Irrigation is the controlled provision of water in order to successfully cultivate a crop under the following conditions:
Because climate and soil conditions are specific to each area, it is not possible to give an in-depth account of irrigation systems and scheduling, but only to describe the more important terms, guidelines and practical implications.
The rate at which water infiltrates the soil is an inherent property of the soil. It is highest in dry sandy soil and lowest in wet clay soil.
An irrigation system should supply water to the soil at the rate at which it can absorb it.
Excess water leads to reduced air supply (oxygen) in the soil, runoff, erosion and between-row weed contamination.
After water has moved downwards through the soil profile (through gravity and soil water gradients, i.e. redistribution), this point represents the highest limit of the soil’s capacity to store water.
Practical implications:
The field water capacity of clay soil is higher than that of sandy soil, and also higher in layered than in uniform soils of the same structure because layered soil limits drainage.
This is often used as a measure that indicates the crop is suffering water stress; irrigation should should be done before this point.
Practical implications:
By the time wilting is visible in a plant, the physiological processes are already impaired. It is accepted nowadays that maximum growth and production is only realised if the soil moisture potential is kept high and if sufficient moisture is available to the plant through the whole growing season.
Evapotranspiration: This determines the pattern and rate of evaporation from an open piece of ground, together with the rate of transpiration from the leaf surfaces of the plants.
Practical Implications
While the lucerne seed is germinating and the seedling is becoming established, evaporation from the soil is higher than from the plants, and is dependent on the climate, on the movement of soil water and on soil temperature.
In established stands where the leaves shade the soil, the transpiration rate of the crop is more important than evaporation in the water use of the crop.
Because plants need CO for photosynthesis (the amount depends on production potential) they have to transpire, with the result that they will necessarily lose water vapour through their stomata.
This is the total of all water fractions which are applied, lost and stored in the root zone, during a given period.
Accumulation : Additions (rain, irrigation and upwards capillary flow into the root zone, minus losses (runoff, downwards drainage, evaporation and transpiration).
Practical Implications
The calculation of additions (rain and irrigation) is practical and possible, while the calculation of losses requires specialised equipment and trained personnel. Well developed models are available in SA from which reliable estimates can be made.
To start with, the rate of water uptake is dependent on the ability of the roots to absorb water from the soil, as well as the ability of the soil to transport water and to provide water to the roots at a rate sufficient for the needs of the plant for transpiration and growth.
The following therefore play an important role:
This is described as the procedure which the producer uses, which determines when and how much water must be applied and is the result of the monitoring of:
The above-mentioned factors are not always measurable by the producer, as expensive measuring instruments and soil- and crop-specialists are needed.
Expert advice is readily available and is generally required by producers who have high expectations (sustainable production) of their farming systems.
Where new stands are to be established, the soil must be irrigated to field capacity before the final preparation of the seedbed. This preparation must then be completed as soon as possible.
After sowing, the soil must be kept damp with light irrigation, until the lucerne has emerged.
Lucerne requires about 1200 mm water/yr to give good production. The volume of water given during irrigation should be sufficient to wet the soil to its full potential (maximum that soil can absorb = field water capacity) while the frequency of irrigation (time between irrigations) should be left as long as possible.
It must be kept in mind that lucerne is usually cut throughout the growing season (unlike annual crops such as maize, wheat or beans) and that water requirements vary with the season, so that irrigation should be adjusted accordingly.
About 150 mm water is needed between cuts, depending on the soil and the climatic conditions. It would be best to schedule irrigation so that the lucerne gets two equal amounts, just after cutting and again 14 days later.
In winter there should be a single irrigation just after the last cut is removed, and another about 6 weeks before the beginning of the new growing season.
Overhead irrigation is a common method of irrigating lucerne. There are disadvantages if pathogens and fungi occur in the area.
Disadvantages of overhead irrigation:
A productive stand of lucerne uses a large quantity of nutrients, and a complete fertilisation program is therefore necessary to ensure a long-lived stand.
