Answers

Answer for Activity 1-3

Explain the human centred and species centred views of what constitutes a 'weed'.
The human centred view of a 'weed' is any plant that is unwanted under a particular set of circumstances (i.e. it is a nuisance to humans or interferes with human activity). The species centred view of a 'weed' is based on a specific set of characteristics which make a particular species highly successful in certain environments.

List at least eight ecologically significant characteristics of weeds.

• a high seed output.
• some form of seed dormancy.
• seeds have an extended longevity.
  
• seeds have simple germination requirements.
  
• rapid seedling growth.
• ability to tolerate environmental extremes.
  
• highly adapted for dispersal.
  
• have vegetative methods of reproduction (eg. rhizomes, tubers, stolons, bulbs).
• have a very adaptive life cycle.

How does the formation of local races in weeds influence their success?
The formation of local races allows weeds to become well adapted to the environment in which they are found and can increase their success in that environment.

What is the nature of the relationship between the distribution of aliens in Australia and their areas of origin?
In native ecosystems, most weed species are plants that come from a region of the world with a similar climate. However, weeds of agricultural systems may be widespread and often adapt to a wide range of climates.

What are the different impacts that weeds have in amenity and nature reserves compared with arable and horticultural crops?
In amenity and nature reserves weeds may: look unsightly; block waterways; reduce the conservation value of areas; interfere with ecological processes; exclude native plants; reduce biodiversity; increase the fire hazard and intensity of fires.

In arable areas and horticultural crops weeds may: reduce the productivity of crops and pastures; interfere with grazing and harvesting operations; poison or injure livestock; contaminate agricultural products; harbour pests and diseases; lower the sale price (i.e. value) of a farm; cause human health problems.

What advantages does correct weed identification provide for effective weed management?
Correct weed identification is possibly the most important prerequisite in the management of weeds because it: allows access to the world's literature on the impact and control of a particular species; enables proper communication with other weed managers; enables the application of proven control measures and prevents the application of ineffective control measures.

Answer for Activity 2-1

Describe the benefits and drawbacks of legislating weed control.
Legislating weed control enables more effective control of widespread weeds, may reduce the rate of spread of new weeds and may enable the eradication of new weeds. However it can be anti-competitive and inefficient (i.e. bad legislation may result in a costly misdirection of resources).

Describe the three main stages of weed invasion?

• Early stage - where the weed is relatively new and not very widespread (i.e. the area infested by the weed is relatively small).
• Mid stage - where the weed is beginning to spread but its distribution is nowhere near its potential range.
• Late stage - where the weed is widespread and near to its potential range.

In which stage of weed invasion might it be feasible to implement eradication strategies for a weed?
Eradication is generally only feasible in the early stage of weed invasion where the area infested by the weed is relatively small.

Why will eradication be unsuccessful in the other stage(s)?
It is usually too costly to eradicate a weed that is present over a large area and all infestations of the weed will be difficult to find. Efforts to eradicate a weed in a local area will often be unsuccessful if the weed is common elsewhere in the region because the weed will easily reinvade from nearby areas.

Answer for Activity 3-1

Describe five types of losses that a farmer will suffer due to weeds.

• Losses in the yield of crops due to increased competition for light, water and nutrients (i.e. reduction in income because of reduced quantity of product).

• Lower carrying capacities in pastures due to lower biomass of pasture plants (i.e. once again a reduction in income because of reduced quantity of product).

• Contamination of agricultural produce leading to a reduction in the quality of the product and a lower income from the product

• Controlling weeds may increase yield and quality but also leads to an increase in production costs.

• Weeds may impact upon the management of farm resources and cause a farmer to switch to a less profitable alternative.

Describe how short and long term planning horizons can influence weed control decisions.
A shot term planning horizon will generally cause a focus on control decisions that give the greatest benefit in the shortest period of time, while long-term planning horizons will focus on management options that have a more prolonged effect. The decision to apply herbicides is common in short term planning as it gives an almost immediate effect, however this effect does not last for very long. Biological control can only be considered in long term planning as it takes a long time to implement and have an affect on weeds, however the effect it has is very long-term (usually indefinite).

What is the difference between a benefit/ cost analysis (BCA) and a gross margin analysis (GMA)?
A BCA is usually undertaken on a regional or national level and all benefits and costs are identified and valued (including externalities). It also includes investment benefits that may occur in the future.

