Rice (Oryza sp.) is a cereal with great importance in the world. It is the second most cultivated cereal, and the third most consumed and exported in the world (FAO, 2013). Senegal is the second-largest importer of rice in West Africa after Nigeria (MAH, 2004). These imports continue to create a disequilibrium in the trade balance of the country with currency outflow of about 100 billion CFA per year (Gueye, 2004). To mitigate these adverse effects on the economy, the State took the option of developing irrigated rice cultivation in the regions of Saint-Louis and Kolda and the consolidation of rainfed rice cultivation in the south (Ndione, 2010). In 2014, Senegal made self-sufficiency in rice a priority through PRACAS (program to accelerate the pace of agriculture in Senegal) (MAER, 2014). However, biotic and abiotic constraints constitute the actual brake of rice production. Among these biotic constraints, weeds constitute a crucial problem, and their control is a fundamental element for the enhancement of production factors (Rodenburg and Johnson, 2009). Uncontrolled evolution of weeds can lead to a total loss of rice production, although such circumstances are exceptional (Johnson, 1997). In Africa, these losses are estimated at 2.2 million tons per year for an estimated cost of 700 billion CFA francs (Rodenburg and Johnson, 2009; Le Bourgeois et al. 2014; Ayodele and Olubunmi, 2017). Therefore, weed management constitutes a significant concern of Senegalese farmers (Noba, 2002; Mbaye, 2013). However, to design the weeds control strategy, it is imperative to characterize the weed flora and understand its chemical, physical, mechanical, and manual strategies management. In lowland and rainfed rice production systems at Casamance (southern Sénégal), herbicides are a profitable investment low fertility soils (Posner and Crawford, 1991). However, in the valley of the Senegal river, the resistance of certain weeds to herbicides is still increasing (Mballo, 2019). Thus the choice of the variety of rice and the method of sowing is a way to make the crop more competitive against weeds (Saito et al. 2010; Rodenburg and Johnson, 2009; Rodenburg et al. 2011).Boraud et al. (2015) found that in rice cultivation in central Côte d'Ivoire, obtained the best yield with only chemical weed killers. The use of chemical weed killers has grown in all parts of Africa (Ipou Ipou et al. 2016) and has become an important alternative to agricultural labor. However, chemical control has limits because of the resistance phenomena observed (Johnson, 1997; Lopes Ovejero et al. 2013) and can have consequences on human health (sterility, cancer, etc.) and the environment (Konan et al. 2014; Kouadio et al. 2014). Faced with these shortcomings in the management of grass cover, it is therefore necessary to offer rice growers alternative control methods, combining in particular less harmful chemical weed control and the sowing method for a better efficiency of the fight against weeds of rice. This study aims at contributing to the management of weeds in irrigated rice cultivation in the valley of the Senegal river for sustainable rice production. The specific objective of the study was to assess the combined effect of Eros Gold and Londax herbicides and sowing methods on rice weeds in ISRA agricultural research station at Fanaye.
Materials and Methods
Location of the study site
The study was conducted at the experimental station of Fanaye (16°33 North and 15°46 West, Department of Podor) in the agro-ecological zone of Middle Valley. It is located at about 160 km North-East of Saint-Louis and 410 km from Dakar. The station covers an area of 40 ha from which only 17 ha is used and located on the right bank of an arm of Senegal river. It is an excellent rice-growing area that host all the rice varietal selection experiments and herbicide efficacy trials on rice weeds. The soils of the site are hydromorphic to gleys or pseudogleys. They are heavy soils with clay content greater than 30%, locally called "Faut Hollaldé". The climate is Sahelian with a short rainy season of 2 to 3 months (July-September) and a dry season spanning 9 to 10 months (October-June). Temperatures are relatively high throughout the year with maximum temperatures varying between 31.6 °C and 42 °C and a minimum of 15.7 °C to 25.5 °C. The average annual rainfall is around 220 mm (ANACIM, 2017).
SAHEL 108, a variety of irrigated rice approved in Senegal was used. It is a variety of Oryza sativa, in the group of Indica varietal with a cycle of 105 days in overwintering and 117 days in the hot, dry season. It is characterized by a potential yield of up to 10 t ha-1 (ISRA, 2012).
