Weed control

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Better cereal yields and lower cost production



This is a practical guide to the use of shallow cultivation for seed bed
preparation and seeding. Shallow cultivation is essential for cereals after
medic in order to ensure regeneration. It is a low cost means of seeding
for cereal, grain legumes and vetches.

This chapter provides the economic justification for shallow cultivation.

Deep plough and cultivation is entrenched in the WANA region. The
technology is wasteful and costly.

This is an overview of deep ploughing and shallow cultivation.

Once the decision has been made to use shallow cultivation it is absolutely
essential to have the proper implements. These are simple and cheap. Their basic design principles are described.


Cultivation, hay production and rotations are the main methods of
controlling weeds in the WANA region. Herbicides have a role. Practical
problems are discussed.

The response of cereals to nitrogen fertiliser in the WANA region is erratic. This is explained and strategies developed to overcome the problem. Phosphate placement can also increase yield responses.

Mechanical harvesting is the main method of harvesting cereals in the
WANA region. The machines imported from Europe and North America
perform badly as they are designed for high yielding, damp crops.
Australian adaptions will improve efficiency in low to medium yielding
crops with short, brittle straw.

Even a modified harvester will not work efficiently on small farms, around
olive trees and with many types of cereal crops. The stripper is a genuine small scale machine suited to these conditions.

Using shallow cultivation will often require more weight on tractors. Why
and how?

Small farmers often employ contractors to carry out cultivation, seeding and
harvesting. This is expensive and various forms of group ownership provide
a low-cost alternative.





(Traditional rotation)




Cereal crop sown

Cereal crop sown

Cereal crop sown

Cereal crop sown


Cereal crop grows

Cereal crop grows

Cereal crop grows

Cereal crop grows


Cereal crop matures

Cereal crop matures

Cereal crop matures

Cereal crop matures


Cereal crop harvested
Stubble grazed by livestock

Cereal crop harvested
Stubble grazed by livestock

Cereal crop harvested
Stubble grazed by livestock

Cereal crop harvested
Stubble grazed by livestock


Weeds germinate naturally

Medic regenerates from seed
produced 18 months earlier.
No cultivation of the land

Land cultivated and sown to
vetch or similar forage

Land cultivated and sown to
grain legume such as lentils or
chick peas.


Weeds grazed. Low stocking rate.

Medic pasture grazed. High stocking rate.

Grazed or more often left for

Grain legumes grow.


Land cultivated for fallow

Medic grazed. Pods produced
for future regeneration.

Cut for hay.

Grain legumes mature.


Bare soil vulnerable to

Pods and stubble grazed.

Stubble grazed.


Stubble grazed.


Cereal cycle begins again.

Cereal cycle begins again

Cereal cycle begins again

Cereal cycle begins again

Why weed control is important?

  * Weed control increases yield by eliminating competition for moisture, fertility and light for both crops and pastures.

   * Weed control increases the value of grain by eliminating weeds seed contamination.

Methods of weed control  

    * Grazing by livestock - sheep and goats mainly but also cattle.

    * Cutting for hay.

    * Harvesting weed seeds.

    * Cultivation.

* Cultivated or herbicide fallow.

    * Herbicides.

The farming year and the weed program.

    Farmers should try to attack weeds and weed seed at every point in the year.

They should try to develop a program for weed control that uses all the means available.

They should be flexible in their decision making. For example, if a crop has a poor yield potentuial perhaps it would be better to cut it for hay if that provides better weed control for the next crop or pasture.

* Spring.

    This is the period when the weeds grow strongly and produce seeds which contaminate the following crops and pastures.

    * Medic pasture.

    Weeds are controlled in the medic pasture by grazing.

Control will never be complete. If there is a high percentage of grass in the medic pasture it should be grazed hard in winter below the 8 cm optimum level. Once it has been grazed hard it should be allowed to recover otherwise the medic production will suffer badly.

The objective is to allow the medic to flower and produce seed for regeneration in the following seasons. ( Grazing green medic )

Grazing will control tall growing weeds such as grasses. Grazing will only control them and prevent them from becoming dominant.

It cannot eliminate them otherwise the medic would be eliminated too.

Grazing is however effective as part of a total weed control program.

Medic can be cut for hay but this is difficult see  Hay production  and cannot be relied upon for the weed control program.

Grass control for the next cereal crop

The above weed control program may not be sufficient if a cereal crop is going to be grown the next season.

Heavy grass infestation in the medic will:

Reduce the amount of nitrogen produced by the medic pasture for the next cereal crop.

Carry over cereal root diseases.

Produce weed seed which will germinate in the cereal crop.