A soil analysis is necesssary if one wants workable recommendations as to fertilisation.
Most analyses determine pH (lime), and levels of phosphorus (P), potassium (K) and certain other secondary nutrients and trace elements.
The recommended levels differ according to soil type, soil and yield potential, and management requirements.
Lucerne has a high nutrient requirement relative to other crops because of its high productivity and nutritional value.
The following shows the amount of nutrients which it removes from the soil per ton dry material produced. (Undersander et al 1994).
| Nutrient Elements | Removal per ton dry material (kg) |
| P | 2.70000 kg |
| K | 21.00000 kg |
| Ca | 13.00000 kg |
| Mg | 2.70000 kg |
| S | 2.70000 kg |
| B | 0.04000 kg |
| Mn | 0.05000 kg |
| Fe | 0.15000 kg |
| Zn | 0.02000 kg |
| Cu | 0.00450 kg |
| Mo | 0.00009 kg |
These nutrients need to be supplemented, else it must be expected that the production and nutritional value of the lucerne would drop with time.
Trace elements such as B, Mn, Fe, Zn, Cu and Mo are removed to a lesser degree, but still play a vital role so that shortages can result in considerable losses.
Because many nutrients which are removed come from the soil, it is not recommended that a fertilisation program be worked out on the basis of nutrient removal.
Rather it should be done on the basis of soil analyses, as this is the most reliable method of preventing shortages.
Visible symptoms can be used to determine nutrient requirements for following harvests.
However, by the time that symptoms become visible on the plant, the nutritional deficiency may be so advanced as to result in serious loss of production and the weakening of the stand.
The symptoms may also be the result of environmental conditions, diseases and other problems not related to nutrient deficiency.
Plant tissue analyses can determine the nutrient status of the crop before any symptoms are noticeable.
They do not measure nutrient levels in order to make recommendations as to a fertilisation program, but when they are combined with a soil analysis, a full program can be worked out.
Tissue tests can also be used to determine levels of sulphur and micro-elements. The table shows the amount of each nutrient in tissue which is sufficient for lucerne production at first flowering.
Nutrient |
Low |
Adequate % |
High |
|---|---|---|---|
| Nitrogen | <3.00 | 3.00 – 5.00 | >5.00 |
| Phosphorus | <0.26 | 0.26 – 0.70 | >0.70 |
| Potassium | <2.41 | 2.41 – 3.80 | >3.80 |
| Calcium | <0.50 | 0.50 – 3.00 | >3.00 |
| Magnesium | <0.31 | 0.31 – 1.00 | >1.00 |
| Sulphur | <0.26 | 0.26 – 0.50 | >0.50 |
Dpm |
|||
| Boron | <30 | 30 – 80 | >80 |
| Manganese | <25 | 25 – 200 | >200 |
| Iron | <30 | 30 – 250 | >250 |
| Zinc | <20 | 20 – 70 | >70 |
| Copper | <5 | 5 to 30 | >30 |
| Molybdenum | <1 | 1 to 5 | >5 |
Legumes have the special ability that, in symbiosis with Rhizobium bacteria, they can fix atmospheric nitrogen which they then use to manufacture plant proteins.
The presence of an effective race of nodule-forming bacteria (which are specific for most pasture legumes) is necessary to obtain maximum nitrogen fixation.
Advantages of this association are summarised as follows:
No N fertiliser is necessary as long as the N-fixation process is working effectively.
Soil fertility can be increased by the release of N into the soil.
Grasses grown in rotation or as a mixture with legumes can utilise remaining field N.
The reduced competition for available N allows the grass in a mixture to utilise soil N optimally.
The crude protein content of the grass component is often the same or higher in a grass-legume mix as in a pure stand with N-fertilisation.
There is no pollution of water sources with inorganic N.
Development and maintenance of root nodules:
Rhizobium invades the plant via the root hairs, after which the cell walls of the root hair form an infection tube within which the bacteria moves.