The herbicides used for the study were Eros gold and Londax. Eros Gold is a new formulation in soluble granules of Pyrazosulfuron 0.75% + pretillachor 30%. It works by inhibiting the synthesis of acetolactate (also known as acetohydroxyacid), blocking the biosynthesis of branched-chain amino acids of valine, leucine and isoleucine and thus cell division and fatty acid production. This inhibition leads to immediate arrest of weed growth. Londax or Bensulfuron methyl or benzoacid is a non-selective systemic herbicide with an active ingredient Pyrimidinyl sulfonylurea. Absorbed by the foliage and the roots, it has a rapid action on the meristematic tissues and controls in rice fields the annual broadleaf weeds.
The study was set up according to a split-plot experimental design with two factors (Herbicides and sowing methods) and three replicates. Londax and Eros Gold herbicides (primary factor) were applied to direct sowing and transplanting (secondary factor). Three (3) blocks of 60 m2 were established. Each block was subdivided into six (6) elementary plots of 15 m2. The elementary plots were separated in the block with dyke of 50 cm. 1m irrigation canals separated the blocks.
Treatments based on Eros Gold and Londax 60 DF were applied at early post-emergence stage (2-4 leaves stage) of weeds. For each herbicide, the recommended dose was applied (1.75 kg/ha for Eros Gold and 80 g / ha for Londax 60 DF). For the control plots, no herbicide was applied (untreated plot or T0).
A basal fertilizer with organic matter and DAP (NP0) was applied before plants emergence or transplanting to ensure good germination of direct sowing and resumption of transplanted rice plants. NPK 10-10-20 fertilizer and urea were applied every 21 days at the doses of 90 kg/ha and 300 kg/ha, respectively. Broadcast sowing was carried out with pre-germinated grains at a density of 120 kg/ha. Regarding the transplanting method, the plants were grown in a nursery for a month before being transplanted with 15×10cm2 of crop geometry. Plants were irrigated twice a week with a water slide of 5 cm during the vegetative phase and 10 cm during the reproductive phase while respecting the drainage as needed.
Measurements of Phytosociological parameters of weeds
Inventory of weed flora
The floristic survey technique used was a field survey, which allows to inventory the species of the plot exhaustively (Maillet, 1981; Le Bourgeois, 1993). For each survey, the species present were recorded by family group (Grasses, Sedges, Dicotyledons) and abundance-dominance scores were assigned according to Braun-Blanquet (1952) scale. Three inventories were carried out: before rice heading stage (early seedling stages of weeds), at rice heading stage (adult plant stages of weeds) and the rice maturation period (senescent weed plant stages).
Identification of species encountered
Most of the species encountered were identified in the field. However, unidentified plants were sampled and sent to Dakar Herbarium for identification. Species were identified using the floras (Berhaut, 1967, 1971; Le Bourgeois and Merlier, 1995; Akobundu and Agyakwa, 1989; Johnson, 1997). The nomenclature used is that of Lebrun and Stork (1991-1997). For biological types, we used the classification of Lebrun, (1966), Raunkier (1934). This classification distinguishes 6 biological forms which are: nanophanerophytes (P), chamephytes (C), hemicryptophytes (H), geophytes (G), therophytes (T), and parasitic plants (Par). For the geographical distribution, the information came mainly from the flora of Hutchinson and Dalziel (1972) and Traore and Maillet (1992).
R software was used for the analysis of variance of the parameters measured, and the means were separated using the Student-Newman-Keuls test. From the plots surveyed, the following elements were calculated: Centesimal frequency (FC) or distribution of a species in the population represents the percentage of plots surveyed where the species was found (Godron, 1968); Average abundance/dominance index (ADm), calculated in relation to the number of plots surveyed in which the species was present (Le Bourgeois, 1993). The abundance/dominance indices were matched in the class of cover and average cover (Table 1). PCA was performed to determine the distribution and specific composition of species at the elementary plot level.
Table 1. Transformation of Abundance-Dominance into average percentage recovery (Gounot, 1969).
Results and Discussion
Specific composition of weeds flora
Table 2 presented the list of weeds species inventoried in the station of Fanaye. Twenty-eight (28) species distributed in 25 genera belonging to 16 families were identified in this study. The Poaceae (8 species), Cyperaceae (3 species), Malvaceae (3 species) and Asteraceae (2 species) families were the most common. All other recorded families were represented by one species only.