The production of weed seed can be reduced by slashing or mowing the medic pasture in the spring. If this is done carefully it is possible to cut off the green grass seed heads without cutting off the medic. Thus the seed production of the grass is reduced compared to the medic.

Alternatively selective herbicides can be used to control the grasses without reducing the production of medic.

These selective herbicides are expensive and are only justified where there is a heavy infestation of grass in the medic pasture.

To be effective the herbicide must be applied early.

Time of application of herbicide to the medic pasture to control grasses.

Yield of wheat in the following season. qx/ha.

Late December

22 qx

Mid January

16 qx

Early February

12 qx

No herbicide

12 qx

From my own experience as a farmer the application of herbicide to the medic and then a subsequent application of a selective herbicide to the cereal crop as a pre-emergent was an effective starting point for an almost grass free medic-cereal rotation.

In subsequent years it was not necessary to apply the herbicide to the medic - only to the cereal crop.

The yield of wheat on treated fields increased from 20 to 30 qx per ha.

The high cost of controllinbg grasses in a medic crop with selective herbicides can be justified if such a substanial increase in yield can be achieved and if it is effective in reducing the grass population over a number of medic-cereal cycles.

Grass control with fallow.

Most farmers and almost all small farmers in WANA will have doubts about herbicide control of grasses in the medic pasture. There are the obvious problems of cost but there are also problems with herbicide application equipment and skills.

Fallow offers an alternative method of controlling grasses in the Zaghouan 4 rotation. There is a loss of pasture production compared to selective herbicides and loss of nitrogen production by the medic.

If the field as been in medic for two years before there should be plenty of soil organic amtter and soil nitrogen. The loss of pasture production might compensation for the herbicide cost but there is a greater risk of failure with herbicides given the current level of inexperience of most farmers.

The fallow must be cultivated early to be effective in controlling cereal root diseases.

This is possible under the Zaghouan 4 rotation as half the farm is in medic pasture and destroying the medic pasture on a further 25% of the farm does not create an acute feed shortage for the livestock.

The use of fallow need not be permanent. After a couple of cycles of the Zaghouan 4 rotation the farmer may decide that the classic medic-cereal rotation would be better suited to that field now the grass weeds are under control. If they become a problem again in the future he may return to the Zaghouan 4 rotation.

   * Vetch and oat.

    Vetch and oat are usually cut for hay.

It is a most effective means of controlling weeds provided the hay is cut before weeds such as wild oats and annual rye grass have dropped their seeds on the ground.

After the hay has been cut the land should be heavily grazed to prevent any re-growth of weeds from producing seed.

    * Grain legume.

    Grain legumes are the opposite of vetch and oats.

They cause weed problems rather than reduce them.

Grain legumes are not strong competiors with weeds.

The weeds grow unchecked in the spring.

With medic grazing will control weeds to some extent.

With vetch a well timed hay cut will remove weeds but with grain legumes there is no spring control.

Farmers need to select areas with low weed infestation for grain legumes. They need to be realistic about them. If the crop appears to be failing due to weed infestation it may be better to take drastic action and cut for hay or graze rather than leave the crop to produce abundant weed seeds.

    * Cereal crop.

    A cereal crop in the spring should be a strong competitor for weeds provided that soil nitrogen levels are adequate.

Again the farmer needs to be realistic about the yield.

There is a good market for cereal hay in the region and this option may produce a return similar to harvesting for grain and provide weed control as well.

If the farmer intends to grow grain legumes after the cereal crop it would be most important to control the weeds. If he decides to sow vetch and oasts it is not so important.

   * Fallow.

    In  Rotations compared we have discussed the reasons for eliminating the fallow from the rotation.

The overwhelming reason is that farmers cannot afford to use the land for two years to produce a single (and not very productive) cereal crop.

Having eliminated the fallow as a regular part of the rotation it is possible to look at fallow as an occasional practice which will control weeds and mobilise nitrogen.

Fallowing can be carried out by cultivation or with herbicides. The Zaghouan 4 rotation uses fallow in the medic rotation.

* Summer

    * Harvesting the weed seeds and controlling the germinating weeds

        Many of the weed seeds are harvested by the cereal harvester.

They are separated out in the harvester from the straw and blown out the back of the machine over the sieves.

It is comparatively easy to harvest these seeds with a catching device. The material passing through the harvester is collected in a cart. When the cart is full it automatically dumps the material in a heap.

In Hay production we suggest that this material is carted by the farmer to the farm house and used to feed livestock in autumn, winter or during a drought.

Most of the weed seeds will be collected and carted off with the harvested material but some will fall on the ground.

