference Akobundu1976; IRRI 1978) when combined with at least one weeding early in its life cycle. Such weed management strategies could result in increased yield with limited labor costs. However, optimizing such cropping practices among farmers may be limited due to non-uniform transplanting, wrong weeding timing, and the lack of technical knowledge, which remains a major bottleneck in bridging rice yield gaps in Nigeria.
In the Sudan Savanna agroecology (Figure 1) of Nigeria, the broadcast seeding method is reported to aggregate higher weed biomass when compared with the dibbling-and-drilling method (Alhassan et al. Reference Alhassan, Dadari, Babaji and Shebayan2015a; Ismaila et al Reference Ismaila, Kolo, Odofin and Gana2015; Phuong et al. Reference Phuong, Denich, Vlek and Balasubramanian2005). On the other hand, increasing seeding rates in lowland rice fields aid the formation of a dense leaf canopy, which helps to suppress weed growth at a later growth cycle (Arce et al. Reference Arce, Pedersen and Hartzler2009; Mahajan et al. Reference Mahajan, Gill and Singh2010). It is a common practice that most farmers broadcast rice in a lowland field in a bid to reduce production costs. Increasing seed rates under such a situation may reduce weed pressure. However, such a result may be difficult to achieve in drought stress conditions (Kirkland et al. Reference Kirkland, Holm and Stevenson2000). Studies on the effective practice of narrow spacing and increased seeding rate coupled with the best seeding method could be further evaluated under water-limited lowland field conditions across agroecological zones in Nigeria.
Crop Rotation
Weeds compete well with crops that have similar growth requirements, such that any cultural practices designed to enhance crop yield may also benefit the growth and development of weeds (Omafra Reference Omafra2006). Therefore, in a continuous monocropping system, adopting the best chemical and cultural practices as a weed management strategy may be unfavorable in the long term. The practice of crop rotation can significantly reduce the density of peculiar or problematic weed species in any continuous cropping system (Ball Reference Ball1992; Gbanguba et al. Reference Gbanguba, Daniya, Kolo, Ibrahim, Ismaila and Umar2020; Marenco et al. Reference Marenco, Maria and Santos1999). For instance, a six-time reduction of the weed seed bank was reported in a rotated cropping system when compared with a monocropping system in a study conducted over 7 yr (Forcella and Lindstrom Reference Forcella and Lindstrom1988). Hence, synergizing crop rotation with other control practices (where it is possible) is key to developing a long-term weed control program across water-limited rainfed lowlands in Nigeria.
Rotation of vegetables and other arable crops (Gbanguba et al. Reference Gbanguba, Ismaila, Kolo and Umar2011) in the late season followed by rice the following season could be beneficial in displacing herbicide-tolerant weed species in lowland ecosystems. For example, studies from the Guinea Savanna area in Nigeria have shown that cassava/legume intercrop, followed by lowland rice, reduced weed population and weed dry matter, and increased rice yield when compared with higher proliferation of troublesome weeds (L. hexandra, E. colona, and E. stagnina) when lowland rice was grown after a long fallow period (Gbanguba et al. Reference Gbanguba, Ismaila, Kolo and Umar2011, Reference Gbanguba, Daniya, Kolo, Ibrahim, Ismaila and Umar2020). In addition, rotating other crops with rice in the lowland ecology may benefit increased weed diversity, such that herbicide efficacy and other weed management strategies are enhanced in the long term. Despite such potentials of crop rotation as an integrated weed management (IWM) approach, there is a paucity of information on the economics and yield benefit of crop rotation for sustainable weed control in rainfed lowlands. Hence, on-farm field demonstration exercises may be necessary to actively engage farmers on the need to complement other weed management strategies with crop rotation in rainfed lowland fields in Nigeria.
