ROTATIONS  COMPARED

 

WEED CONTROL

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Cereal- fallow rotation.
 

  * Cereal phase.

        Weed control is usually good because of the previous fallow. The cereal-fallow rotation is an effective means of controlling weeds - and little else. In fact the major benefit of the rotation is weed control. Soil fertility is low and soil erosion is high.

    * Fallow phase.

    Most weeds germinate in the autumn and are grazed by livestock during the winter.

In the spring the land is cultivated or ploughed to a depth of 20 cm before the weeds produce seed.

The weeds are effectively controlled.

If there is any further weed growth the fallow is cultivated again. The fallow is an effective means of reducing the reserves of weed seeds in the soil.

    The above description is the ideal fallow.

In the real world of farming the fallow is at times cultivated late because farmers are desperately short of feed for their livestock.

Spring should be the period of abundant pasture production.

The fallowing of the natural weedy pasture turns the spring abundance into an acute shortage. Farmers may delay the cultivation of the weedy pasture which then produces seeds that fall on the ground.

Another weakness is that the ploughing does not always kill all the weeds. Farmers save money and time by not cultivating a second time to complete the weed kill.

 Summer weeds can also grow on the fallow.

These weeds are difficult to control with grazing as most of the weed species that have become adapted to summer fallow are unpalatable to livestock.

Summer weeds are also well adapted to utilise any stored moisture in the fallow. In fact they are able to remove more soil moisture than cereals.

A bad infestation of summer weeds will not only remove all the moisture available in a fallow but reduce the level to such an extent that the first rains will still not be accessible to the cereal crop.

Control by cultivation will create further erosion potential.

Control by the use of herbicides is difficult and costly.
 

Cereal - vetch rotation

    This is discussed next because this rotation can be an effective means of weed control.

    * Cereal phase.

    If the vetch has been cut for hay in the previous year weed control the following cereal crop is reasonably good. The vetch stubble after the hay cut should be heavily grazed also.

    * Vetch phase.

    Vetch is often sown with oats. The combination of oats and vetch is a strong competitor with weeds in winter and spring.

The crop is then cut for hay before the weeds produce seed or at least before the seeds drop on the ground. The seeds are removed with the hay.

The vetch and oats and weeds then grow again. This re-growth in controlled with heavy grazing.

A possible weakness in the system is the late cutting of the hay. In this case weed seeds will drop before the hay is made.

Cereal - medic rotation.

    * Cereal phase.

    Experience over the last 25 years with the medic-cereal rotation in the WANA region has shown that weed control is variable.

In a few examples there has been a large increase in the weed burden that has cancelled the increase in fertility leaving cereal yields unaltered.

   * Medic phase.

    Weed control in a medic pasture requires more management skill.

In the spring the farmer needs to graze the pasture to a height of about 10 to 15 cm in order to reduce the tall growing grass weeds.

This will encourage the medic to compete and hopefully dominate the pasture.

The aim is to produce abundant medic seed and a minimal amount of weeds seeds through carefully controlled grazing.

It can be seen that it is a balance in which it is almost impossible to stop weeds producing some seed. Total weed control can only be achieved by extremely heavy grazing or cutting for hay. Either of these operations would also prevent the medic from producing seed.

    Some case studies 

FAO in Algeria

  When the medic rotation was introduced into Algeria in the early 1970's by FAO grazing management was totally ignored.

The FAO planners believed that medic could be used as a "green manure" to fertilise the cereal crop and pasture was ignored as a source of feed for sheep.

Some farms had no sheep. Grazing management advice was not included in the project.

The results were a disaster. Almost all the farms abandoned the medic rotation within a couple of years as the medic was over-run with weeds.

Far from a medic dominant pasture it was a weed dominant pasture. The carry-over of weed seeds to the cereal crop caused cereal yields to fall. Only one or two farms mastered the system and achieved high cereal yields while increasing their livestock numbers by five times.

Libyan projects

    Other Australian managed projects, particularly in Libya, introduced grazing management and showed that large increases in livestock output could be linked to increased cereal yields on a more consistent basis.

