According to long-term weed surveys, maize fields contain 43 weed species in 38 genera and 22 families. The main weeds are crabgrass, barnyardgrass, green foxtail, goosegrass, redroot pigweed, purslane, Asian copperleaf and purple nutsedge. Some weeds, such as crabgrass, purple nutsedge, creeping thistle, Asiatic dayflower, velvetleaf, field bindweed, knotgrass, cocklebur and alligator weed, are difficult to control with conventional herbicides.
The following introduces these weeds and their control strategies.
1. Asiatic dayflower
To control Asiatic dayflower chemically, application should be kept to the two- to three-leaf stage (before branching); the earlier the timing, the better the result, which reduces the need for applications at the branching and later stages. After maize sowing, at the six-leaf stage of maize, using atrazine + nicosulfuron, atrazine + nicosulfuron + fluroxypyr, or fluroxypyr + MCPA gave 97%-99.5% control by plant count and 90%-95.5% control by fresh weight; overall efficacy ranked fluroxypyr + MCPA > atrazine + nicosulfuron + fluroxypyr > atrazine + nicosulfuron. In addition, bentazone is highly specific against Asiatic dayflower.
Northeast China: in maize fields with heavy post-emergence Asiatic dayflower, apply 20% fluroxypyr alone at 1,000-1,500 mL/ha. Or mix 20% fluroxypyr 500-600 mL + 4% nicosulfuron 1,000 mL. Apply at the 3-5 leaf stage of maize and the 2-5 leaf stage of the weeds.
2. Velvetleaf
Velvetleaf, commonly called "maguo", is poorly controlled by nicosulfuron- and atrazine-based products. Because nicosulfuron-atrazine is now the most widely used post-emergence maize herbicide in Northeast China, velvetleaf has broken out over large areas.
Fluroxypyr: it can be used in maize to control broadleaf weeds and is highly effective against broadleaf weeds such as velvetleaf and cocklebur that nicosulfuron-atrazine controls poorly. In fields with severe velvetleaf, add 30 mL of 200 g/L fluroxypyr per mu to nicosulfuron-atrazine; this controls common grass and broadleaf weeds while being highly specific against broadleaf weeds such as velvetleaf and cocklebur.
Syngenta's product "Lumax": formulated as 3% mesotrione + 24.7% S-metolachlor + 10.8% atrazine (containing the safener fenclorim). Lumax combines three active ingredients that complement each other for highly efficient weed control and an extremely broad spectrum; it controls almost all annual grass and broadleaf weeds and is highly specific against velvetleaf. Dose: 400-450 mL per mu. Method: 30 L water per mu, sprayed evenly on the soil surface, then shallow incorporation, sowing and film mulching.
Shandong Zhonghe Chemical Co., Ltd.'s product "Mingtu": formulated as 58% mesotrione-acetochlor-atrazine suspension emulsion (SE). Mingtu can effectively control 65 species of grass and broadleaf weeds, including green foxtail, crabgrass, goosegrass, Asian copperleaf, velvetleaf and black nightshade, as well as weeds resistant to glyphosate, triazines, sulfonylureas and PPO-inhibiting herbicides. It kills emerged velvetleaf directly and has secondary residual activity on ungerminated velvetleaf, which continues to die after emerging when exposed to the herbicide. It has a long residual effect; pre-emergence application for soil closure can last up to 60 days. The active ingredients remain stable on the soil surface when sprayed in dry conditions and their herbicidal activity can be reactivated by rain; if rain occurs immediately after foliar treatment and washes the herbicide to the soil surface, it again exerts soil herbicidal activity.
Dose: 200-300 mL per mu. Method: broadcast spray over the whole field at the 0-3 leaf stage of maize.
Other optional herbicides include: flumetsulam, rimsulfuron, thifensulfuron-methyl, metribuzin, flumioxazin, bentazone, dicamba, mesotrione, halosulfuron-methyl, foramsulfuron, primisulfuron-methyl and clopyralid.