A GMA is a simple budgeting tool that is usually undertaken on the single farm level and only considers a short period of time. It is simply the difference between the gross income earned and the costs of production incurred in obtaining revenue from an activity.

What is an opportunity cost and how might a farmer suffer from an opportunity cost caused by weeds?
An opportunity cost is the cost of something in terms of an opportunity foregone and the benefits that could be received from that opportunity. In other words, the economic difference between the opportunity that has been taken, and the most valuable foregone alternative.

An example of an opportunity cost caused by weeds might be a situation where a specific weed problem (eg. wild oats in a wheat crop) is so bad that it causes a farmer to have to leave his fields fallow for a season or grow a less profitable crop instead of the crop he would prefer to grow.

Answer for Activity 9-1

Answer for Activity 9-2

Describe the difference between competition and interference.
Competition occurs when two or more neighbouring organisms seek a common resource (e.g. nutrients and water) whose supply falls below their combined demand.

Interference is a term that describes all changes in the environment brought about by the proximity of individuals. It includes competition as well as other effects, such as the role of toxins (i.e. allelopathy), changes in conditions (eg. protection from wind) and influences on the behaviour of predators.

Name three resources that weeds compete for.

• water
• light
• nutrients

Describe the relationship between weed density and crop yield. How might this relationship vary for different weeds?
The relationship between weed density and crop yield loss is hyperbolic (i.e. exponential) in shape. That is, the yield of a crop decreases asymptotically with increasing density of weeds.
It may vary depending on the time of weed emergence, weed size and weed development (i.e. phenology).

Define the three different types of seed dormancy.

• Innate dormancy - is a dormancy caused by innate physical barriers that prevent germination (eg. hard seed coats or the presence of chemicals within the seed) or the need for further physiological development (eg. after-ripening).

• Induced dormancy - is a temporary dormancy that is caused when the seeds are exposed to suppressive conditions (eg. high temperatures). This dormancy continues after temperatures change and prevents germination during the wrong time of year. It is usually broken by temperatures opposite of those that induced it and remains until conditions for germination are right.

• Enforced dormancy - occurs when environmental conditions are unfavourable (eg. cold temperatures, lack of moisture or oxygen). When limitations are removed, the seeds germinate freely.

What factors are essential for the germination of weed seeds? How does burial of seeds affect germination?
This question is best answered in the following article: "Reproduction by seed" by H.A. Roberts. In Weed Control Handbook: Principles. Blackwell Scientific Press, Oxford pp. 11-27 (1982).

Answer for Activity 9-3

Cape weed Arctotheca calendula prickly pear Opuntia stricta paterson's curse Echium plantagineum skeleton weed Chonfrilla juicea bitou bush Chrysanthemoides monilifera perennial veld grass Ehrharta calycina wild oats Avena fatua wireweed Polygonum aviculare rushes Juncus spp wild radish Raphanus raphanistrum doublegee Emex australis

Answer for Activity 10-1

Describe the three main objectives of tillage.

• to remove vegetation (including weeds) that may interfere with the sowing operation and become competitors for the emerging crop or pasture.
• to prepare a seed bed for the sowing of a crop or pasture, such that the contact between the seeds and the soil is enhanced to facilitate rapid and even germination of the sown seed.
• to facilitate infiltration of moisture in soils that have a compacted layer at the soil surface (i.e. a crust) caused by livestock or previous cultivation and/or to remove a compaction layer at depth (i.e. a hardpan) that is restricting water movement down the soil profile.

Describe the impact of the various tillage implements on soil, existing vegetation and weed seeds.
The mouldboard plough turns over sod (i.e. completely inverts the upper soil layer), which buries existing vegetation and weed seeds. However, the clods are fairly large and weed roots may not be destroyed allowing some of the weeds to re-grow.

The disc plough has a slicing action that partially inverts the soil and not only buries vegetation and weed seeds, but also tends to break up roots, preventing them form re-growing.

The chisel plough creates narrow furrows and breaks up the soil into smaller particles to the depth of the ploughing operation. This implement will control small existing weeds effectively provided full soil disturbance is achieved, however vegetation and weeds seeds can be left near the soil surface.

The action of the rotary hoe shatters and thoroughly mixes the soil. It buries a large proportion of the vegetation and weeds seeds and effectively handles existing weeds.