Bacteria are meanwhile released into the root cytoplasm and become surrounded by a membrane. In this way the Rhizobium differentiates into different cell types, called bacteroides, and a root nodule develops.
Atmospheric N diffuses via the root hairs into the bacteroides, is converted to ammonia and made available to the plant.
A protein called leghaemoglobin is produced by the cells of the root nodule, and transports oxygen to the rhizobia.
If the nodule is broken open, it will be seen to have a characteristic red or pink appearance. This is due to the presence of protein, indicating an effective nitrogen-fixing process. A nodule that is white, green or brown indicates that the Rhizobium is inactive or only minimally active, while a black nodule is a once-effective nodule that has died.
The bacteroides are dependent on the photosynthesing plant for their carbohydrates. The carbohydrates, with oxygen, are used by the bacteroid to supply the energy necessary for N-fixation (among other processes).
The energy for conversion of atmospheric N into ammonia must therefore be supplied by the plant, and in some instances can affect dry mass production.
Lucerne-specific Rhizobium functions best at pH 6.4-7.2.
N-fertilisation, or the presence of high N levels in the soil, are two of the most important factors reducing N-fixation.
Fertilisers containing copper and zinc inhibit the growth of Rhizobium.
Molybdenum is necessary for the fixation process and can be applied with fertiliser in small quantities. A slightly higher than normal quantity in the plant can be toxic to animals, and therefore fertiliser levels must be carefully considered. Certain types of Mo-containing fertiliser can inhibit nodule formation.
The use of herbicides, fungicides and insecticides may have a negative effect on the survival of Rhizobium. If necessary, care must be taken not to apply these at sowing time, but only after the association between the legume and the Rhizobium is well established – about a month after establishment.
Irrigation trials conducted by the ARC-Range and Forage Institute in 1994-1996 (Roodeplaat Research Station) showed that SA Standard can fix 286 kg/ha N in its initial year. In the following year a figure of 414 kg/ha N was obtained.
The percentage fixed N in the plant was 20-50% in the first year and 55-85% in the second. Further conclusions which may be drawn from the experiment are:
For the first 2 tons/ha material cut in the first year, about 35 kg/ha N were fixed. For every ton/ha above 2 it can be considered that about 15 kg/ha N more were fixed.
For the first 2 tons/ha material cut in the second year, about 40 kg/ha N were fixed. For every ton/ha above 2 it can be considered that about 20 kg/ha N more were fixed.
The potential nitrogen fixation ability of lucerne per cut during the first two years of cultivation.
DM production |
Potential N-fixation (kg/ha) |
|
|---|---|---|
| Initial year | Second growing season | |
| 2 | 35 | 40 |
| 3 | 50 | 60 |
| 4 | 65 | 80 |
Lucerne is attacked by insects throughout the year. They feed on the stems, leaves, roots and seeds of the plant and can have a serious effect on production.
Which insects occur in a stand of lucerne varies greatly between years, depending on climatic conditions and the occurrence of their natural enemies.
Lucerne should be inspected regularly for the presence of harmful insects, which can do serious damage to the stand before any control measures can be implemented.
It is important that insecticides are correctly and responsibly applied. Apart from being expensive, unnecessary use can lead to the development of resistance to that insecticide, it may build up to toxic levels in the lucerne, pollinators may be killed and game, domestic animals and human health may be affected.
A prerequisite for successful insect control is correct identification of the insect. It is also important to understand what problems it may cause.
The goal of successful control is to suppress the unwanted insect population before it can cause damage, while not affecting desirable insects.
The beetles are oval, about 4 mm long, brown or black in color, and have well-developed rearmost “thighs” (femora) with which they can jump if disturbed
The beetles feed on the leaves and chew several small holes through part of the thickness of the leaf, which look like whitish “windows”.
This can do much damage to seedlings, but older plants are only affected if there is also drought stress. The larvae feed on the roots of the plants
Parathion is recommended as an insecticide.
