Current status of biological control of paddy weeds in Vietnam

Rice is a staple food in Vietnam and accounts for > 7.7 × 10⁶ cultivated ha, which provide 35.5 × 10⁶ t of rice, of which 4.2 × 10⁶ t were exported in 2004. The enlargement of the cropping area and the enhancement of rice yield have rapidly increased the amount of agrochemicals, including herbicides, in crop production in Vietnam. From 1990–2003, the percentage of herbicides in total pesticides has increased ≈ 10-fold to 30.2%. In addition, the improper use of herbicides caused environmental hazards, unsafe agricultural products, and human health problems. Biological management integrated with traditional weed control techniques might help to reduce the dependence on synthetic herbicides and build eco-friendly, sustainable agricultural production in Vietnam. This paper reviews the efforts in establishing a strategy for biological management of weeds that was conducted in recent years by Vietnamese weed scientists. This has included cropping system management, water and soil management, integrated pest management, and utilization of plant allelopathy as major components of the strategy. Many plants with strong allelopathic potential can be a source for biological weed suppression and soil fertility improvement. The utilization of allelopathic properties in rice might also help to provide new rice cultivars with weed-suppressing characteristics.     

Weed suppression by Passiflora edulis and its potential allelochemicals

Passion fruit (Passiflora edulis) is grown in the tropics for its edible fruits and for its many ethno‐pharmacologic and pharmacological properties. Our study revealed that the plant contains a strong allelopathic potential. In a bioassay, aqueous extracts of P. edulis strongly suppressed germination and growth of lettuce, radish and two major paddy rice weeds, Echinochloa crusgalli and Monochoria vaginalis. In glasshouse and field experiments, P. edulis also strongly inhibited the growth of paddy rice weeds. Application of 2 t ha^?1 dry plant material of P. edulis reduced weed biomass by 70% and increased rice yield by 35% compared with the unweeded control. Ten newly identified substances in P. edulis extracts, including coumarin, long‐chain fatty acids and lactones, may be responsible for the inhibitory activity of P. edulis. Coumarin and the lactones showed greater inhibition of germination and growth of E. crusgalli than the fatty acids. The authors suggest that P. edulis may be used as a natural herbicide to reduce the dependency on synthetic herbicides.     

三乙烯四胺二硫代氨基甲酸钠对黄铜矿氧化的影响

酸性矿山废水(AMD)的形成对环境造成严重危害,而从源头上控制其产生是一项重要的处理措施。本研究利用课题组前期研制的钝化剂三乙烯四胺二硫代氨基甲酸钠(DTC-TETA)对常见硫化矿物黄铜矿进行处理后,通过不同体系的摇瓶实验比较,研究该钝化剂对黄铜矿的化学氧化和生物氧化过程的影响,并利用SEM、XRD和XPS对黄铜矿进行分析,探讨钝化剂的作用机制。结果表明,钝化剂处理过的黄铜矿经过20 d的化学、生物氧化后,产生的铜离子浓度分别是其相应对照的17%和48%,说明钝化剂能有效抑制黄铜矿的氧化速率。SEM结果表明,黄铜矿表面在钝化剂处理后,经过氧化受到的侵蚀明显减少,这是由于钝化剂在黄铜矿表面形成包膜有效隔绝氧化剂和细菌对黄铜矿的接触。样品的XRD和XPS分析结果说明了经钝化剂处理过的黄铜矿在氧化过程中可形成一些产物,但这没有改变黄铜矿的氧化进程。因此,黄铜矿经钝化剂DTC-TETA处理后其氧化速率受到抑制,表明表面钝化法是一项处理硫化物矿物避免产生AMD造成危害的可行技术,这对于从源头上控制AMD具有重要意义。     

Phosphorus sorption in soils of the Mekong Delta (Vietnam) as described by the binary Langmuir equation

Phosphate adsorption isotherms were determined for 20, mostly very acidic, soils from the Mekong Delta. The experimental data were well described by a binary Langmuir equation which considers two groups of sorption sites that differ in their P bonding energies. The maximum P-sorption capacities of these sites were related to the soil properties by simple linear correlation and by stepwise multiple regression. Results suggest that high energy sites are on Al-oxihydroxides or small Al-substituted Fe-oxides and, to a lesser extent, on poorly ordered Fe-oxihydroxides. On the other hand, the P-sorption capacity of low energy sites is mainly related to clay content, and it increases as pH decreases. These sites are also positively correlated with organic carbon and poorly crystalline Fe-oxihydroxides. However, as these two variables are closely correlated with each other, organic matter is likely to be considered as an indirect factor of P fixation through its association with Fe-oxihydroxides and not as an important source of P-sorption sites. The maximum sorption capacity, i.e. the sum of sorption capacities of the two groups of sites, is well described (r²= 0.88) by an equation that takes into account the four variables identified above: Al-bearing oxihydroxides, poorly ordered Fe-oxihydroxides, clay content and pH. Grouping the soils according to the orders of Soil Taxonomy, the P-sorption capacity increases in the following sequence: ultisols < entisols < inceptisols. A P concentration often considered adequate for plant nutrition is 0.2 mg P 1^?1 solution, and only the high energy sites are involved in sorption at that concentration. Thus an equation including only Al- and Fe-oxihydroxides could be used to fix P norms in these soils.     

