Abstract: Background and Objective: Rice (Oryza sativa L.) is the most important staple food in the world and information on the effects of chemical weed control on rice grain quality is insufficient. Hence this study examined the effects of 2 herbicides pretilachlor and 2,4-dichlorophenoxyacetic acid on the proximate and mineral compositions of Oryza sativa L. Materials and Methods: Field experiment was conducted in evaluating the effect of herbicides on the proximate and mineral compositions of Oryza sativa. The experimental setup was a completely randomized block design with 7 treatments and 4 replicates. The treatments include; T1: Weedy check, T2: 1.5 L ha–1 pretilachlor, T3: 2.0 L ha–1 pretilachlor, T4: 2.5 L ha–1 pretilachlor, T5: 6.0 L ha–1 2,4-dichlorophenoxyacetic acid, T6: 8.0 L ha–1 2,4-dichlorophenoxyacetic acid and T7: 10.0 L ha–1 2,4-dichlorophenoxyacetic acid. Results: Using one-way analysis of variance, the result showed that pretilachlor and 2,4-dichlorophenoxyacetic acid at all application rates considered in this study significantly increased (p<0.05) the proximate parameters of harvested rice grains except for moisture content which was found to increase significantly only in 2,4-dichlorophenoxyacetic acid treated plots. Regardless of the application rates, the 2 herbicides considered significantly enhanced all the mineral contents investigated in this study except for iron content which showed a significant decrease in all the application rates. Conclusion: The results concluded that applications of pretilachlor and 2,4-dichlorophenoxyacetic acid in rice cultivation did a lot more than weed management as they significantly enhanced proximate and mineral compositions of harvested rice grains except for moisture and iron content.
INTRODUCTION
Rice (Oryza sativa L.) is ranked the most important staple food crop for over 50% of the world population1. It is extensively cultivated in both the sub-tropical as well as the tropical regions of the world. On a world scale, its cultivation reaches 154 M ha with an annual production of around 426 Mt, meaning an average productivity2 of 3.9 t ha–1. The continuously growing world population and the consequent increment in the demand for rice have necessitated the need for the realization of the crop’s full production potentials3. Several biotic and abiotic factors have been identified to restrict the crop’s potential performance. Some of these factors include drought, flood, erratic rainfall, extreme temperature, soil erosion, soil nutrient imbalance, high phosphorus fixation, weed competition, pest and diseases infestation4. The most severe of all the aforementioned factors is weed competition, which accounts for the greatest yield loss2. Uncontrolled weed growth in lowland and upland rice for yield loss was reported in the range5,6 of 12-81%. Several weed control methods have been reported ranging from cultural control method4,7, manual weeding8,9, mechanical weeding10,11 and chemical control method12,13. However, the chemical control method has been reported to be the most effective, less time consuming2, less tedious14 and offers an advantage of a better labor efficiency15. The influence of herbicides on the yield of rice have been well documented by several studies14,16,17 but the knowledge on how these herbicides affect proximate and mineral compositions of the harvested rice seeds is still fragmentary. The present study was evaluating the effect of two different herbicides (pretilachlor and 2,4-dichlorophenoxyacetic acid) on the proximate and mineral compositions of Oryza sativa (L.).
MATERIALS AND METHODS
Study site: The experiment was conducted at the University of Ilorin Botanical Garden, Southern Guinea savanna zone at the 4°38.920'E-4°39.971'E and 8°27.810'N-8°28.230'N.
Experimental layout and treatment details: The experimental field dimension was 17 by 7 m containing 24 unit plots each measuring 2 by 2 m with a net-plot spacing of 0.5 m. The arrangement was a completely randomized block design with 7 treatments and 4 replicates. The treatments include; T1: Weedy check, T2: 1.5 L ha–1 pretilachlor, T3: 2.0 L ha–1 pretilachlor, T4: 2.5 L ha–1 pretilachlor, T5: 6.0 L ha–1 2,4-dichlorophenoxyacetic acid, T6: 8.0 L ha–1 2,4-dichlorophenoxyacetic acid and T7: 10.0 L ha–1 2,4-dichlorophenoxyacetic acid.
Soil analysis: Mechanical analysis by the hydrometer method was employed to determine particle size distribution of the soil. The soil pH was determined by the use of a pH meter. Electrical conductivity was resolved using a conductivity meter. Organic matter, ash content and organic carbon concentration were determined using Walkley-Black method18. Using an ammonium ion-selective electrode, cation exchange capacity (CEC) was evaluated through ammonium saturation of exchange sites and estimation of saturated ammonium. Base saturation was calculated as19:
Soil calcium, magnesium, potassium and sodium were determined by flame photometry.
