The aim of a weed control program is to reduce the number of unwanted plants so that that damage as a result of weeds is reduced to a level that is negligible or at least tolerable.
The spectrum of weeds present is determined by factors such as the previous crop grown and the degree of contamination that existed, sowing date, sowing methods, etc. Problems are usually caused by annual weeds, with the exception of invasion by grasses, which are usually perennial.
In the case of established lucerne stands, factors like drainage, soil pH, availability of nutrients, physical damage by animals, grazing intensity and climate play a major role in the vigour of the stand. A vigorous stand usually does not have weeds and therefore seldom requires weed control.
Management practices as a part of weed control
The occurrence of weeds is often the symptom of incorrect management of the lucerne, especially grazing/cutting, fertilisation and irrigation. Effective weed control is linked to a uniform, dense stand of lucerne. Management practices which favour the lucerne and encourage its growth reduce weed competition.
Dense stands can be obtained by correct fertilisation, drainage where necessary, provision of water where it is a problem, disease and pest control, correct frequency of cutting, use of cultivars suited to the environment and other practices.
Preventative practices (before and at sowing)
Moisture and temperature are the most important environmental factors determining the time of sowing.
Autumn plantings are preferred because one can be certain of reduced weed competition at this time, and because by the next spring the lucerne will be well-established and better able to compete with annual grasses.
Lucerne can actually be planted at any time of the year under irrigation in frost-free areas as there are registered weedkillers to control the weeds.
Sowing depth is an important factor because, if the seeds are planted too deeply, emergence is poor and the resulting seedlings are weak, the plants grow more slowly and weeds can easily become a problem because the lucerne is unable to compete with them. The normal planting depth for lucerne is 5-7 mm, and seed must not be planted deeper than 15-20 mm.
Method of establishment
The method of establishment is important. Broadcast sowing generally gives better suppression of weeds than establishment of lucerne in rows, because the whole surface is covered.
If a weed problem should arise in a broadcast-sown land, then the only method of control is cutting and/or chemical control. If weeds become a problem in row-sown lucerne, then mechanical or hand-hoeing can be used.
Practices such as minimum-cultivation are sometimes applied. This consists of establishing the lucerne without ploughing so that there is minimal soil disturbance.
If applied correctly it will keep unwanted plants to a minimum. It must be kept in mind that if the soil nutrient status, specifically the soil pH, is incorrect, then minimum-cultivation is not an option, as lime must be mixed with the soil.
Lucerne is a long-term crop and weeds can impair the stand w.r.t. both quality and yield. Seedbed preparation is very important and the seedbed must be fine, firm and weed-free so that seed can germinate rapidly and develop into healthy plants. The footprint test is general: if one walks on the prepared seedbed the footprint must be well-defined but not deeper than the sole of one’s shoe, i.e. there must be a degree of firmness.
The pH of the soil must be corrected before sowing and any nutrient shortages alleviated. In general it is the case that a low soil pH (4.5 or lower) favours unwanted grasses. Lucerne is more sensitive to pH and to a shortage of phosphate than grasses. Low N, P and K can encourage certain undesirable plants.
The application of N also often has a stimulating effect on unwanted plants while disadvantaging the lucerne. Low levels of phosphate (P) are often associated with broad-leaved weeds. Lime usually has a favourable effect on all plants. It must also be kept in mind that 80% of the phosphate and 90% of the potassium is re-circulated by regular grazing, but that these nutrients are removed from the system if lucerne is grown for hay.
The use of good quality seed with a certificate of purity and germination percentage is necessary to prevent contamina-tion of the lands with weed seed. Certified seed has only a low percentage of weed seed permitted and its germinability is guaranteed. Dodder is a major problem in lucerne and once the lucerne land becomes contaminated with dodder it can only be partially cleaned by present-day methods.
It must be established that lucerne is the correct choice for the land, and that another crop is not perhaps more suitable. Lucerne is sensitive to water-logged conditions.
Poor drainage can create unsuitable conditions for its establishment while encouraging unwanted grasses.
Management after establishment
In the past it was recommended that weeds be controlled in lucerne by cutting treatment. This method is beginning to loose its importance in intensive grazing programs. The cutting program is effective in controlling broad leafed weeds by regularly cutting lucerne at the correct time, i.e. at very early flowering stage. Cutting the weeds when they are in the early flowering stage will prevent them setting seed, but it is ineffective against weeds that carry their seeds near to the ground.
It may even favour weeds such as herderstassie/shepherd’s purse (Capsella bursa-pastoris), chickweed/common sterremuur (Stellaria media) and voelduisendknop/knotweed (Polygonum aviculare).
Cutting after establishment but before lucerne has reached the flowering stage, is disadvantageous for it as root reserves have not yet built up and it may greatly reduce the vigour of the lucerne.
Annual weeds that are cut will make new shoots below the cutting height. This can be controlled by doing one cut high up and another lower down to remove the new growth. However, if this method is used against perennial weeds, it will take years to get rid of them.