Effect of treatments on the density of weed populations per family groups
Variation of the density of weed populations of different family groups according to the observation periods
Density evolution of Cyperaceae, also known as sedges, Poaceae (grasses) and dicotyledon families depending on the observation periods, is presented in Table 3. There was no significant difference between the observation periods for the density of grasses and sedges. However, a significant difference was noted between the observation periods in dicotyledons. The density of dicotyledons was lower before and at heading stages as compared to the maturity period of rice.
Average density variation of weed populations of different family groups according to the treatments
For each family group (sedges, grasses and dicotyledons), the analysis of variance (Table 4) indicated a significant difference between weed densities in the different treatments. The sedges were less abundant in broadcast sowing plots treated with Eros Gold (15.39 individuals / 0.25m²). Eros Gold effectively controlled weed grasses in broadcast sowing plots with an average density of 15.72 individuals / 0.25 m². Londax was most effective to control grasses in transplanted plots. For dicotyledons, transplanted plots treated with Londax presented the lowest grass cover (18.22 individuals/0.25m²) as compared to other treatments.
Distribution of weed species according to the observation periods and treatments
Distribution of weed species before rice heading stage (early stage of weeds)
The distribution of variances of weed species before heading stage of rice is presented in Table 5. The percentage of cumulative information on the two (2) axes was 95.59%. The horizontal axis, which explained 86.48% of the information, allowed to discern the efficacy of the different treatments (sowing methods and herbicides) on weed species and the vertical axis (9.11%) showed the resistance of certain species according to the treatments.
Table 2. List of inventoried weed species with the number of families and genera.
M: Monocotyledons; D: Dicotyledons.
Table 3. Evolution of density of different family groups of weeds according to the observation periods.
Figure 1 presents the distribution of weed species based on the abundance and dominance in the plots according to the different treatments before rice heading stage using Principal Component Analysis (PCA). Analysis of Figure 1 showed that some species: (C. iria and E. colona) were resistant to the two herbicides used. Therefore, they were abundant in either treated or untreated plots. However, E. colona resisted only in direct sowing plots treated with Londax and C. iria in plots treated with Eros Gold. On the other hand, D. retroflexa, A. auriculata, L tetranda, P. amarus, A. indica, F. litoralis, C. olitorius, P. laetum, E. cilianensis, and D. horizontalis seemed to be sensitive to the herbicides regardless of the sowing methods. At the seedling stage, C. iria was better controlled with Londax while Eros Gold controlled E. colona. All other species encountered were well controlled by both herbicides.
Table 4. Average density evolution of weed populations of different family groups according to the treatments.
Figure 1. Distribution of weed species in the different treatment plots before heading stage of rice.
Distribution of weed species at the rice heading stage (adult stage of weeds)
Table 6 shows the distribution of variances of weed species at rice heading stage. The first two axes represented 74.5% of the information of the total inertia of variance. The vertical axis represented about 48.97% of the information. It showed that Londax effect as opposed to those of Eros Gold and the horizontal axis representing 25.54% of the information allowed to isolate the weed species that were sensitive to herbicides from those which were resistant.
Table 6. Distribution of variances of weed species at rice heading stage.
The degree of infestation of weed species in the different elementary plots during the heading of rice is presented in Figure 2. Analysis of this figure showed that: C. difformis, C. iria, S. alba, E. japonica and C. olitorius were the resistant species to both herbicides at adulthood. However, C difformis was more resistant to Eros Gold, E. japonica resistant to Londax. At the same time, C. iria, S. alba and C. olitorius had the same trend of resistance to both herbicides. Resistance at the maturity of the species seemed to be regardless of sowing methods. Also, Eros Gold and Londax effectively controlled the rest of the weed species at adulthood: E. colona, D. retroflexa, A. auriculata, L. tetranda, P. amarus, A. indica, F. litoralis, P. laetum, E. alba, E. hirta, P. oleracea, O. barthii, I. acanthocarpa, B. aurita and C. fasicularis. It is also observed that C. iria and E. colona, which were resistant at the young plant state, became susceptible to the herbicides at adulthood.
Figure 2. Distribution of weed species in elementary plots of different treatments during the heading stage of rice.