The weed seeds will be concentrated in a small patch (perhaps a few square metres) where the farmer can control them by hand. For example the farmer can spray the small areas by hand with a knock down total herbicide in the winter using a small back pack sprayer.

The collection of weed seeds is particularly important when herbicides are used.

Many weeds become resistant to herbicides.

Once resistance reaches a certain level it can quickly dominate the weed population. The resistant weeds grow in the crop and harvesting their seed will help to keep them from becoming dominant.

* Autumn decision time

    The most important aspect of the weed control program is flexibility.

Farmer should try to adjust their program to control the weeds.

For example if a cereal crop had a high level of wild oats and rye grass farmers should not sown grain legumes. Instead they should sow oats and vetch.

If a medic pasture was under-grazed and had excessive weeds it may be better to keep it for another year, control the weeds and increase the nitrogen levels.

* Autumn weed control

    * Cultivation.

        This is a most effective means of weed control.

The conventional system is to cultivate the land twice with a scarifier and harrow and then use a seeder which also carries of a further cultivation and harrowing

See Cultivation and seeding  for more details on different tillage packages.

     * Cultivation and herbicides.

    Various levels of herbicide use can be included in the conventional package.

The most common is to control broad-leafed weeds with a post-emergent herbicide. This is cheap and effective as there are cheap generic chemicals available.

The application takes place after the cereal crop has reached the tillering stage. As the chemicals are so cheap a second late application can be made for late germinating weeds. These will cause little yield reduction but can cause contamination of the grain.

    Herbicides can be used to replace some of the cultivation.

Herbicides are particularly effective against grass weeds. There are the sulfonylureas, triflurin or diclofop groups available at reasonable prices.

They can be used with deep ploughing but the cost is high because farmers pay for both cultivation and herbicide.

    * Herbicides alone.

    Herbicides can be used for total weed control.

Either a knock down herbicide or one of the above pre-emergent selective herbicides. 

The crop is then sown using a No Till system. That is with a specially modified seeder with sowing tines only  or more commonly a Direct Drill system where the land is cultivated and sown by the seeder in a single operation. Also see  Cultivation and seeding

    The arguments for and against herbicides are discussed below but in this context there are two approaches:

    + Emergency use.

    If the rains are late or if they are excessively heavy farmers will find that they are behind in the preparation of the land for sowing.

With heavy rain it is a good guess (but by no means secure) that late crops will mature.

With late rains in autumn a poor finish in spring is a high risk.

The application of herbicides and direct drilling will reduce the time for seed bed preparation and weed control by three to four weeks. It could make the difference between a good crop and total fail in these circumstances.

    + Regular use.

    This is an economic decision.

Some farmers are finding a different balance between cultivation and herbicides.

The various uses described above require a full complement of tractors and implements.

Some farmers are working on the basis of a greatly reduced investment in machinery and the greater use of herbicides instead.

One of the possible problems of continual herbicide use is weed resistance.

Using herbicides  

*  Arguments for and against.

       * Sustainability.

    The major argument used against herbicides is a general one about sustainability.

Herbicides are dependent on petro chemicals and fossil energy.

They are part of the trend of pushing more and more fossil energy into agriculture.

While this is true it has to be put into perspective.

The amount of energy in herbicides is less than the amount in artificial nitrogen fertilisers.

The replacement of nitrogen fertiliser with legume nitrogen should be a much higher priority.

Herbicides also replace cultivation which uses energy.

In total herbicide systems they replace tractors and implements. The energy balance of herbicides is most complex.

     * Reduced soil erosion.

    This is also a sustainability issue.

The major cause of soil erosion in WANA is excessive cultivation.

Herbicides if used with shallow cultivation reduce cultivation and therefore erosion.

As soil erosion is the most important sustainability issue in the cereal zone of the WANA region the argument for herbicides is strong on sustainability grounds.

    * Control of specific weeds.

    Some weeds are extremely difficult to control by any other means.

For example we had a heavy infestation of oxalis in some fields on our farm in Australia. The weed grows from a small bulb in winter.

It is impossible to control with cultivation.

Cereal production is almost impossible.

The oxalis can poison sheep.

Using a systemic herbicide once every ten years produced magnificent control of the weed and converted land that was of low value for pasture or cereals into good quality land.

    * Earlier sowing of cereal crops.

    This a powerful reason for the use of herbicides.

Early sowing providing weeds are controlled is a proven method of increasing cereal yields in the cereal zone of WANA.

It is particularly important in the greater part of the WANA zone with a rainfall less than 500 mm.

Where the season is longer and the rainfall more reliable early sowing is not so vital.