What are the differences between the terms minimum and zero tillage? How do weeds respond to these two tillage systems?
Minimum tillage is a system where there are fewer and/or faster cultivation passes at a shallower depth than under normal ploughing. Zero tillage is a system of directly drilling the seed into the soil with little or no prior land preparation.

Weed control generally requires more careful management under minimum tillage, in particular grass weeds and volunteer cereals can become more of a problem. They are generally encouraged by minimum tillage or direct drilling (zero tillage) if weed seed has been shed in the previous crop. In these systems cultivation is relied on less for weed control and the effective use of herbicides is more important.

Answer for Activity 11-1

Define selective grazing. What is the effect of selective grazing on a pasture?
Selective grazing occurs when the grazing pressure is not uniform or constant over the whole pasture. For example, livestock may selectively graze different plant parts and different plant species more than others.

Weeds are rarely selected over the more desirable pasture species and so the effect of selective grazing is normally to disadvantage the desirable pasture species and cause an increase in weed species.

List some direct effects of grazing on plants.

• grazing reduces plant growth and effect the development of a plant.
• grazing affects reproductive growth and reduces the reproductive potential of a plant.
• grazing reduces the carbohydrate reserves of a plant.

What types of weeds are best grazed by goats rather than sheep or cattle?
Woody weeds (i.e. shrubs) in rangelands are best grazed by goats because of their preference for browse species. Thistles and some other herbaceous weeds may also be best grazed by goats.

How can rests and mob stocking be used to manage some weeds. Give an example.
Mob stocking can be used for a shot period of time on pastures to force stock to eat plants that may otherwise be avoided. For example, mob stocking can be used over spring and summer to control infestations of perennial thistles (eg. Cirsium arvense). In this situation new shoots of the thistles are exposed to grazing and the carbohydrate reserves of the thistles may be depleted, thereby leading to less vigorous growth of the thistles in the following season.

What factors might constrain the successful use of mob stocking for controlling some weeds?

• mob stocking requires sufficient stock being available to apply a high intensity of grazing pressure to the required area.
• forcing stock to graze all plants may lead them to graze potentially poisonous plants and so it cannot be undertaken in situations where highly toxic species are present in the pasture.
• mob stocking may not be very successful in situations where large and well established weeds are present, as they have a greater ability to recover.

Answer for Activity 12-1

What is the impact on weeds of a simple compared to a complex crop rotation?
A simple crop rotation has only one crop of each crop type used in a set sequence and is the most
common type of crop rotation. A complex crop rotation is where crops within the same crop type vary from year to year. A more complex crop rotation is usually better for the management of weeds as it does not allow certain weeds to adapt to the situation and build up their population over time.

Name three characteristics typical of highly competitive cereal cultivars.

• greater height.
• more tillering.
• higher early vigour.
• extensive leaf display.
• an efficient root system.

What tactics other than choice of crop and cultivar can be used to provide a more competitive crop?

• increasing the planting density of the crop (i.e. increasing the seeding rate).
• altering the source, timing and placement of soil nutrient (i.e. targeted fertiliser application strategies).

How might changes in sowing date be used to influence weed numbers?
Delaying the sowing date of a crop may be used where the weeds present have a very narrow emergence pattern (eg. annual ryegrass). The weeds are allowed to germinate and then sprayed with herbicide prior to planting the crop, thereby greatly reducing weed numbers in the crop.

Answer for Activity 13-2

Answer for Activity 14-1

Group A necrosis of meristematic region Group B reddening of leaf margins and stem Group C bleaching of leaves Group D deformation of roots and shoots Group G bleaching of pigments Group I twisting of stems and petioles

Answer for Activity 15-1

Describe the essential steps in efficient and accurate application of herbicides.

• Identify the weed and define situation (identifying which weed species need to be controlled allows you to accurately determine what is the best stage of growth to be targeted).

• Select the best herbicide (determine which herbicide will be most effective has the most appropriate use pattern, mode of action, formulation, effect on the environment, residual life in the soil, etc.).

• Read the label (follow the directions on the label and ensure that you know your legal obligations in using the herbicide).

• Define the target (taking into account the biology of the weed and the mode of action of the herbicide, determine where to target the herbicide to maximise its effectiveness).

• Select the best equipment for application of the herbicide (determine which equipment is most readily available, easy to use, effective and economical for your situation).

• Adjust and calibrate equipment (adjust equipment to maximise effectiveness and determine the dose and application rate required).