Distribution of weed species during rice ripening (senescent stage of weeds)
Table 7 shows the distribution of variances of weed species at maturity phase of rice. The two axes are chosen together represented 68.86% of the total inertia information of variance. The horizontal axis alone represented 44.67% of the information. This axis allowed to isolate species resistant to the herbicides. The vertical axis representing 24.19% of information and indicated the effectiveness of herbicides.
Table 7. Distribution of variances of weed species at maturity phase of rice.
Analysis of Figure 3 indicated that all of the following species were more abundant in plots treated with Eros Gold and control plots regardless of the sowing methods: E. colona, D. retroflexa, L. tetranda, P. amarus, A. indica, F. litoralis, C. olitorius, P. laetum, S. alba, E. albonia, E. japa, E. japan, P. oleracea, O. barthii, I. acanthocarpa, B. aurita, C. fasicularis, L. abyssinica, B. sufriticosa, S. dulcis, H. indicum, A. sessilis.
Figure 3. Distribution of weed species in elementary plots of different treatments during rice ripening.
The number of taxa encountered in this study appears to be low as compared to those of the weed flora of irrigated rice fields of the valley of Senegal River. The total number of species recorded in all phytosociological surveys was 90, distributed in 27 families. The weed flora of irrigated rice fields of Daloa at central-western of Côte d'Ivoire revealed a total of 148 species belonging to 102 genera distributed in 40 families (Sylla et al. 2017). The differences of results in the valley of Senegal River could be due to the fact that the study was carried out in a much larger number of plots (127 plots) or larger surface area of plots as compared to those in Daloa with an area of 110.25 m² per elementary plot (10.5 m x 10.5 m). On the other hand, the differences were considerable as compared to those of Boraud et al. (2015) in rice cultivation at the center of Ivory Coast where 16 species were identified, distributed in 14 genera and belonged to 10 families. The results showed that the transplanted and direct sowing plots treated with Londax had the lowest grass cover of dicotyledons (18.89 individuals/0.25m²) and (20.56 individuals/0.25m²), respectively as compared to those treated with Eros Gold. Therefore, Londax controls dicotyledons effectively in transplanted rice plots as well as in broadcast sowing plots. In untreated plots, either transplanted or broadcast sowing had the highest level of weeds. The use of herbicides after weed emergence is more effective when the weeds are still at the seedling stage (Johnson, 1997). In contrary, C. iria and E. colona were resistant to Eros Gold and Londax herbicides at that stage. These results corroborate with those obtained by Botella (2005), which states that the genus Echinochloa seems according to some producers no longer to be controlled by certain herbicides on the market. Furthermore, laboratory tests have confirmed cases of resistance of E. colona and E. obtusiflora to sulfonylurea products.
Transplanting rice plants gives a considerable advantage for weed germination over direct sowing. Weeds are less voracious in direct sowing systems than in transplanting (Johnson, 1997). The two herbicides used, namely Eros Gold and Londax, are based on pretilacolor and bensulfuron, respectively. And according to (Johnson, 1997), sedges are sensitive to herbicides based on these active ingredients. Thus, herbicides (Eros Gold and Londax) effectively control the majority of species in the adult state: E. colona, D. retroflexa, A. auriculata, L. tetranda, P. amarus, A. indica, F. litoralis, P. laetum, E. alba, E. hirta, P. oleracea, O. barthii, C. iria, I. acanthocarispa. Even C. iria and E. colona, which were resistant as a young plant, became fully susceptible to herbicides at adulthood. Therefore, at maturity, the degree of leaf absorption increases with the increase in leaf area. Grard et al. (2012) showed that Londax effective against perennial broadleaf weeds. However, Eros Gold effectively controls weed grasses in broadcast plots with an average density of 15.72 individuals / 0.25 m². Londax is more effective in transplanted plots. The transplanted and broadcast control plots have the highest level of grass cover. Rahman et al. 2014 have shown that some residual herbicides used for weed control in corn crops can sometimes persist in the soil. Which negatively affects crop rotation (Mehdizadeh, 2019).
This study evaluated the effectiveness of two herbicides Eros Gold and Londax according to two sowing methods, namely broadcast sowing and transplanting. The efficacy of Eros Gold for weed control is generally more effective on sedge and grass families, while Londax is more effective on broadleaf weeds. The effectiveness of both herbicides appears to be more pronounced with broadcast sowing. The species E. colona and C. iria appear to be resistant to both herbicides.
Conflicts of Interest
The authors have declared no conflicts of interest.
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