Detailed Explanation of Key Technical Points for Herbicide Use in Soybean Fields

In recent years, the area treated with herbicides in soybean fields has increased year by year, and problems such as poor weed control and crop phytotoxicity caused by human factors frequently occur in production. Therefore, the following introduction is provided on herbicide use techniques in soybean fields.
I. How to scientifically use herbicides in soybean fields.
Select suitable herbicide varieties according to the weed species in the crop field. Any herbicide variety has a certain spectrum of weed control: some control broadleaf weeds, some control narrow-leaf weeds (grass family, sedge family), and some control both narrow-leaf and broadleaf weeds. However, even the best herbicide cannot effectively control all weeds in the field. Therefore, before using a herbicide, it is necessary to clarify what weeds are present in the field and to distinguish which are the main weeds and which are the secondary weeds. The herbicide should be selected on the principle of mainly controlling the main weeds in the field while also controlling secondary weeds. For example, using Bentazon, Pursuit to control annual broadleaf weeds in soybean fields such as lambsquarters, amaranth, smartweed, cocklebur, elsholtzia, velvetleaf, black nightshade, etc. is very effective, but when perennial broadleaf weeds such as field sowthistle and creeping thistle are also abundant in the field at the same time, selecting Bentazon, Pursuit to control broadleaf weeds will not work. Instead, herbicides such as Flumetsulam, Gengtianyi, Clomazone, Jinliandao should be selected to control broadleaf weeds in the field, so that field sowthistle and creeping thistle in the field can be controlled. Therefore, before using a herbicide, it is necessary to investigate and understand the weed species in the field and apply the appropriate herbicide in a targeted manner. Do not simply apply whatever herbicide others are applying.
　　
II. Soil characteristics determine the dosage and efficacy of soil-applied herbicides. Soil characteristics mainly refer to four aspects: soil texture, organic matter content, soil moisture status, and seedbed preparation quality. Clayey soils and soils with high organic matter content have strong adsorption capacity of their colloids (colloidal particles are the fine organic and inorganic particles contained in soil), and herbicides are adsorbed relatively more, so the high rate within the recommended dosage range should be used as appropriate to ensure weed control efficacy. Sandy soils and soils with low organic matter content have weaker adsorption capacity and are prone to leaching, so the low rate within the recommended dosage should be used as appropriate to avoid phytotoxicity.
Good soil moisture status is conducive to desorption of herbicides adsorbed by soil colloids so that they are released into the soil solution and absorbed by weeds. When soil moisture status is poor, herbicides firmly adsorbed by soil colloids cannot be released into the soil solution to be absorbed by weeds, and weed control efficacy is poor. Therefore, if soil moisture status is poor and there are no irrigation conditions, do not force soil-closure weed control, but instead switch to post-emergence stem and leaf treatment.
During soil-closure treatment, seedbed preparation quality must be good. The soil should be leveled and harrowed fine with no large clods, so that after spraying a herbicide film can form on the soil surface, and when weeds emerge and contact the herbicide film they die. If seedbed preparation quality is poor, it will affect the formation of the herbicide film, thereby affecting weed control efficacy.
　　
III. Correct selection of spraying period. In chemical weed control, when soil treatment is suitable, use soil treatment; when soil treatment is not suitable, wait for post-emergence stem and leaf treatment. When applying herbicides, be sure to seize the timing and carry it out promptly. Most soil-applied herbicides are effective against weed seedlings. If applied too late, some weeds have already emerged and weed control efficacy will not be very good. Generally speaking, application must be completed within 3--5 days after sowing. If the soil surface is dry at the time of application, the herbicide is easily tightly adsorbed by soil particles, and irrigation or rainfall is needed to leach the herbicide into the 0--5 cm soil layer to exert its efficacy, or mechanical soil incorporation is needed to reach the 2--5 cm soil layer to exert its efficacy. Post-emergence stem and leaf herbicides should be applied somewhat later, but not too late either. Generally speaking, application is done at the 2--3 leaf stage after soybean emergence and the 2--4 leaf stage of weeds. If applied too early, weed emergence is uneven, and later-emerging weeds still require another application or other weed control measures (of course, except for agents such as Pursuit and Clomazone that can be used both for soil treatment and for stem and leaf treatment). If applied too late, weeds are large and their resistance is strong, so efficacy will be significantly reduced, and for some herbicides applied too late, the dosage must be correspondingly increased appropriately, which is also not safe for the crop.