Timing of Operations
A major setback in the implementation of an IWM strategy is the poor timing of operations (Alagbo and Akinyemiju Reference Alagbo and Akinyemiju2018a). Weed control at an early crop growth stage is important to enhance rice yield (Ajala et al. Reference Ajala, Muhammad, Yakubu and Sanda2019b; Nojima Reference Nojima, Matsunayashi, Ito, Tarase, Nomoto and Yamada1966). Weeds are better controlled during periods of crop establishment (when crop plants and weeds are tender) provided planting or transplanting takes place within a short time after land preparation (Adigun et al. Reference Adigun, Lagoke and Adekpe2005; Imeokparia and Okusanya Reference Imeokparia and Okusanya1997). For instance, early weed interference in irrigated rice could result in potential yield loss if not addressed (Okafor Reference Okafor1982). Unfortunately, poor smallholder farmers underrate the timeliness of operation (Alagbo and Akinyemiju Reference Alagbo and Akinyemiju2018a). Rice must remain weed-free during the critical period of biomass development (Ajala et al. Reference Ajala, Muhammad, Yakubu, Adamu and Busari2019a) to support panicle initiation and grain filling. Once the tillering is adversely affected by weeds, the yield will be reduced (Ajala et al. Reference Ajala, Muhammad, Yakubu and Sanda2019b; Imeokparia and Okusanya Reference Imeokparia and Okusanya1997). Achieving such timeliness would imply that farmers have the required weed control inputs ahead of time. Therefore, the timely intervention of government and nongovernmental organizations (NGOs) in providing an enabling platform for smallholder farmers to explore the available IWM technology will successfully bridge the rice yield gap, thus, enhancing rice productivity.
Physical Weed Control
Tillage Practices
Land preparation (either mechanical or manual) is important to provide the weed-free seedbed needed for effective weed control at the initial cropping stage before broadcasting or transplanting rice (Jiya et al. Reference Jiya, Daniya and Matthew Kolo2019; Okafor Reference Okafor1986). The knowledge and feasibility of good tillage practices among rice farmers are crucial for sustainable weed management in the lowland.
For decades, the practice of no-till systems has been common among smallholder rice growers in Nigeria. It serves as an alternative to conventional or minimum tillage, which is believed to be labor and capital intensive due to the scarcity of tractors (Ajah Reference Ajah2014) and increased cost of manual operations. Many studies have supported that high labor costs and weed density reductions could be achieved in lowland rice if a no-till system is adopted (Akobundu Reference Akobundu1987; Jiya et al. Reference Jiya, Daniya and Matthew Kolo2019; Magani and Shave Reference Magani and Shave2011; Zakaria and Dzomeku Reference Zakaria and Dzomeku2018). Particularly, the use of pre-plant (nonselective) herbicides such as glyphosate and paraquat to clear vegetation is widely adopted in Nigeria for this purpose (Akobundu Reference Akobundu1987; Alagbo and Akinyemiju Reference Alagbo and Akinyemiju2018a). Under such practices, the increased prevalence of noxious perennial grasses (Table 1) is becoming more serious. A no-till system is predicted to result in a shift in the weed community structure over a while (Buhler Reference Buhler1992). The successful proliferation of perennial weeds under no-till conditions has encouraged a low weed species diversity in lowland ecosystems (Sans et al. Reference Sans, Berner, Armengot and Mäder2011; Shrestha et al. Reference Shrestha, Knezevic, Roy, Ball-Coelho and Swanton2002). This possible eco-shift in the weed community under no-till situations is still not addressed in most rice fields in Nigeria. Hence, introducing minimum or deep tillage practices may further enhance the burial of surface seeds and better efficacy of herbicides and/or alternative weed management approaches.