• Apply at the right time under favourable conditions (apply at the right growth stage and when the plant is not stressed, and in weather appropriate conditions that will minimise off-target damage).

• Take appropriate safety precautions (wear/use appropriate safety equipment and handle the herbicide in an appropriate and safe manner).

• Clean up (decontaminate the operator when finished and use the relevant cleaning procedures for the equipment and herbicide involved; return the herbicide to the right storage conditions).

• Update records and monitor results (keep records for legal and management requirements and monitor the effectiveness of the herbicide that has been applied).

Describe three methods of herbicide application.

• broadcast (distributed uniformly over a large target area).
• spot spraying or wiping (directed on to target weeds)
• herbigation (metered into irrigation water).
• injection (directly injected into the stem of plants or into the soil).

What are the important characteristics of spray droplets of less than 100 µm or greater than 300 µm diameter, that make these droplets unsuitable for post emergent herbicide applications?
If the droplet size is too small (i.e. less than 100mm) the droplets will drift long distances, thereby decreasing the amount of herbicide reaching the target area and increasing the likelihood of off-target damage. If the droplet size is too large (i.e. greater than 300mm) the droplets will not provide adequate coverage on the leaf surface of the target weeds (i.e. the percentage of target surface area covered will be too low for the herbicide to be very effective).

What processes contribute to herbicide drift and how can drift be minimised?
Herbicide drift can be much higher if the equipment used, operating procedures used, and the weather conditions apparent during application of the herbicide are not suitable.

Herbicide drift can be minimised by avoiding the production of fine droplets in sprays, correctly modifying application equipment, avoiding the application of the herbicide in adverse meteorological conditions, including buffer zones around the area to be sprayed, and by the enforcement of suitable legislative restrictions by relevant authorities.

Describe the features of herbicides that will lead to strong and weak adsorption to soil components? How will soil properties influence adsorption?
Herbicide chemistry is a major factor influencing adsorption and charged herbicides (those having either a negative or positive charge) will have a stronger adsorption to soil components. The amount of organic matter in the soil can also affect adsorption of certain herbicides, with higher levels of organic matter increasing adsorption.

Describe how a herbicide such as atrazine enters plants and its fate within plants?
Atrazine is a PS II-inhibiting Group C herbicide that has extensive soil activity with absorption into the plant often occurring mainly through the roots. Group C herbicides are normally applied to bare, moist soil and may need rainfall or irrigation after application for maximum effect. Atrazine is particularly effective against young broadleaf weeds and is rapidly metabolised by maize and sorghum crops by glutathione conjugation mediated by glutathione-S-transferases.

Answer for Activity 16-1

How do the initial gene frequency for resistance and the size of weed populations treated with herbicide interact in the evolution of herbicide resistance?
The evolution of herbicide resistance is more likely to occur where larger weed populations, that have a higher initial frequency of herbicide resistant genes, are persistently treated with the same herbicide.

Explain why there might be more resistance to Group B herbicides than to Group D herbicides in Australia.
Group B herbicides generally have a very specific mode of action that targets a single enzyme. This makes them very effective, however it only requires the plant to develop a modification to this single enzyme, such that binding of the herbicide is reduced or eliminated, for resistance to develop. Group D herbicides do not have such a narrow range of effect and act by inhibiting tubulin polymerisation and the formation of microtubules. Therefore it is more difficult for a plant to develop resistance to such chemicals.

Describe the relative importance of pollen and seed movement for the spread of herbicide resistance.
In reality, most weed populations are relatively well separated and pollen flow between populations is likely to be a minor contributor to the spread of herbicide resistance. In contrast, gene flow by introduction of seed can be a major contributor to the spread of herbicide resistance between weed populations. The spread of herbicide resistant weed seed between fields and farms has been documented in southern Australia.

Describe three biochemical mechanisms that can endow herbicide resistance in weeds.
Resistance due to target site insensitivity - this is where modification occurs to the enzyme normally inhibited by the herbicide, such that binding of the herbicide is reduced or eliminated.

Resistance due to increased detoxification of the herbicide - this is where resistant plants are able to detoxify the herbicide at a more rapid rate than can susceptible individuals of the same species.

Resistance due to altered transport of herbicide into or out of the cell cytoplasm - this is where considerably less herbicide reaches the sites of action in the chloroplasts of resistant plants.