In addition, sometimes in order to seize the farming season, it is common to encounter rain at the time of spraying, especially in large-area production. Because different herbicides differ in their physicochemical properties and formulation types, the required interval from spraying to rainfall varies. Before spraying, close attention should be paid to the local weather forecast, and every effort should be made to finish spraying one day before rain. In this way, weeds can fully absorb the agent, and after absorbing the agent, the agent's translocation within the weed body can be accelerated. Also, after rain, weeds grow and develop quickly and have poorer resistance, so weed control efficacy is also good. Under normal circumstances, for Lactofen, rain 0.5 hour after spraying does not affect efficacy; for Quizalofop-p-ethyl, Fluazifop-p-butyl, Sethoxydim, Clethodim, Jindou, Pursuit, rain 1 hour after spraying does not affect efficacy; for Fomesafen, Bentazon (Basagran), Baocheng, rain 4 hours after spraying does not affect efficacy; for Flumetsulam and Bentazon, rain 6 hours after spraying does not affect efficacy.
　　
IV. Consider preceding and succeeding crops to select appropriate herbicides and avoid damage to succeeding crops from long-residual herbicides. In recent years, due to neglect of the toxic soil residues of herbicides, damage to sensitive succeeding crops has occurred from time to time, involving many crops such as rice, vegetables, and cucurbits. At present, the long-residual herbicides used in soybean fields mainly include Pursuit, Chlorimuron-ethyl, Clomazone, Flumetsulam, etc. In use, special attention should be paid to their residual dynamics in soil and their safety to different succeeding crops.
To plant corn, choose land where Chlorimuron-ethyl was not used the previous year. To plant rice (including seedling bed soil), choose land where Imazethapyr and Chlorimuron-ethyl were not used the previous year. To plant wheat, choose land where Clomazone was used in the previous crop at no more than 50 g active ingredient per mu. To plant sugar beet, choose land where Chlorimuron-ethyl and Imazethapyr were not used in the previous 4 years, where Flumetsulam and Imazethapyr were not used in the previous 2 years, and where highly permeable Fomesafen was not used the previous year. To plant vegetables (tomato, eggplant, pepper, Chinese cabbage, radish), choose land where Chlorimuron-ethyl and Imazethapyr were not used in the previous 3 years. To plant potato, choose land where Imazethapyr and Chlorimuron-ethyl were not used for 3 years. To plant pumpkin, watermelon, zucchini, and cucumber, choose land where Chlorimuron-ethyl and Imazethapyr were not used in the previous 3 years.
In addition, the use of long-residual herbicides should also be viewed in relative terms. Some herbicides have very good control efficacy, and the harm of long residual activity can be avoided by changing the application method. For example, Pursuit is a long-residual herbicide. If we use it only for stem and leaf treatment, and during stem and leaf treatment mix it at half dosage with other herbicides having a similar weed control spectrum, or reduce the amount of the original active ingredient by adding spray adjuvants, or adopt stem and leaf band application at the seedling stage to minimize the amount of herbicide entering the soil, then long residual activity problems will not occur.
　　
V. Scientific mixing of herbicides and the issue of spray volume. In order to broaden the weed control spectrum, herbicides should often be used in mixtures. The main issue to note in mixing is that the several mixed herbicides should not have antagonistic effects. If there is an antagonistic effect that reduces efficacy, it is best not to mix them. For example, 25% Jinliandao aqueous solution has good efficacy against dayflower, field sowthistle, field horsetail, creeping thistle, etc., but it cannot be mixed with sulfonylurea herbicides (Chlorimuron-ethyl, Baocheng, Baoshou, etc.). If the mixed agents have a significant synergistic effect, the dosage should be appropriately reduced as appropriate. If the mixture has neither antagonistic nor synergistic effects, agents with different weed control spectra should be used at normal dosage, and herbicides with the same weed control spectrum may be used at half dosage as appropriate.
At present, the main problem with spray volume in production is that under drought conditions it is mistakenly believed that spray volume should be increased, raising spray volume from the traditional 150--200 L/ha to 300--400 L/ha. In fact, spray volume is not better the larger it is. For pre-emergence application, about 200 L/ha is appropriate. If 1000 L is sprayed per hectare, it is only equivalent to 0.1 mm of rainfall, which is neither economical nor helpful for exerting efficacy. For post-emergence stem and leaf treatment, spray volume should likewise not be too large, because when spray volume is large, droplets tend to be large, and if droplets are too large, the spray liquid easily rolls off the weed leaf surface and loses efficacy. A spray volume of about 100--130 L/ha is appropriate. When spraying, if the soil is severely dry and the relative air humidity is below 65%, for post-emergence stem and leaf application, add to the spray volume 0.5%--1.0% of the plant oil-type spray adjuvant Yaoxiaobao or Xindebao. This can not only reduce the dosage by 30%, but also achieve stable efficacy.