Herbicides must be used with shallow cultivation to achieve early sowing.

With deep ploughing the seed bed requires many cultivations to break clods and level the ground. These additional operations have to be carried out even if they are not needed for weed control purposes. They take time.

    * Lower cultivation costs

    Herbicides if used correctly with shallow cultivation can reduce cultivation costs substantially.

With deep ploughing there is little advantage in terms of cost as the land must be worked many times after deep ploughing to break down clods and produce a level seed bed. Herbicides are an additional cost not a replacement for cultivation.

Making the decision.

     The most important aspect of the decision to use herbicides is to make it early.

Farmers should try to avoid the situation of trying to control weeds by a series of cultivations, failing and then turning to herbicides.

Such a program has all the costs of both cultivation and herbicides.

    Some case studies:

    * Heavy rye grass infestation.

    Let us say that a farmer grows a grain legume crop that is heavily infested with rye grass in the spring.

The rye grass produces huge amounts of seed.

The farmer then decides to sown cereals in the following autumn.

The rye grass seedlings are controlled by cultivation but there are waves of germinations and good control is difficult using cultivation alone.

Perhaps it would have been better to grow something else but if cereals are important a selective herbicide can be most effective in controlling rye grass and sowing early.

    * A wet season.

    Let us say that the season is excessively wet and the cultivation program is delayed.

Instead of digging small seedling out of the ground and knocking them around to remove the soil from the roots there are well established plants.

These will be more difficult to kill.

It will take time.

Farmers may find it better to apply a total knock down herbicide and eliminate two or more cultivations and three weeks delay.

See  Cultivation and seeding  for the concept of priority cultivation and seeding.

Practical problems.  

     Generally the practical problems of herbicide use have received a low priority in the WANA region.

Herbicides have been used on research centres.

The response has been good.

They have been taken out to farms and the results have been variable. Frequently the operator is blamed for the poor on farm results.

In fact there are many problems on farms that do not exist on research centres. It is important to recognise these rather than blame the operator for the poor operation of the sprayer.

    * Boom stability.

    Deep ploughing causes waves and rough seed beds.

On research centres these are usually levelled with many more cultivations.

Farmers do not have the time, resources or money to make good seed beds from the results of deep ploughing.

Spray equipment is badly affected. The boom swings and the application of the herbicide is poor.

Farmers reduce speed which reduces the problem but creates additional cost.

    * Spray volume and water availability.

    Generally volumes of 100 litre per ha of water have been used on research centres.

This is easy if there is a ready supply of water from a tap but many farms do not have plentiful water so readily available.

A reduced volume of under 25 litre per ha is a more practical solution but like shallow cultivation it necessary to determine whether this is possible with the nozzles etc. on the sprayer or whether the sprayer has to be modified.

This is a little exaggerated as most farms now have some water supply that does not involve carrying the water on a donkey. Water supplies are however difficult and high application rate of 100 litres per ha. are beyond the capacity of many farms.

    * Water quality.

    Water quality can also affect the efficiency of the herbicide.

Water quality is often poor on farms or the best water must be reserved for human use.

    * Calibration.

    Calibration is not easy with the standard sprayers used in the WANA region.

The farmer has to adjust the ground speed of the tractor to a certain PTO speed for the pump.

The pump provides a steady volume and the ground speed determines the amount per hectare.

If the ground is rough or if for any other reason the ground speed is changed the whole calibration is altered.

Unfortunately as so often there are trade offs.

The high volume applications of over 100 litre/ha are not so sensitive to poor calibration (10 litre more or less per ha is not a large error) but it is difficult for farmers to find and transport such quantities of water.

Lower volume applications are more practical from the water view point but need better calibration. (10 litre more or less is a large error for a 25 litre per ha rate of application.)

Types of spray equipment.

    The farmers' units work on two simple principles:-

   * PTO drive.

    The standard units work on the basis of a pump that moves the water and herbicide from the tank under pressure to the boom.

The pump is driven by the PTO on the tractor.

The volume and pressure levels are determined by the PTO speed.

The application rate over the ground is then determined by the speed the unit travels over the ground.

The unit must be calibrated for a certain gear of the tractor and a certain engine speed.

If the gear is changed the relationship between PTO speed and the ground speed changes.

If the engine revs change the relationship alters also.

More modern units have electronic monitoring devices which will provide the operator with volumes and application rates but these are expensive and well beyond the means of the average farmer in the WANA region.

    * Ground drive.

    The ground drive sprayer unit overcomes the calibration problem by driving the pump from the wheels of the trailed spray unit or the ground speed PTO.

The sprayer is automatically calibrated.