Planting operation and cultural management: Manual weeding was done on the experimental plots. This was followed by ploughing to loosen the soil in preparation for planting. Viable rice seeds obtained from the African rice center were pre-treated with 2.5 g of dress force per kg rice grain before planting to prevent soil borne diseases. The pretreatment chemical contained 20% w/w thiamethoxam, 2% w/w difenoconazole and 20% w/w metalaxyl-M. The seeds were planted and raised on a nursery bed for 21 days. Thereafter, they were transplanted at the rate of two seedlings per hole and a spacing of 20 by 20 cm. Rice spikelets were harvested from matured plant by manual cutting the panicle from the parent plant. Thereafter, the husk was removed from the harvested spikelets and the rice grains were kept in an air-tight container for mineral and proximate analysis.
Proximate analysis of harvested rice grains: Proximate composition of harvested rice grain was analyzed according to the methods of the Association of Official Analytical Chemists20. Moisture content of the harvested rice grains was determined by drying 2 g of sample in an oven at 105°C for 5 h. Ash content was determined by incinerating 2 g of sample in a muffle furnace at 600°C for 3 h, till a greyish ash was formed. Crude fibre was accounted for by extracting 2 g of milled rice sample with hexane in a thimble for 6 h to exclude the fat sample. Subsequently, the sample was hydrolysed using 1.25% tetraoxosulphate (VI) acid to liberate digestible nutrient in the fat. Filtration was then carried out and the resulting filtrate was combusted in a muffle furnace at 600°C for 30 min. It was then cooled in a desiccator and weighed. Crude protein was calculated by multiplying factor (N%×6.25) following a nitrogen (%) determination by the Kjeldahl method using 2 g of milled seed samples. Crude fat was determined by manually extracting 2 g of milled seed samples with 150 mL petroleum ether in a soxhlet extractor at a boiling point of 60-80°C. Total carbohydrate was determined by deducting the sum of ash, crude protein, crude fibre, crude fat and moisture from one hundred.
Mineral analysis of harvested rice grains: Total nitrogen was assayed using the micro-Kjeldahl procedure21. Total phosphorus was determined by the ammonium molybdate/vanadate yellow colour method following ternary acid perchloric-nitric-sulphuric acid wet digestion22, total boron was determined by azomethane-hydrogen method following ternary acid digestion23. Atomic spectrophotometer was used in determining total cations: calcium (Ca), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn). Elements such as sodium (Na) and potassium (K) were analysed using flame photometry following a wet digestion with perchloric-sulphuric ternary acid23.
Statistical analysis: Data were analysed using one-way analysis of variance (F-test) with Statistical Package for Social Science (SPSS) software version 23.0. Significant test results followed the Duncan multiple range test (DMRT) at 5% level of probability for identification of important contrasts.
RESULTS
Soil physico-chemical properties: The results in Table 1 showed the physico-chemical properties of the experimental plots. The soil textural class was found to be loamy with a pH of 7.3 which is higher than the standard soil pH requirement for rice cultivation. The soil had moderate levels of organic matter, moderately high in total nitrogen. It was high in available phosphorus and potassium (Table 1). The exchangeable calcium was low when compared with standard land quality requirements for rice cultivation in Southern Guinea Savanna Ecological Zone of Nigeria.
Effect of herbicides on the proximate composition of rice: The proximate composition of the harvested grain as influenced by the herbicides is presented in Table 2. Significantly higher moisture content was recorded in T5 and T6 treatments while significant least moisture content was recorded in T2, T3 and T4 treatments. Ash content, crude fibre and crude protein of harvested grains were significantly higher in T3 plants. The T1 plants had generally the lowest values for all the proximate parameters assessed.
Effect of herbicides on the nutrient composition of harvested rice grains: Regardless of the treatment rates, overall grains harvested from herbicide treated plants recorded a significantly higher (p<0.05) nitrogen (except T3), phosphorus (except T4), potassium, calcium and magnesium content when compared to the T1 plants (Table 3).