3. Field bindweed
Field bindweed, also called field convolvulus or small morning glory, belongs to the Convolvulaceae; it is a root-suckering weed with trailing stems and many branches at the lower part, and white horizontal creeping roots underground. It prefers moist, fertile black soils; it is much branched and leafy, twining around itself with roots running flat or obliquely in the soil at 50-60 cm and, in summer and autumn, producing new overwintering buds on near-surface roots. Field bindweed does considerable harm to maize. When it breaks out it grows in patches, densely covering the ground and twining upward, strongly inhibiting crop growth and causing lodging.
Control methods:
(1) Cultural control: plough to a depth of not less than 20 cm and strengthen field management with frequent hoeing and harrowing;
(2) Chemical control: 20% fluroxypyr EC at 50-70 mL per mu, mixed with 30-45 kg water, applied as a foliar spray at the 2-4 leaf stage of the weeds.
Other optional herbicides: bentazone, 2,4-D isooctyl ester, MCPA, glyphosate and glufosinate.
4. Field horsetail
Because the vascular (conducting) tissue of field horsetail is particularly poorly developed, translocated herbicides are hard to move within the plant, so contact herbicides are the first choice for controlling it.
Optional herbicides: glufosinate, fluroxypyr-meptyl + MCPA, 2,4-D isooctyl ester, dicamba (Banvel), glyphosate and primisulfuron-methyl.
5. Field sowthistle
When field sowthistle dominates after maize emergence, apply at the 3-5 leaf stage of maize: 48% bentazone EC at 2,250 mL/ha; 25% 2,4-D butyl ester-sulcotrione EC at 3,000-4,500 mL/ha; 4% nicosulfuron at 1,500 mL/ha; or 80% nicosulfuron at 75 g/ha as a foliar spray; these also control several other weeds. In addition, 25% bromoxynil octanoate EC, 30% clopyralid AS and 30% topramezone SC give good control of field sowthistle.
6. Creeping thistle and greater creeping thistle
For creeping thistle in maize fields, fluroxypyr, bentazone, carfentrazone-ethyl or clopyralid can be used. Small creeping thistle can be controlled with nicosulfuron + fluroxypyr, MCPA-sodium, or mesotrione; if creeping thistle exceeds 15 cm, add 20 mL clopyralid per mu. Clopyralid is highly specific against deep-rooted weeds of the Asteraceae and Fabaceae. When the weeds are older, use a mixture of nicosulfuron and MCPA, keeping the MCPA dose low and spraying mainly the weeds while avoiding the new leaves of maize as far as possible, to prevent herbicide injury to maize.
7. Black nightshade
Black nightshade, commonly called "heitiantian" and also known as bitter nightshade, wild eggplant, crow's eye, dog's teat, wild grape and Tianbao bean, is an annual herb of the Solanaceae. It is widely distributed throughout the country, can flower and fruit several times a year, and occurs commonly in Heilongjiang. It mainly damages crops such as maize, peanut, beans, tobacco, cucurbits, vegetables, tubers and cotton, and does relatively severe harm to maize, generally causing 13%-23% yield loss.
According to the occurrence pattern of black nightshade, chemical weeding in fields - especially chemical control of black nightshade in maize - has three suitable application timings:
First, pre-sowing application as a soil treatment. Suitable herbicides include 80% flumetsulam (Kuocaocing) water-dispersible granules, 75% thifensulfuron-methyl (Baoshou) dry suspension, 75% isoxaflutole (Bainongsi) water-dispersible granules, 38% atrazine suspension, 48% alachlor (Lasso) EC and 90% acetochlor (Harness) EC.