Tunisia

    Other countries such as Tunisia took up the medic rotation but grazing management remained a problem.

    * Extension agents were more concerned with over-grazing than under-grazing.

They did on realise that damage could be caused by under-grazing. They believed low stocking rates were a "safe" option for the survival of the medic.

    * There were few attempts to plan the area of medic in relation to the flock.

The area of medic was at times greater than the farmers could possibly graze effectively with their existing flocks. Under-grazing was in fact built into the project plans.

Farmers in desperation cut hay from their medic pastures which partly solved the weed problem but destroyed the capacity of the medic to produce seed for future regeneration.

    * On some projects farmers without sheep were provided with medic pasture.

medic pasture were established on some farms that had no sheep on the assumption they would be supplied later. Often these sheep arrived late (in the spring rather than the autumn) and the medic failed due to under-grazing. Weed seed production was high

    * The Zaghouan 4 rotation.

On the IFAD project at Zaghouan in Tunisia in 2002 we developed some new rotations to overcome the problems of grazing management and weed control in the cereal crop. We have called it the Zaghouan 4 rotation. They are described in more detail elsewhere.

Year 1. Cereal crop is sown with medic pods broadcast over it.

Year 2. Medic pasture regenerates from pods sown over cereal crop.

Year 3 Medic pasture regenerates from pods produced in previous year.

Year 4 Medic pasture regenerates in autumn from pods produced in Year 2 and 3.

The land is cultivated and fallowed in the spring, This will destroy the medic and prevent it producing pods for future years. It will also destroy the weeds.

This fallow is more effective than most fallows in controlling weeds. The farmer is not under pressure to provide feed for his sheep.

With the traditional fallow - cereal rotation there is an acute storage of feed in the spring once the land has been cultivated for the fallow. With the Zaghouan 4 rotation half the land is in medic pasture. There is ample feed in the spring.

The farmer can cultivate earlier before the weed have produced seed. He can cultivate again to kill any re-growth of weeds as the area is much smaller and the cost lower.

The fallow is also effective in mobilising the soil organic matter and releasing organic nitrogen. This was one of the advantages of the fallow but when it is used for many years there is no more soil nitrogen to mobilise. The 2.5 years of medic provide ample soil organic matter.

Year 5 Repeat of Year 1. Cereal crop is again sown with medic pods.

This cereal crop should produce an excellent yield as it has 2.5 years of medic pasture to add fertility to the soil and a cultivated fallow to control weeds.
 

The Zaghouan 4 rotation can be used by small farmers as a permanent rotation.

 The Zaghouan 4 rotation can be used as a transitional rotation. It can be used for three or four cycles during which time the farmer develops his grazing management skills. In terms of sustainability it is a great improvement on the traditional cereal-fallow rotation. The area of fallow is reduced from half the farm to a quarter. The soil structure and fertility is improved, so the erosion during the fallow phase is considerably reduced.

    Alternatively the Zaghouan 4 can be used as periodic "clean up" rotation.

One of the difficulties facing the farmer adopting the classic medic rotation (cereal -medic year after year) is that if weed control is poor the problem can accumulate over the cycles of the rotation.

Poor weed control in the medic produces a large carry over of weeds and fewer medic pods. The cereal crop is poor. The medic regeneration is poor and the farmer decides to return to the cereal-fallow.

Under the Zaghouan 4 rotation a fallow provides a periodic "clean up" and the farmer can then try the classic medic - cereal again in the knowledge that he has a back up method of weed control if he fails to manage the weeds effectively through grazing and shallow cultivation.

 Cereal - grain legume rotation

    * Cereal phase

    Grain legumes are not a cleaning crop and weed control problems in the cereal crop grown after grain legumes can be serious.

    * Grain legume phase.

    Grain legumes are not strong competitors with weeds during the autumn and winter.

In spring the weeds grow and produce seed.

The reserves of seed will carry over to the next cereal crop.

Grain legumes should be grown on land that is comparatively free of weeds. They should be grown in rotations with cereals that allow cleaning crops such as vetch and oats hay to be included from time to time to reduce weeds.