Overgrazing, especially under wet conditions, may lead to soil compaction, and encourage many undesirable plants which are better adapted to soil with a poor structure and waterlogging than lucerne.
Certain grazing patterns can also lead to weed problems. For instance, heavy grazing during the winter, followed by light grazing in the summer, will increase the occurrence of weeds.
If lucerne is grazed, it will be aided by the cutting of unpalatable plants after the stock has been removed. It is well known that sheep suppress some undesirable plants.
If lucerne is irrigated, it is necessary to prevent it becoming waterlogged as this leads to an increase in grasses and sedges (Cyperus).
Chemical weed control
Chemical weed control comprises the use of herbicides which control a narrow or broad range of weeds. Herbicides can either be applied over a broad area or in spots. When applied widely, the whole area which is to be sown or where plantings have been established is sprayed. Spot application is used where dodder is the problem.
The following aspects are very important for correct chemical control:
- Correct identification of the important weeds
- Time of year when these germinate
- The depth of germination influences the use of soil-applied herbicides, especially those which are applied before planting and before emergence.
- Growth stage at which the weed is controlled — this is especially important during the establishment phase.
- The vegetative system, its depth in the soil and the reaction of the weed to fragmentation is of major importance in weeds that are vegetatively propagated.
- Soil type must be taken into account as clay soil needs a higher dosage than sandy soil where the herbicide is worked into the soil before planting time.
- The direction and strength of the wind must be noted so that crops bordering the field are not affected by the herbicide.
- Herbicides are poisonous, and with unwise use can cause permanent damage to people, plants and animals.
- Read the pamphlet carefully before using the herbicide.
- Always consult an expert before beginning spraying.
Pre-planting herbicides need a clean, firm seedbed. They are applied on the ground and then worked thoroughly into the soil in the areas where most weed seed would germinate. It is of cardinal importance to seal the seedbed after working the herbicide in. This application should take place five days before planting to ensure that the lucerne is not harmed.
There is only one registered chemical herbicide which is applied before planting. The active substance is EPTC and it is usually applied where yellow and purple uintjies (Cyperus esculentus and C. rotundus) are a problem. Examples are, among others, Eptam super and Extender which are both emulsifiable concentrates.
Pre-emergence treatment is any treatment which is carried out after the lucerne seed has been planted but before the lucerne or weed seedlings have emerged. The herbicide is usually applied over the whole surface. Sometimes it must be worked into the soil at irrigation. Propyzamide is one example.
Post-emergence treatment is any chemical herbicide treatment after the lucerne or the weeds have already come up. Post-emergence herbicides consist of either contact or systemic herbicides. It is possible to combat grasses growing in among the lucerne. Examples of herbicides are diclofop-methyl, EPTC, fluazifop-P-butyl, proprop, quizalofop-ethyl, etc. Always consult an expert before beginning with spraying.
Combating broad-leaves weeds in lucerne is a serious problem. There are substances for this purpose but the requirements for their use are very specific. For instance, the lucerne must not be too young or the herbicide must be applied after cutting.
Active ingredients are, among others, bromoxynil, fenozaprop-ethyl, etc. Broad-leaved weeds can also be sprayed with mixtures. Active ingredients of these include 2,4-DB and MCPB.
Herbicides registered for use in lucerne
Registered for use on:
|Bromoxinil||Annual broad-leaves weeds in lucerne and in lucerne/grass mixtures|
|Imazamox (Imidazolinone)||Annual broad-leaved and grass weeds|
|2.4-DB (sodium salt)||Annual broad-leaves weeds in lucerne|
|Fluazifob-P-butyl||Annual and perennial grasses in lucerne|
|MCPB||Annual broad-leaves weeds in lucerne|
|Paraquat||Annual broad-leaves weeds and dodder in lucerne (older than 6 months)|
|Proprop||Annual and perennial grasses in established lucerne|
|Propisamide||Annual grasses in irrigated lucerne (Karoo region only)|
|Annual grasses in lucerne (winter rainfall region only)|
|Quizalofop-ethyl||Annual and perennial grasses in lucerne|
Mechanical weed control
Mechanical weed control ranges from hand implements to expensive hoeing implements, and varies from labour-intensive weed control measures to the use of modern machinery.
The most effective method of mechanical weed control is seedbed preparation. In this way the seed reserves of the weeds in the soil are exhausted. The best results from cultivation are obtained when weeds are about 50 mm high.
If the weeds are too tall, shallow cultivation is useless. Cultivation of dry soil will hasten the death of weeds but in damp soil it will only spread them. Untimely rains can prevent mechanical weed control and weeds may then get out of hand.
Mechanical weed control should therefore be regarded only as supplementary to chemical control, and the combination will lengthen the time of suppression of the weeds.
Other problems with cultivation are that after establishment it can only be used where lucerne is grown in rows, and also that weeds inside the rows are not destroyed. Furthermore, hoeing can damage the lucerne roots. Hand hoeing is very labour-intensive, time-consuming and expensive, and is therefore only used on small areas. A combination of hand- and mechanical-hoeing is recommended in this case.
Dodder is controlled by burning infected patches with diesel burners.