The white-fringed beetle is a serious problem in South Africa. The larvae are white, broad, banana-shaped grubs that remain in the soil. The adult is a flightless beetle, about 12 mm long, grey with fine whitish lines along the length of the wings, and a characteristic broad white stripe down the length of each side.
The eggs are laid in clusters on plant parts or on other objects on or near the soil surface. The adult beetles are very common in the summer, but may still be encountered in the winter. They can live for up to 5 months and lay 1000 eggs each. The eggs need contact with free water to hatch. If they do not get this they may still remain viable for some months.
Larvae develop during the winter, usually 10-20 mm under the soil surface, but occasionally up to 160 mm deep. Larval development is usually complete by spring, but can be extended through the following summer and winter until the pupae hatch and the adult beetles appear. Although they cannot fly, these are spread in feed, equipment and implements to other areas where they start new infestations. There are no males and new colonies can arise from a single beetle.
The larvae attack the roots of the plant and can cause considerable damage. They have a serious effect on the longevity of the stand, as the plants are weakened, become wilted and eventually die off .
The adults feed above ground but seldom cause serious damage. White fringed beetles, even at a low infestation rate, can cause considerable loss of yield. At a high rate of infestation, root losses can run to over 80%.
Because lucerne is a perennial crop, and because of the succession of generations of the insect, there is a possibility that lucerne may already be heavily infested by its second season. Die-off may then reduce plant numbers to such an extent that the stand becomes uneconomical.
Populations can be limited by rotating with other grains or grasses, or a fallow summer before planting with lucerne may be applied. The insect can only be combated by certain soil-applied insecticides, for instance, those with gamma-BHC’s, methyl bromide and chlorpicrin as active ingredients.
Aphids are small, soft, delicate insects with long legs and antennae. They are usually green with or without transparent wings. They form colonies and suck sap from the soft growth of the plants. There are three species on lucerne in SA, namely the spotted alfalfa aphid, the blue-green aphid and the pea aphid. A fourth species, the black or cowpea aphid, has been noted in South Africa, but there is as yet no information about its spread or its effect on lucerne here.
The spotted alfalfa aphid is relatively small (1.4-2.2 mm long) with long antennae, short cornicles, and a tail. It is shiny and pale yellow or green-white with red eyes and three pairs of brown spots on the back of each segment of the abdomen. Adult female aphids may or may not have wings, and the wings have dull areas along the nerves. The spotted aphid jumps if disturbed, an unusual behaviour in aphids, releases large quantities of honeydew and starts by invading the undersides of the leaves.
Serious spotted aphid infestation results in the leaves turning yellow, reduced growth, lower yield and eventually plant death. The damage may also reduce the nutritional value of the lucerne. The aphids choose strong, rapidly growing plants with a high turgor as found under high humidity conditions. They prefer hot weather, but not dry conditions.
The use of resistant cultivars is recommended. There are insecticides for aphid control, with the active ingredients dimethoate and omethoate.
The blue-green aphid is about 2-3 mm long. The wingless form is blue-green, but can also be blue-grey. The winged form has a brown to dark brown “chest”. The antennae are an even brown. This aphid runs quickly and produces many more winged than wingless forms. They are all female and produce live young.
The blue-green aphid causes a great deal of damage to lucerne. It feeds on the soft sappy leaves and stems. This causes the leaves to curl up and turn yellow, growth is seriously dwarfed, and the plants recover only slowly after cutting. The aphids form colonies around the apical growing points, but as the population increases, they also group around the softer stems and under older leaves.
The use of resistant cultivars is recommended. There are also insecticides for the control of aphids, with the active ingredients dimethoate and omethoate. These insects are able to rapidly build up resistance to insecticides, and may re-infest a treated land. They can cause great economic losses at relatively low population levels.
The pea aphid is a large aphid and grows up to 4 mm long. The wingless form is pale green or yellow-green, sometimes with a pinkish tint. The antennae and cornicles are long and slender. The immature forms are lightly powdered with wax. The winged form has a pale brown “chest”. The legs and antennae have brown areas around the articulations which distinguish them from the blue-green aphid. This aphid gives off little honeydew.