Alfalfa, rice by-products and their incorporation for weed control in rice

Two alfalfa varieties, namely Rasen and Yuba , and rice by-products of Koshihikari ( Oryza sativa L. cv. Japonica ), including hull and bran were used for weed control in rice. Rasen and Yuba were sequentially evaluated to have the strongest allelopathic potential among eight common alfalfa varieties in Japan. Rasen , Yuba and rice by-products in a laboratory experiment exhibited suppression of emergence of lettuce ( Lactuca sativa L.), barnyardgrass ( Echinochloa oryzicola Vasing.), and monochoria ( Monochoria vaginalis Presl var. plantaginea Solms-Laub.). In a paddy field experiment, at 15 days after application, a dose of 1 ton ha⁻¹ of alfalfa, rice by-products and the incorporated alfalfa-rice by-products significantly inhibited weed growth and reduced weed species, except for rice bran. However, at 40 days after application, the greatest weed control was sequentially recorded with Yuba (95.2%), Rasen (90.5%) and the incorporated Rasen -hull (88.3%). Rice hull exhibited stronger (51.7%) weed control than the negligible weed control of rice bran (25.1%). Inhibition of both Rasen and Yuba on weed emergence after application became more vigorous, whereas those of rice by-products were gradually devitalized. Rasen had a maximum increase of rice yield (80.6%) compared to the control and it was slightly higher (9.6%) than the herbicide treatment. Findings showed that Rasen was the most promising material for weed control among those studied. Results denoted that weed control capability of allelopathic materials had an impetuous effect on rice growth and yield.     

Behavior of simetryn and thiobencarb in rice paddy lysimeters and the effect of excess water storage depth in controlling herbicide run‐off

Eight small‐scale lysimeters with different excess water storage depths (EWSDs) were used to investigate the behavior of two herbicides, simetryn and thiobencarb, under paddy conditions. The concentration of simetryn dissipated similarly in all the lysimeters, while the thiobencarb concentration varied significantly because thiobencarb can adsorb onto the dissolved organic matter in a manure slurry, which was applied to six of the lysimeters. The herbicide losses (the percentage of the applied mass) from the lysimeters were reversely proportional with the EWSD. The correlation was stronger for simetryn than for thiobencarb. An appropriate EWSD is required to effectively prevent herbicide run‐off from the paddy field, especially when a rainfall event occurs soon after herbicide application.     

Impacts of tillage and application methods on atrazine and alachlor losses from upland fields

The effects of tillage practises and the methods of chemical application on atrazine and alachlor losses through run-off were evaluated for five treatments: conservation (untilled) and surface (US), disk and surface, plow and surface, disk and preplant-incorporated, and plow and preplant-incorporated treatments. A rainfall simulator was used to create 63.5 mm h⁻¹ of rainfall for 60 min and 127 mm h⁻¹ for 15 min. Rainfall simulation occurred 24–36 h after chemical application. There was no significant difference in the run-off volume among the treatments but the untilled treatment significantly reduced erosion loss. The untilled treatments had the highest herbicide concentration and the disk treatments were higher than the plow treatments. The surface treatments showed a higher concentration than the incorporated treatments. The concentration of herbicides in the water decreased with time. Among the experimental sites, the one with sandy loam soil produced the greatest losses, both in terms of the run-off volume and herbicide loss. The US treatments had the highest loss and the herbicide incorporation treatments had smaller losses through run-off as the residue cover was effective in preventing herbicide losses. Incorporation might be a favorable method of herbicide application to reduce the herbicide losses by run-off.     

Allelochemicals of barnyardgrass‐infested soil and their activities on crops and weeds

Barnyardgrass is one of the most noxious paddy weeds in the world and causes great trouble to many crops. In a bioassay, the aqueous extract of paddy soil infested with barnyardgrass showed phytotoxic action against the growth of the tested crops and paddy weeds, of which rice was the most suppressed among the crops. In contrast, barnyardgrass was the least affected paddy weed. By the use of a separation resin, 18 compounds belonging to terpenes, derivatives of cinnamic acid and ferulic acid, long‐chain fatty acids, and steroids that were potentially involved in the phytotoxic activities in barnyardgrass‐infested soil were isolated and identified by gas chromatography–mass spectrometry analysis. Of these, the quantities of linalool, 4‐terpinenol, coumaran, methyl phenethyl ketone, and methyl ester cinnamic acid were 1.42, 0.37, 0.02, 3.12, and 4.59 μg g^?1 of the wet soil, respectively. The herbicidal activity was varied among these qualified test compounds, in which methyl phenethyl ketone and methyl ester cinnamic acid were more herbicidal than coumaran, linalool, and 4‐terpinenol. A mixture of these compounds was also the least inhibitive against the growth of barnyardgrass, but exerted strong suppression against that of rice and monochoria. The present study demonstrates that barnyardgrass possesses strong phytotoxic properties and releases plant growth inhibitors into the soil to compete with rice and other paddy weeds in its vicinity by a chemical pathway.