Agriculture for Engineers 4 (3+1)

Lesson 28. Weeds Control

28.2 MAJOR ELEMENTS OF WEED MANAGEMENT:

1. Crop Husbandry Control of Weeds
2. Physical Control of Weeds
3. Herbicidal Control of Weeds
4. Biological Control of Weeds
5. Crop Breeding for Weed Control
6. Non-Living Mulches
7. Burning and Flaming

28.3 CROP HUSBANDRY CONTROL OF WEEDS:

Good crop husbandry is more than half the weed control envisages on farmland. While directly it induces a healthy growth of crop, indirectly it maintains a crop environment that is as detrimental to weeds as possible.

28.3.1 Proper crop stand and early seedling vigour:

Ready and uniform germination of crop seeds and their development into vigorous crop seedling leaves less space for the weeds to grow amongst the crop plants. Uneven and low crop populations and week crop seedling, on the contrary, permit thick growth of weeds. A vigorously growing crop aids weed control by weakening the weeds by offering competition.

Important steps in obtaining good germination and optimum stand of crops are (1) selection of most adapted crops and crop varieties, (2) the use of high viability seeds, (3) pre-plant seed and soil treatment with pesticides, dormancy breaking chemicals, and germination boosters, (4) adequate seed rates and (5) proper planting time and method. Despite these practices if some gaps are still seen in the crop rows, these must be filled as soon as possible. Row spacing of crops should be as narrow as agronomic recommendation will allow so that the crops close in early. Unnecessarily deep plantings of crop seeds should be avoided as it results in slow germination, and often, in weak seedling.

28.3.2 Selective crop stimulation:

Selective crop stimulation can be achieved in many ways. To start with, correction of soil condition to favour crop growth by the application of soil amendments like gypsum or lime, as the case may be is an important step towards favouring crop growth. Addition of farm yard manure or synthetic soil conditioners to very light or very heavy soils may be useful in improving crop growth by ameliorating the soil structure and thus, maintaining better air-water relationships.

Application of suitable fertilizers and manures in adequate quantities improves plant growth very much. But when these are applied uniformly to soil, the may benefit the crops and weeds. Therefore to make fertilizers available mainly to the crop, these should be banded or side dressed. Weeds growing 20 cm or more away from the fertilized bands usually fail to make use of even a mobile nutrient like N. such weeds remain stunted in comparison to the close growing weeds. Foliar application of fertilizer to wide row crops like maize sugarcane, and cotton, also amounts to their selective stimulation.

28.3.3 Proper planting method:

Any planting method that leaves the soil surface rough and dry will discourage early weed growth. For example, in India when winter grains are planted in seedbeds prepared after a pre-sowing irrigation, it leaves the top 3 to 5 cm of soil above the crop seeds in rough and dry tilth. Such physical condition of soil defers the germination of the weeds. By this time the crop plants are sufficiently grown-up to fight weeds. In variance with this sound practice of planting crops, when a farmer plants his crops in dry soil and irrigates the field soon thereafter, he lands himself into serious weed problems.

In the summer season, furrow planting of crops is a very useful method of reducing the weed problems. It is so because in this method the irrigation water is restricted initially to the furrows. This leaves the inter-row crop spaces dry where weeds fail to germinate. After the crop seedlings are well established, the fields can be irrigated, uniformly.

In transplanted crops the farmer gets opportunity to prepare a weed free field for the placement of healthy crop seedlings. This gives a definite advantage to the crop over the later germinating weeds.

28.3.4 Proper planting time:

With the availability of photo-insensitive varieties of many crops, manipulation of planting time of crops so as to avoid the first heavy flush of weeds should be easy. For instance, when rainy season crops like maize and cotton are planted about 15 days before the break of monsoon with the help of a presowing irrigation, the crops germinate in weed free environment. And by the time the weeds germinate with the onset of rains, the crop seedlings are well up.

28.3.5 Crop rotation:

Crop rotation are effective in controlling crop-associated and crop-bound weeds such as Avena fauna ‘wildoat’ and Cuscuta spp. ‘dodder’, respectively. Wild oat can be driven away from small grain fields by using pea and gram as break crops for 2 to 3 years. Dodder on the other hand, can be eliminated from lucerne by turning the land to grain crops for some time.

28.3.6 Stale seedbed:

A stale seedbed is one where 1 to 2 flushes of weeds are destroyed before planting of any crop. As earlier stated, most weed seeds germinate from top 3 to 5 cm of surface soil. If a finally prepared seedbed is withheld from planting and it contains adequate moisture in its top 3 to 5 cm of soil, a flush of young weed seedling will appear on it, in about a week’s time. These weed seedlings can be destroyed either with a contact herbicide like paraquat or by spike tooth harrow, spring tooth harrow, weeder-mulcher, and sweeps. Depending upon the time available, one or two flushes of weeds can be destroyed in this manner before planting of crops in the stale seedbeds. The main advantage of stale seedbeds is that crops germinate in weed free environment.