The aphids feed in dense colonies on the undersides of leaves and on young shoots and stems. Sometimes the plants wilt and subsequent leaves, flowers and pods are deformed. The growth of the lucerne may be seriously weakened. These aphids may be vectors of more than 30 different viruses.
The use of resistant cultivars is recommended. There are insecticides for aphid control, with the active ingredients dimethoate and omethoate.
Adult mites and nymphs can easily be seen with the naked eye and are about 1 mm long with pear-shaped, velvet-black bodies and legs.
The larvae are longer. Populations large enough to cause serious damage occur from the first winter rains till early summer, then disappear because of the high temperatures and dryness. Eggs with especially hard shells survive the summer. The mites prefer damp conditions, but avoid free water. Arctotheca calendula (gousblom) is a host, but good weed control will help to prevent buildup of damaging populations. The mites congregate together when they are feeding, otherwise they group together under loose soil and leaves.
The mites cause silver-white patches on the leaves, especially along the main veins of the lowermost leaves which eventually wilt and turn yellow. Seriously infested plants have a crumpled and deformed appearance. Young plants may die off. The plants look as if they were killed by frost.
Systemic chemical control is necessary. Insecticides containing dimethoate and omethoate are recommended. Biological control with Anystis sp., a predator mite, has been successful in some areas, but chemical control remains important.
The lucerne brown mite is green-brown with pale red legs. Newly hatched larvae are bright red, but become green once they begin to feed. They overwinter both as eggs and in other stages of the lifecycle.
In the spring, the overwintered eggs hatch, and adult females also lay eggs which become part of the same generation. Larvae and nymphs which have overwintered grow to maturity and lay eggs which form part of a late spring generation. Breeding and development continues till autumn, but can be stopped by high temperatures. The mites drop to the ground if disturbed.
Initial symptoms are chlorotic streaks and finely crinkled strips on the leaves. Later the leaves become yellow or brown and may wilt completely.
The mites can be controlled with a variety of miticides, e.g. dimethoate and omethoate.
The young larvae or caterpillars vary from nearly black to dark beige in color and become lighter as they age. The colour of older caterpillars can vary from green to brown to red-pink. The underside is dirty white. The presence of a broad white to pale yellow stripe down each side is characteristic. The full-grown caterpillars are about 30-40 mm long, with hairs on the body.
The rear wings are paler, each with a characteristic dark brown to black fleck. The moth is 15-20 mm long when sitting and has a wingspan of 35-40 mm. The eggs are yellow-white and become brown just before hatching. The pupae are in thin cocoons in nests about 25-75 mm below ground level.
In cold areas the larvae are dormant in winter, but along the coast and in the sub-tropical areas undergo continuous development, so that there is moth and larval activity throughout the year except for midwinter. Temperatures above 35 °C are fatal for developing pupae and often result in a lessening of activity in midsummer. Most activity takes place in the spring, early summer and again in the autumn.
The caterpillars feed on the young leaves and hollow out the inflorescences. They may also destroy the growing tip of the plant. They can do a great deal of damage in a short time.
Epidemics may break out suddenly and unexpectedly. Natural enemies are insufficient to control such plagues and insecticides must be used. Beta-cyfluthrin, lambda-cyhalothrin and methomyl may be used to combat the caterpillars.
The caterpillar is about 40 mm long and approximately the same color as the young leaves and stems of the lucerne. The pupae are also green and are attached to the stems of the plant. The adult butterfly has a wingspan of 40-50 mm and is orange-yellow in color, the colour being brighter in males than in females.
The wings of the females may sometimes be off-white rather than yellow, but these have the same black pattern on them. The eggs are small, oblong, yellow and individually attached to the stems of the plant.
The eggs hatch throughout the year, rapidly in summer and slowly in winter. There may be 6-7 generations in a year.
Adults are very active during sunny periods and this initiates the time of serious infestation.