The faster the unit moves over the ground the faster the pump turns.

More water and chemicals are pushed through the boom.

Obviously there are limits as excessive speed will increase the pressure to levels that are higher than the design limits of the nozzles.

Extremely low speed will also cause problems as lack of pressure will mean the spray fans do not form.

Within these extreme limits there is a wide range where calibration is automatic. The rate of application can be altered. It is done by changing the cog on the pump.

One difficult with the ground driven units is that they miss the first 50 m of crop. This represents the time needed for the pump to build up pressure.

Farmers have overcome this minor fault by adding a hand crank and building up the pressure by hand before moving the tractor.

This is a ground-drive sprayer on my own farm in Australia. The near wheel drives a piston pump. The chain and gear wheel can be seen clearly. To change the application rate another gear wheel is fitted. The pump is obsured by the boom. The suction hose from the tank to the pump has been disconnected for cleaning.

The spherical tank reduces the swaying of the sprayer on rough ground. It is also excellent for cleaning. The water and herbicide drain down to the outlet at the bottom even on a slope.

Currently there are five different models available with tank capacities from 1000 to 4000 litres.

For more details.

    * Linkage.

    These are small cheap units mounted on the linkage.

They are driven by pumps fitted to the PTO and therefore have all the problems of calibration.

All modern tractors have a "ground speed PTO" so there is no reason why the ground drive pump could not be used. It simply has not been done,

    * Trailed.

     Trailed units have the advantage of carrying more water.

They can be operated by a PTO pump or a ground driven pump.

Some units have a spherical tank which gives additional stability.

    * Specialised units.

    These are used by contractors and provide a high level of spraying efficiency but are expensive to purchase.

Below are some high-tech spraying units that are only suited to contractors or very large farm.

Sensor weed sprayer

This sprayer is used to apply herbicide to isolated summer weeds. The boom passes over the ground. On the front of the boom is a red and infrared light that shines down onto the ground. When the boom passes over a weed the light is reflected back to a sensor which turns on an individual spray nozzle.

Obviously there is a considerable savings in the amount of herbicide that is used. Alternatively a more expensive but more effective herbicide can be used. Spraying speed is increased as there is less filling of tanks.

The disadvantages are that it cannot be used to spray weeds in a crop and the current machines are expensive. For small farmers hand spraying of summer weeds is a feasible option.

Direct injection

The direct injection sprayer eliminates the need to mix chemicals in the tank. The water and chemicals are stored in separate containers and the chemical is injected into the water just before it enters the spray line. The immediate benefits are reduced cleaning time, no manual mixing of chemicals and no leftover mixed and diluted chemical that has to be disposed of. Operator safety is improved as contact with the concentrated chemical is reduced. Multiple pumps can be fitted which allow an almost instant change to different chemicals or mixes.

Height control

Height control sensors are fitted to the ends of the boom and they feed information to hydraulic rams that control the height of the boom over a crop. If the boom hits the ground it can cause considerable damge. The height control sensor allows operators to set the boom height at the optimum level without fear of damage. They can also operate at higher speeds and concentrate on calibration without giving most of their attention to the height of the boom.


Human health and the environment  

    There is justifiable concern about the use of herbicides on human health and the environment.

Herbicides in general are not as serious a pollution and health problem as insecticides or most fungicides.

Pollution and health problems are mostly associated with the concentrated chemicals.

Containers should not be re-used nor disposed of where they can contaminate water supplies.

Some practical measures to reduce pollution and protect human health are descibed below.

    * Returnable containers.

    A returnable container which carries a substantial deposit is the best means of ensuring the containers are not re-used for water storage or disposed of in a careless way.

The re-use of containers is most cost-effective once the collection system has been established.

There is a need for joint action by farmers, local government and chemical suppliers to establish a common collection system.

If each chemical supplier establishes a separate system (they will only do so if forced by law) the cost will be high.

It will also make it more difficult for small suppliers to enter the market and provide cheap chemicals to farmers.

Some containers may not be reusable.

They can also be returned using the same system.

They carry a tax which is then returned in part to the farmer. The remainder is used to dispose of the container safely.

    * Pouring devices.

    There are many simple and cheap gadgets that reduce the danger of chemical spillage and splashing during the transfer of the concentrated chemical from the container to the spray tank.

These should be made more widely available and farmers encouraged to use them to protect their own health and save on chemicals.

    *  Clean water for washing.

    This is one of the simplest low tech means of reducing contamination problems.

All spray units should have a small container of clean water so the operator can wash off any concetrated chemical quickly if a spill or splash occurs.

Operators should of course wear protective clothing and gloves.