28.3.7 Smother cropping:

The smother crops germinate quickly and develop large canopy, capable efficient photosynthesis, in relatively short period. They possess both surface and deep roots. Also called competitive crops, they suppress the weed seedling by excluding light beneath and utilizing large quantities of nutrients from the soil, rapidly.

28.3.8 Summer fallowing:

Farmers in India, as in many other tropical countries, have used for decades hot months of April, May and June to expose their lands to sun in order to control many soil-borne pests, including weeds. Roots, rhizomes and tubers of shallow rooted perennial weeds like Bermuda grass and nutsedge are desiccated when these are brought to surface by tillage and exposed to air temperature of 40 to 45 °C.

28.3.9 Minimum tillage:

Deep and frequent tillage may be useful for some reasons, but it serves to (i) bring more of dormant weed seeds and rhizomes to the soil surface, and (ii) preserve the new ones deep inside the soil for the future. Both these things are undesirable.

28.3.10 Lowering area under bunds:

Soil bunds made in fields for the purpose of irrigation are ideal places for the rapid growth of weeds. Soil dug or scraped for making these carries numerous weed seeds that germinate readily on them soon after the fields are irrigated, and establish into thick weed populations. These weeds from potential source of every year weed seeds on the farm since the application of herbicides and cultivations are usually restricted to the net cropped ground.

28.3.11 Flooding and Drainage:

Flood kills weeds by excluding air from their environment. Some weed species are more susceptible to it then the others. Flooding is a world-wide crop husbandry method of controlling weeds in rice fields. In some parts of Madhya Pradesh (India), deep flooding of fallow fields with rain water is practiced continuously for 2-3 months. After that the water is let out and the winter grains are planted. The practice, locally called ‘Haveli’ is considered very effective in controlling weeds besides conserving moisture. The technique however can be used as a weed suppression measure only in limited situations.

In variance with flooding, drainage is used for controlling aquatic and semi-aquatic weeds in rice fields, canals, and ponds. In rice fields, where both terrestrial and aquatic weeds may be common, a judicious combination of the two can be practiced.

 28.4 THE PHYSICAL CONTROL OF WEEDS

28.4.1 Pre-Plant Tillage Control of Weeds:

Pre-plant tillage is usually conducted in two phases, viz., the primary tillage and the secondary tillage. The primary tillage is conducted effectively with either a soil inverting plough or a disc. The secondary tillage, on the contrary, is performed with lighter implements like harrows, cultivators, weeder-mulchers, and conjugated rollers.

28.4.2 Primary Tillage Control of Weeds:

Burying the existing weeds, which in turn, eases the post-plant tillage and the application of pre-plant herbicides? Bringing the weed seeds to soil surface for germination and their subsequent destruction by suitable, secondary tillage  operations and incorporating pre-plant herbicides.

In the case of perennial weeds, delaying or obviating the need of post-plant tillage.

The role of secondary tillage in weed control is mainly to dislodge the germinating weed seedlings.

28.4.3 Post-Plant Tillage:

For a long time, besides the control of weeds, at least three more functions were assigned to the post-plant tillage, more popularly called row cultivations. These functions are (i) conservation of soil moisture, (ii) enhanced soil aeration, and (iii) mixing of fertilizer and manures with the soil. The development of chemicals to control weeds made it possible to separate the weed control effects of row-tillage from its other effects.

28.4.4  Row-Cultivation Implements:

i. Spike-tooth Harrow:  Spike tooth harrow, also called drag harrow and peg-tooth harrow, uproots the germinating weed seedlings, breaks the soil crust, and stirs the soil to 1-5 cm depth.

Fig 1. Spike tooth harrow

 

ii. Spring-tine Harrow:  Spring-tine harrow, also called spring-tooth harrow, consists of elliptical spring tines with triangular and sharp free ends. It is used for same purpose as spike tooth harrow but it may stir the soil up to 7.5 cm depth.

Fig 2. Spike tooth harrow

iii. Rotary-Hoe Cultivator:  Rotary-hoe cultivator is tractor-drawn implement designed to run fast, accomplishing adequate soil movement to dislodge the weeds. It is used to break hard crust that usually forms over the crop seeds after a rain but during this process it also destroys the sprouting weeds. Also, a rotary-hoe cultivator is employed to destroy germinating weeds and grasses in the rows as well as those growing close to the young crop plants.