The caterpillar prefers new growth rather than adult plant material and the succession of generations is therefore synchronised with the cutting and regrowth of the lucerne.
Young larvae feed at first beside the main veins, leaving only the veins of the leaves behind.
Older larvae eat from the margins inwards and thus eat the whole leaf.
Light infestation can go unobserved because the caterpillar is camouflaged, but may lead to losses of up to 10%.Serious infestation can at some times of the year lead to losses up to 50%.
Natural enemies control lucerne caterpillars reasonably well under certain conditions. A polyhedrosis disease that attacks the caterpillars can reduce their population considerably.
A bacterial insecticide can be used as a control measure. Some chemical insecticides are effective against outbreaks of the caterpillars but must be used with circumspection because they have a negative effect on biological control.
alphacypermethrin, Bacillus thuringensis var. kurstaki, carbaryl, cyfluthrin, cypermethrin, cypermethrin-high cis, deltamethrin, lambda-cyhalothrin, mercaptothion, permethrin, tralomethrin, trichlorfon.
This is the most important pest of dryland lucerne in the winter rainfall region of the Cape. They are soft, small, 1-2 mm long, grey-white, wingless, and provided with a strong forked jumping organ which allows them to jump high into the air when disturbed.
Infestation can become serious under humid conditions, and with irrigation is almost continuous. In dryland lucerne in the winter rainfall areas, the eggs hatch after the first rains, and 4-5 overlapping generations can be completed in one rainy season. When conditions are dry, the eggs do not hatch, and they can build up in the soil, sometimes at an advanced stage of development. The highest population density would usually occur in the autumn when all the eggs hatch at the same time.
Very young nymphs chew small holes through the epidermis into the underlying mesophyll tissue. This results in a finely speckled appearance to the lucerne shortly after infestation. Older nymphs and adults remove large areas of leaf epidermis, eat all of the underlying mesophyll and leave only the bottom epidermis.
This causes typical “windows” in the leaves. In serious cases only the veins of the leaves remain. The insects also eat the stems of lucerne, which then wilt. Lucerne yield can be reduced by up to 20%.
There is a measure of biological control by predator mites. Treating seed with systemic insecticides can control both earth fleas and the lucerne brown mite at the same time. Spraying lucerne in autumn and spring is also effective. The active ingredients of the insecticides used are dimethoate, mercaptothion and omethoate.
The larvae are small, white, legless grubs with segmented bodies about 2 mm long. The pupae are white and soft. Adults are small, highly active wasps about 2 mm long, with yellow-brown claws and transparent wings. The eggs are laid inside soft young seeds. Some larvae develop immediately and become pupae inside the seed, emerging as adults in the same season. Others remain dormant in the seed through the winter, emerging as adults the following spring. There can be up to 3 generations per year.
The entire contents of the seed are eaten and only the thin outer hull left. Cleaned seed from infested lands contains many empty hulls filled with live insects that emerge later. This influences the germination percentage of the seed and therefore its quality. It is also a source of infection when the seed is sown.
Hay from an infected land must be cut and removed before the seed is ripe. Only clean seed should be sown.
Adults are 30-50 mm long, brightly coloured and produce a stinking yellow liquid when disturbed.The wings are often underdeveloped and yellow-green with red markings. The head is dark with orange or red eyes and the thorax-shield yellow-green or bluish.The abdomen has contrasting yellow, orange, blue and black patterns. Young stages look like adults, but are mainly black with yellow or white stripes. The hoppers swarm together, but the adults do not form swarms. The adults are slow and easily caught by hand. Eggs are laid in autumn and overwinter in the soil until they hatch with the first spring rains.
Both hoppers and adults may cause severe defoliation in localised areas.
Locusts are considered difficult to control and are not easily destroyed by insecticides. It is important to control the swarms while they are still small and situated in their overwintering areas, before they can get into the lucerne. If they do get into the lucerne, spray infested patches with carbaryl to protect the lucerne.