Mechanical Weed Control
Introducing mechanical weed control in lowland rice fields is necessary to complement conventional weeding practices (herbicide and hand weeding). Manual mechanical weeding tools such as rotary weeders can help farmers to save weeding time (Rodenburg et al. Reference Rodenburg, Saito, Irakiza, Makokha, Onyuka and Senthilkumar2015) and remove herbicide-tolerant weeds. For instance, rotary weeders such as straight-spike and twisted-spike floating weeders were reported to reduce weeding time by 32% to 49% and by 32% to 56%, respectively, with better weed control efficacy compared with hand weeding (Rodenburg et al. Reference Rodenburg, Saito, Irakiza, Makokha, Onyuka and Senthilkumar2015). Nevertheless, the adoption of mechanical weeders under rainfed field conditions may be challenging (Senthilkumar et al. Reference Senthilkumar, Tesha, Mghase and Rodenburg2018) considering poor leveling of land, inadequate flooding, and non-uniform seeding practices by smallholder farmers. A recent study from Tanzania shows that more than 80% of farmers adopt rotary weeders under irrigated rice fields compared with 20% in rainfed lowlands (Senthilkumar et al. Reference Senthilkumar, Tesha, Mghase and Rodenburg2018). Similarly, farmers in Nigeria also encountered difficulty in adopting the ring hoe technology in upland rice fields due to non-uniformity in seeding, even though it was perceived to be best in saving weeding time (Johnson et al. Reference Johnson, Rodenburg, Tanaka, Senthilkumar, Ahouanton, Dieng, Klotoe, Akakpo, Segda, Yameogo, Gbakatchetche, Acheampong, Bam, Bakare, Kalisa, Gasore, Ani, Ablede and Saito2019). These suggest that the easy adoption of mechanical weeders could boost good agronomic practices where capital infrastructures such as rice seeders and transplanters are procured for farmers to seed or transplant rice in a uniform manner. It is therefore important for the government and other NGOs to provide infrastructural support in rural farming communities.
Precision Weed Management
Precision weed management is gaining wider acceptance globally because it promises a significant reduction in herbicide load in the environment (Gerhards Reference Gerhards and Stafford2019). Solving the problem of suboptimal or over-usage of herbicides (Alagbo and Akinyemiju Reference Alagbo and Akinyemiju2018a; Rodenburg et al. Reference Rodenburg, Johnson, Dieng, Senthilkumar, Vandamme, Akakpo, Allarangaye, Baggie, Bakare, Bam, Bassoro, Abera, Madiama, Dogbe, Gbakatchétché, Jaiteh, Kajiro, Kalisa, Kamissoko, Sékou, Kokou, Mapiemfu-Lamare, Lunze, Mghase, Maïga, Nanfumba, Niang, Rabeson, Segda, Sillo, Tanaka and Saito2019), and the long-term persistence of problem weed species in rainfed lowland fields would further necessitate the development of smart spraying technology for future farms in Africa. In high-income countries, offline weed mapping data in connection with a decision support system have been deployed for site-specific weed spraying in winter cereals resulting in higher yield and better weed control efficacy compared with conventional spraying (Gerhards and Quebel Reference Gerhards and Oebel2006).
Recent studies in Nigeria show that deep-weed, learning-based algorithms such as fuzzy logic (Olaniyi et al. Reference Olaniyi, Daniya, Kolo, Bala, Olanrewaju and Mikail2020) and a single-shot multi-box detector (SSD) network models (Olaniyi et al. Reference Olaniyi, Daniya, Abdullahi, Bala, Olanrewaju, Masrour, Cherrafi and Hassani2021) have shown high precision and accuracy in making weed management decisions and thus showing their potential for smart spraying in lowland rice. The SSD neural network-based algorithm resulted in 86% accuracy, 84% precision, and 92% sensitivity performance in object (weeds and rice) differentiation at varying resolutions (Olaniyi et al. Reference Olaniyi, Daniya, Abdullahi, Bala, Olanrewaju, Masrour, Cherrafi and Hassani2021). The fuzzy logic-based expert system depends on weed density signals (thresholds) to make herbicide spraying decisions in lowland rice with a mean absolute error (decision accuracy) of 0.9 (Olaniyi et al. Reference Olaniyi, Daniya, Kolo, Bala, Olanrewaju and Mikail2020). Both studies demonstrate the possibilities of developing an autonomous weed management system for rice production in sub-Saharan Africa.
Integrated Weed Management
IWM systems are perhaps the most appropriate weed control strategy for resource-poor rice farmers in the tropics. Combining various measures (i.e., cultural, chemical, physical, etc.) to control weeds at a subsistence level depends on the technical efficiency of adopted agronomic practices, availability of farm inputs to time, and the economic status of individual rice growers. Any time lag, deficiency, or absence of either or any of these automatically forfeits the possibility to optimize IWM principles for effective weed control at the subsistence level. Such deficits possibly explain the increased yield gap between researchers’ and farmers’ yields over decades (Daramola Reference Daramola2005). Harnessing the potential of recommended IWM strategies under different lowland ecologies across agroecological zones in Nigeria could sustainably bridge the yield gap.