Fig 3. Spike tooth harrow

iv. Wheel-Hoe Cultivator:  The wheel-hoe is suitable for weeding only small vegetable gardens and some flower beds.

Fig 4. Wheel-hoe culivator

v. Blade-Harrow:  It cuts the weeds 7.5 to 10 cm below the ground and leaves them on the soil surface as much, without causing any inversion of the soil.

Fig 5. Blade harrow

vi. Cultivator (horse-hoe):  Cultivator is primarily a secondary tillage implement for seedbed preparation. But many cultivator designs are also used widely for inter-row crops.

Fig 6. Cultivator (horse hoe)

vii.  Rice rotary weeder:  Rice rotary weeder, as the name indicates is a specific tool for weeding rice crops.

Fig 7. Cultivator (horse hoe)

28.4.5  Mowing, Cutting and Dredging & Chaining:

i. Mowing:  Mowing is cutting of a uniform growth of weeds from entire area at the ground level. Its chief purpose is to improve look and accessibility to the area, and in certain cases to prevent multiplication of weed seeds. Also, repeated mowing can weaken the underground parts of perennial weeds.

Mowing is usually practiced in non-crop areas. Lawns, and gardens but it can also prove useful in removing weeds from rows of certain established crops. Common mowing tools and implements are sickle, sword, scythe, machete, lawn mower and reciprocating type rotary-bar mower. Mowing is effective against only erect herbaceous type of weeds. It is preferred over tillage control of weeds on land susceptible to erosion since it does not produce any bare land.

ii. Cutting:  In variance with mowing, cutting is topping of the weeds above the ground level. It is most commonly practiced against brushes and trees with the help of axes and saws. In aquatics, under-water weed cutters are used to cut weeds up to 1 m below the water surface. Both cutting and mowing are short-lived in their effects; the topped weeds re-grow soon from their crown region and underground buds. Therefore, these operations must be repeated often to keep the weeds and grasses low.

iii. Dredging & Chaining:  These two physical control methods are used against aquatic weeds growing in shallow ditches. Dredging constitutes mechanical pulling of weeds with their shallow roots and rhizomes covered in mud. In chaining, on the other hand, a very heavy chain is pulled over the bottom of the ditch with the help of two tractors, one moving on either bank of the ditch. The chain fragments the rooted weeds by its rubbing action, and the weed fragments float to the water surface. From here these can be collected down the stream by nets and hooks.

 28.4.6  Soil Solarisation (Soil Heating):

Soil solarisation, also called solar soil heating, is another method of utilizing solar energy for the desiccation of weeds in fallow fields during the hot summer months. It differs from hot weather tillage control of weeds described earlier in two major aspects (i) It is effective mainly against weeds from seeds. (ii) It usually does not involve any tillage of the field.

The technique of soil soalrisation involves covering the moist soil with polythene sheet of 20-25 mm thickness during hottest part of the year (April-June). This sheet permits the incoming short wave solar radiation through to the soil. However, the out going long wave terrestrial radiation is trapped inside the sheet which results in net rise in temperature by 10-12 degrees celsus. This rise in soil temperature is fetal for many species of weeds including Phalaris minor and Avena fatua etc. In addition to weed control, solarisation also damage many of the hibernating and aestivating insects and disease organisms.

  28.5  HERBICIDAL CONTROL OF WEEDS

Herbicides are chemicals capable of killing or inhibiting the growth of plants. In the last 40 years or so, man has greatly improved upon his weeding efficiency by supplementing the conventional weeding methods with herbicides. It has saved farmers of undue, repeated inter-cultivations and hoeing and has helped him in obtaining satisfactory weed control where physical methods often fail. Today, we have over 400 herbicides in common use for selective and non-selective weed control in different areas. These chemicals vary greatly in their (a) molecular structures, (b) mobility within plants, (c) selectivity, (d) fate in soils, and (e) response to environment.

A. Benefits of Herbicides:

Herbicides were developed in the western world primarily to overcome the shortage of farm labour for weeding crops. However, during the past four decades, slowly the utility of herbicides has also been realized in the labour-rich tropical world, for varied reasons. Important among these are the following:

In monsoon season incessant rainfall may make physical weeding infeasible. Herbicides can be used to ensure freedom of crops from weeds under such a condition. The soil applied herbicides can be of great help in these regions in boosting crop production.

Herbicides can be employed to control weeds as they emerge from the soil to eliminate weed-crop interference even at a very early stage of crop growth. But by physical methods weeds are removed after they have offered considerable competition to the crops, and rarely at the critical time. Thus, herbicides provide benefits of timely weed control.