Second, pre-emergence after sowing as a soil treatment. Commonly used herbicides include 50% propisochlor (Lefengbao) EC, 72% 2,4-D butyl ester EC, 75% isoxaflutole water-dispersible granules, 38% atrazine suspension, 48% alachlor EC, 90% acetochlor EC, 24% nicosulfuron-mesotrione-atrazine (2% nicosulfuron + 4% mesotrione + 18% atrazine) oil dispersion, 63% acetochlor-atrazine (35% acetochlor + 28% atrazine) SE, 42% prometryn-atrazine-acetochlor (10% prometryn + 9% atrazine + 23% acetochlor) SC, 55% alachlor-acetochlor-atrazine (6% alachlor + 24% acetochlor + 25% atrazine) SC, 40% propisochlor-atrazine (16% propisochlor + 24% atrazine) SC, 42% butachlor-propisochlor-atrazine (4% butachlor + 20% propisochlor + 18% atrazine) SC, 60% propisochlor-atrazine-2,4-D butyl ester (25% propisochlor + 25% atrazine + 10% 2,4-D butyl ester) SC, 53% acetochlor-atrazine-2,4-D butyl ester (30% acetochlor + 15% atrazine + 8% 2,4-D butyl ester) SE and 48% 2,4-D butyl ester-atrazine-butachlor (5% + 19% + 24%) SE.
Third, post-emergence foliar treatment. Available herbicides mainly include 22.5% bromoxynil (Buctril) EC, 25% rimsulfuron (Baocheng) dry suspension, 4% nicosulfuron (Yunongle) suspension, 75% thifensulfuron-methyl dry suspension, 25% bromoxynil octanoate EC, 38% bromoxynil octanoate-nicosulfuron-atrazine (13% + 3% + 22%) oil dispersion, 26% nicosulfuron-atrazine (4% + 22%) oil dispersion, 46% mesotrione-propisochlor-atrazine (6% + 15% + 25%) SC, 55% mesotrione-atrazine (5% + 50%) oil dispersion, 54% mesotrione-acetochlor-atrazine (4% + 30% + 20%) SC and 50% rimsulfuron-atrazine (1% + 49%) WP.
8. Purple nutsedge
Purple nutsedge has become a major problem in summer maize fields, with an accelerating upward trend and increasingly serious damage. In lightly infested fields it can reduce maize yield by 10%-20%, and in severe cases by 30%-50%. Purple nutsedge generally begins to emerge in mid-April, with a first peak in early May causing relatively light damage that usually needs no control. A second peak appears in early June and lasts until July, with a rapid onset and heavy damage that strongly affects maize; after August it basically stops emerging. Therefore June-July is the key period for controlling purple nutsedge in summer maize.
Why purple nutsedge is difficult to control:
1) Because purple nutsedge has an underground tuber that stores energy and has extremely strong vitality, and the main tuber produces 10-20 daughter tubers each year; the daughter tubers not only produce new purple nutsedge seedlings but also produce "granddaughter" tubers underground, multiplying endlessly.
2) Because the tubers, bulbs and seeds of purple nutsedge can all reproduce; in the growing season new shoots emerge in about a week and new tubers form in about 20 days.
Evaluation and comparison of the main herbicides for purple nutsedge:
1) Nicosulfuron: good control of young purple nutsedge (before the 3-leaf stage), reaching over 80%; symptoms are that the above-ground stems and leaves first turn yellow and then die, and the underground corms near the plant turn brown and black and gradually rot. But the effect on older purple nutsedge is not ideal - the above-ground heart leaves turn slightly yellow and the underground corms are basically unaffected. The active-ingredient dose of nicosulfuron is generally 4 g per mu, equivalent to about 100 g of 4% nicosulfuron oil SC; in some places 5-6 g is used.
2) MCPA: overall better control of purple nutsedge than nicosulfuron, especially more thorough on the underground corms near the plant, and also good on older purple nutsedge, with a lower cost per mu and high cost-effectiveness. But MCPA is a hormone-type herbicide requiring high application skill; improper use or abnormal weather (high temperature and humidity) can easily injure maize, causing leaf curling, root deformity and brittle stems, so MCPA is less safe to maize than nicosulfuron. The dose of 56% MCPA soluble powder is generally 20-30 g per mu and must not be arbitrarily increased. MCPA is often sold in combination packs with nicosulfuron-type products, and the market is relatively mature in some areas of Henan where purple nutsedge is common and severe.