In Nigeria, cultural practices such as land preparation, crop rotation, and adoption of a weed-competitive cultivar are most widely recommended practices in lowlands in combination with an appropriate chemical weed-control method (Ajala et al. Reference Ajala, Muhammad, Yakubu, Adamu and Busari2019a, Reference Ajala, Muhammad, Yakubu and Sanda2019b; Danmaigoro et al. Reference Danmaigoro, Ishaya and Ibrahim2016; Imeokparia and Okusanya Reference Imeokparia and Okusanya1997; Jiya et al. Reference Jiya, Daniya and Matthew Kolo2019). Emphasis given to each weed control method would depend on the peculiarity of rice ecosystems across each agroecological zone (Figure 1) in which rice is cultivated. For example, the cultivation of rice under hydromorphic soil conditions resulted in the proliferation of problem weeds (Imeokparia and Okusanya Reference Imeokparia and Okusanya1997). Hence, the need for a weed-suppressive IWM strategy.
A farmers’ participatory research engagement is necessary to ascertain the potential of IWM under field conditions. It is equally important that necessary infrastructure facilities and farming inputs are supplied for the timely implementation of field operations.
Conclusions and Recommendations
It is clear that poor flood control systems combined with continuous cropping practices have benefited a shift in weed species from annuals to stable perennials that continually threaten lowland rice ecosystems. Sustainable weed management intervention in lowland ecology systems requires smart integration of cultural, physical, and chemical weed control methods that can mitigate heavy weed pressure in the long run (i.e., weeds are drastically suppressed, and yield is significantly enhanced).
To achieve better weed suppression and enhanced rice yield in Nigerian rainfed lowland environments, incorporating regular rotation with legumes (Gbanguba et al. Reference Gbanguba, Ismaila, Kolo and Umar2011, Reference Gbanguba, Daniya, Kolo, Ibrahim, Ismaila and Umar2020) could reduce the pressure of problem weeds and enhance the biodiversity of weed flora. The practice of conventional or minimum tillage operations as a substitute for no-till practices would further allow the burial of perennial weed propagules and heavy weed seed deposits within shallow soil depths. Promoting such tillage methods would enhance the emergence of uniform weed seeds and promote higher efficacy of the recommended postemergence and preemergence herbicides. Such a technique is necessary to reduce weed pressure and enhance the vigor of weed-competitive cultivars at early growth stages.
Uniform plant spacing, precise seeding rate, and seeding methods are prerequisites for ease of adopting mechanical weeders and optimum plant population as target weed suppression. Availability of rice seeders or a transplanting machine is needed for precise seeding in this case rather than the usual manual transplanting or broadcasting.
Future Outlook
The weed community structure in the key rice-producing areas in Nigeria must have changed with time due to poor agronomic practices. Unfortunately, there are few documented surveys of weed flora dynamics (i.e., current weed species richness, abundance, and diversity) across agroecological zones. Knowing the weed flora dynamics is necessary for site-specific weed management precision. On the other hand, new studies on the biology and ecology of existing problem weeds with multi-propagative mechanisms are needed to guide farmers to adopt appropriate control strategies.
Existing or newly released rice cultivars should be screened for site-specific weed competitiveness across agroecological zones in Nigeria. It is important to validate recommended herbicides with those present in the market (Rodenburg et al. Reference Rodenburg, Johnson, Dieng, Senthilkumar, Vandamme, Akakpo, Allarangaye, Baggie, Bakare, Bam, Bassoro, Abera, Madiama, Dogbe, Gbakatchétché, Jaiteh, Kajiro, Kalisa, Kamissoko, Sékou, Kokou, Mapiemfu-Lamare, Lunze, Mghase, Maïga, Nanfumba, Niang, Rabeson, Segda, Sillo, Tanaka and Saito2019). Future studies of herbicide field trials should factor in poor water management systems under rainfed, inundated, and hydromorphic lowlands. Furthermore, the environmental impacts of herbicides on nontarget lowland communities should be investigated (i.e., future studies should focus on herbicide persistence status in lowland soil, water, and rice grain quality).
Acknowledgement
We thank Dr. Femi Ajibola for proofreading the manuscript. The authors received no grants from any funding agency, or commercial or nonprofit organization. The authors declare no conflicts of interest.