Herbicides can kill many weeds that survive by mimicry, for example, wildoat (Avena Spp.) in wheat and barnyard grass (Echinochloa spp.) in rice. Weeds that resemble crop plants usually escape physical weeding. Herbicides control does not dictate strict row spacing. In physical weed control, on the other hand, the crop rows have to be sufficiently wide to accommodate weeding implements, else, hand weeding and hand-pulling of weeds has to be resorted to.

Herbicides bring about longer lasting control of perennial weeds and brushes than is possible with any physical control method. Many modern herbicides can translocate considerably deep in the underground system of weeds and damage them.

Herbicides are convenient to use on spiny weeds which cannot be reached manually. Herbicides are safe on erodible lands where tillage may accelerate soil and water erosion. Excessive tillage, in any case, spoils soil structure, reduces organic matter content, and depletes moisture status of the soil.

Some other benefits of using herbicides include (a) fewer labour problems, (b) greater possibility of farm mechanization. (c) easier crop harvesting and (d) lower cost of farm produce. In dry land agriculture, effective herbicidal control ensures higher water use by crops and less crop failures due to drought.

B. Limitations of Herbicides:

Like any other method of weed control, herbicides have their own limitations. But with proper precautions these limitations can be overcome, markedly. Important limitations in the use of herbicides are as follows.

In herbicidal control there is no automatic signal to stop a farmer who-may be applying the chemical inaccurately till he sees the results in the crops sprayed or in the rotation crops that follow. Even when herbicides are applied accurately, these may interact with environment to produce unintended results. Herbicides drifts, wash-off, and runoff can cause considerable damage to the neighboring crops, leading to unwarranted quarrels.

Depending upon the diversity in faming, a variety of herbicides must be stocked on a farm to control weeds in different fields. On the contrary, for physical control of weeds a farmer has to possess only one or two kinds of weeding implements for his entire farm.

Above all, herbicidal control requires considerable skill on the part of the user. He must be able to identify his weeds and possess considerable knowledge about herbicides and their proper usages. Sometimes an error in the use of herbicides can be very costly.

 28.6  THE BIOLOGICAL CONTROL OF WEEDS

The biological control of weeds involves the use of living organisms such as insect, herbivorous fish, other animals, disease organisms, and competitive plants to limit their infestations.

An important aspect of biological weed control is that at a time, it is applicable to the control of only one major weed species that has spread widely. With perennial weeds the main objective of bio-control is the destruction of the existing vegetation, in the case of annual weeds prevention of their seed production is generally more important.

 28.7 CROP BREEDING FOR WEED CONTROL

Plant breeders, thus far, have engaged themselves in evolving crop varieties tolerant to specific pathogens, insects, nematodes, and even birds. But hardly any attempt has been made to breed crop plants tolerant to competition from weeds. In recent years attention of a group of Asian and African plant breeders has been drawn towards breeding millet varieties tolerant to Striga sp.

 28.8 NON-LIVING MULCHES

Mulching stunts or kills the weeds by cutting light to them. Straw, hay, dry sugarcane leaves, farmyard manure, rice hull, saw dust, and bark dust are natural, partial mulch materials, which can at best stunt the weed growth. Even when these are spread in thickness of 6 to 12 cm, the perennial weeds grow through them easily. Also, the natural mulches tend to harbour insect pests and disease organisms and obstruct farm operations. Synthetic mulches, namely black paper or polythene film mulches provide stronger mechanical barriers to all kinds of germinating weeds. Black, plastic-coated, craft paper mulches are now preferred on newly prepared fields to suppress weeds. These are made in rolls of different widths which can be spread on the seedbeds and then crop seedlings can be transplanted through holes made into them. Presently mulching with synthetic materials is costly, so its economic use is limited to certain high value vegetables and ornamentals.

 28.9  BURNING AND FLAMING

Burning is the cheapest method of eliminating mature, unwanted vegetation from rangelands and non-crop situations like roadsides and ditch banks in dry seasons. Weed seeds that have already shattered on the ground before flaming will not be killed by it. Burning is limited in its value because it is a potential source of fire hazard. In variance with it, flaming is a momentary exposure of green weeds to as high as 1000 °C temperature from flame throwers. Flaming kills plants by coagulating their cell protoplasm. In some western countries, flaming is used for selective control of inter-row weeds in onion, soybean, grain sorghum, castor bean, cotton, sesame, and certain fruit orchards. For selective weed control, flaming must be done when weeds are in their seedling stage and the crop plants are well established. The flame should be directed towards the weeds under hood covers.

Flaming has proved useful in destroying dodder-infested crops of Lucerne. In grasslands, woody shrubs and patches of perennial weeds can be destroyed by spot flaming. Flaming is also helpful in weeding farm ditches and many other difficult non-crop situations. It is believed that repeated light applications of flame to plant shoots can destroy even roots of the perennial weeds. The process is called searing.