3) Halosulfuron-methyl: this is currently the highly effective herbicide against purple nutsedge; its control is far superior to nicosulfuron and MCPA, and its safety to maize is also better than the former two. It not only controls the above-ground part and the corms near the plant, but through its excellent systemic translocation the herbicide reaches adjacent corms, giving more thorough control and a longer residual effect - truly treating both symptom and root cause. But halosulfuron-methyl is only moderately effective on broadleaf weeds and its spectrum is not broad. Its cost is also high, about 20 yuan per mu, far higher than nicosulfuron and MCPA. In addition, halosulfuron-methyl is still in the promotion stage, and data on its safety to following crops and residues are not yet comprehensive. The product is a 75% water-dispersible granule (the imported product is named XiangfuJing), with a dose of 4 g per mu. Therefore halosulfuron-methyl is more suitable for areas where purple nutsedge occurs heavily or as a rescue treatment after the optimal control period has been missed.
4) Other compounds: mesotrione can kill the above-ground part of purple nutsedge or inhibit growth, but does not remove the roots thoroughly; fluroxypyr-meptyl is basically ineffective against purple nutsedge; carfentrazone-ethyl and rimsulfuron are used in a few places but have not been widely promoted due to herbicide injury or other reasons and are not mainstream; therefore these four compounds are not recommended for purple nutsedge control.
9. Crabgrass
Crabgrass is one of the most widely distributed and most representative grass weeds. It mainly damages dryland crops, especially grass crops such as wheat, barley, maize, rice and sorghum, and turf. Because it belongs to the same family as grass crops, emerges at a similar time and has similar leaves and leaf shape, and its physiological and biochemical selectivity to herbicides is basically the same, it is very hard to find a herbicide effective on crabgrass yet safe for grass crops; crabgrass is therefore listed as a difficult, malignant weed in grass-crop fields and poses a serious threat to their yield and quality.
In no-till maize fields of the Huang-Huai-Hai region, the frequency and damage rate of crabgrass are 99% and 73% respectively, mostly about 70%. The maize fields of the Huang-Huai-Hai region have seven main weed groups, six of which are dominated by crabgrass. As a dominant, malignant and difficult weed of maize fields, crabgrass poses a serious threat to maize production, causing about a 20% yield loss on average.
In the 1980s nicosulfuron was registered in maize fields in China; it is the most economical and effective ALS-inhibitor herbicide for maize fields, giving good control of broadleaf and grass weeds in maize and playing a very important role especially in controlling grass weeds such as crabgrass; it is widely used as the backbone variety in maize fields. As an important post-emergence herbicide for maize, nicosulfuron has been used in China for several decades. Because of heavy, repeated and single use, the resistance problem of nicosulfuron has become increasingly prominent. In recent years, in some maize fields the control of crabgrass by nicosulfuron has declined markedly; applying nicosulfuron at conventional doses can no longer effectively control crabgrass, and this problem is especially prominent in the maize fields of Northeast and North China.
Analysis and countermeasures regarding the re-greening of crabgrass after post-emergence herbicide application in summer maize in 2016
Causes of crabgrass re-greening:
1) Nicosulfuron has been used as a post-emergence herbicide in maize fields for more than 10 years.
2) Safeners reduce the effect of nicosulfuron on grass weeds.
The large-scale, wide-ranging re-greening of crabgrass after post-emergence herbicide application in summer maize in 2016 should be attributed to crabgrass resistance to nicosulfuron; irrational mixtures and the addition of safeners further reduced the effect.
Herbicides currently used to control resistant crabgrass:
1. Post-emergence foliar treatment: mesotrione (active dose above 15 g per mu), nicosulfuron (increased dose), isoxaflutole, foramsulfuron and Qingdao KingAgroot's patented product "Jinyu Purui".
2. Pre-emergence soil closure: amide herbicides compounded with atrazine.