| Table 1: | Soil physico-chemical properties of the experimental soils in comparison to land quality (ranges) for rice cultivation |
| *Source: Aondoakaa and Agbakwuru24 | |
| Table 2: | Proximate composition of rice seed as affected by pretilachlor and 2,4-dichlorophenoxyacetic acid |
Values represent Means±SEM, means with the same superscript along the same column are significantly the same at p<0.05, n = 4, T1: Weedy check, T2: 1.5 L ha–1 pretilachlor, T3: 2.0 L ha–1 pretilachlor, T4: 2.5 L ha–1 pretilachlor, T5: 6.0 L ha–1 2,4-dichlorophenoxyacetic acid, T6: 8.0 L ha–1 2,4-dichlorophenoxyacetic acid, T7: 10.0 L ha–1 2,4-dichlorophenoxyacetic acid | |
| Table 3: | Mineral analysis of rice seed as affected by pretilachlor and 2,4-dichlorophenoxyacetic acid |
Values represent Means±SEM, means with the same superscript along the same column are significantly the same at p<0.05, n = 4, T1: Weedy check, T2: 1.5 L ha–1 pretilachlor, T3: 2.0 L ha–1 pretilachlor, T4: 2.5 L ha–1 pretilachlor, T5: 6.0 L ha–1 2,4-dichlorophenoxyacetic acid, T6: 8.0 L ha–1 2,4-dichlorophenoxyacetic acid, T7: 10.0 L ha–1 2,4-dichlorophenoxyacetic acid | |
The herbicides tend to significantly limit iron content of the harvested grain when compared to the T1 plants (Table 3).
DISCUSSION
The result of the soil analysis is an indication that the soil is capable of supporting the growth of rice given the reference standard land quality requirement for rice cultivation. The values reported for most of the physico-chemical properties were within ranges reported previously while assessing land suitable for rice cultivation24.
It is indicated from the results that plants treated with 2,4-dichlorophenoxyacetic acid (T5, T6 and T7) retained more moisture when compared to pretilachlor (T2, T3 and T4). Moisture content is of importance in storage because the lower the moisture contents of food material, the higher the keeping quality25, therefore seeds harvested from pretilachlor treated plants have a better shelf life when compared to 2,4-dichlorophenoxyacetic acid. The result is in consonance with the moisture content previously reported26. Ash content, crude fibre and crude protein of harvested grains were significantly higher (p<0.05) in T3 plants. This was followed by T7, T4 and T5 plants with significant least ash, crude fibre and protein recorded in the T1 plants. Higher ash content, protein content and crude fibre in all herbicide treated plants as compared to weedy check could be attributed to reduced weed competition for available resources necessary for proper plant growth and yield as suggested by an earlier report27. The values obtained for ash, fibre and protein content in this study are slightly higher than those from an earlier report26. Crude fat and carbohydrate content of harvested grains from the T1 plots were found to be significantly lower compared to all other considered treatments. Significantly lower fat and carbohydrate content recorded in T1 plots could be as a result of serious competition for moisture, light, nutrient and space between the weeds and the rice crop. The values were in conformity with those recorded from a study on mineral content, proximate composition and fatty acids analysis of aromatic and non-aromatic Indian rice26,28.
Significantly lower iron content was recorded in all application rates for both herbicides considered when compared to the weedy check. However, no significant differences were recorded in plants receiving herbicide treatments. The general increase in nitrogen, phosphorus, potassium, calcium and magnesium in all herbicide treated plants could be due to a better crop performance resulting from reduced weed competition for space, water and available nutrient. Workers have attributed increase in rice yield and nutrient composition to a better plant performance owing to reduced competition between the crop and the weed; promoting increased availability of water, light and nutrients29. A researcher has adduced the greater nutrient uptake in spring barley under chemically suppressed weed growth condition to reduced weed competition30. Significant lowest nitrogen, phosphorus, potassium, calcium and magnesium in the weedy check plots could be traced to enormous competition for moisture, space, light and nutrient between the weeds and the rice crops28. Significantly lower iron content obtained in all herbicide treated plots when compared to the weedy check could be due to decreased soil iron availability caused by reduced soil microbial activates. In rice plant, increased soil pH and low soil microbial activity have been described as being vital to soil iron availability31. The importance of microbial biomass in the availability of trace elements in the soil has also highlighted32. The values obtained for mineral content of rice in this study aligns with those reported in a previous experiment26. Further studies need to be carried out to determine that; is the harvested grains containing the herbicides residue in ensuring crop safety for consumers?
CONCLUSION
In this study weed management in rice cultivation using pretilachlor and 2,4-dichlorophenoxyacetic acid within the application rates were significantly enhanced all the proximate parameters except for moisture content which was only enhanced in 2,4-dichlorophenoxyacetic acid. The 2 herbicides at all the concentration were significantly improved all the mineral content investigated in this study except for iron content which showed a significant decrease in all the application rate considered for both herbicides. The 2 herbicides can therefore be regarded as quite indispensable in maintaining the quality of rice while simultaneously combating the menace of weeds in rice cultivation.
SIGNIFICANCE STATEMENT
This study established the suitability of 2 herbicides; pretilachlor and 2,4-dichlorophenoxyacetic acid in serving the dual purpose of weed management and enhancement of the quality of Oryza sativa. This area of study is thus of significance to not only plant biologists but also to agriculturists in the field of weed management.