THE ECOLOGICAL ROLE OF WEEDS IN AGRICULTURE
In modern agriculture we have greatly simplified the field environment. This has speeded up the plant succession patterns and the selection for weeds that are highly competitive and opportunistic. A simple environment is great from an engineering standpoint or for doing scientific studies. But the more elements we remove from a system, the greater are the demands on us to control processes that were once taken care of by a natural balance. A homily would be: 'the simpler the system the greater demand for our interaction.'
In a complex system such as casual gardening this becomes clear. We till the soil, stick in some seeds, and then for the next few months, just pull a few weeds and water if the rain is sparse. Worms stir the soil, rotting material and microbes release nutrients from the soil. Birds and beneficial insects eat the pests and the rain cleans the leaves. Then we harvest the produce.
In contrast, there is hydroponics in a greenhouse, where the system is limited to a plant, some plant support and a nutrient solution. Every hour a person or machine has to pump the nutrient solution through the plant roots. The plant needs to be bathed, protected and coddled like a little baby. The greenhouse is fogged with the most potent insecticides, sprayed with fungicides, and treated for spot pest problems. The plants are tied to supports. Then finally the produce is harvested.
We have to look at weeds as components of an agricultural system. They are just another part with insects, crop, surrounding vegetation, and soil and they are interacting in many, many ways.
But lets pretend. Say that we can eliminate all the weeds from a field, a farm, a county, what then? This is the image that chemical company's suggest in ad-copy for their herbicides. Elimination is on the other side of impossible but lets imagine "What if?"
First off, Mother Nature hates a vacuum. She will try to fill it any way possible. If we used herbicides to kill off the weeds, the herbicide resistant ones would replace the mild mannered ones that we eliminated.
There would be less plant material (biomass) produced because the usual "wasted space" between crop plants isn't taken up by weeds. There would be less residue to turn under at the end of the season.
All around your farm, yard or garden there were weeds. The insects and diseases that hounded them are now looking for a new food source, one that you would rather keep for yourself.
The insects that were eating the local weeds were also feeding a host of birds, beneficial insects and other wildlife. Part of the time they were on your crop plants feasting on insects that would have eaten your food. Now that the weeds are gone, the three months that your crop is in the ground isn't enough to keep the good guys going.
The weeds don't sprout up after you harvest the crop anymore and don't hold the soil and water-soluble nutrients. The rains come, and wash away your soil that took 10,000 years to form, and the nutrients for which you paid dearly.
Those long deep weed roots that you hated had been mining minerals from the soil rock particles, and deeper soil levels. They had been bringing them to the surface in the leaves, stems and flowers of the plant. When you tilled the soil before planting, burying the weeds, these nutrients were released for the crop. Then the deep roots died and rotted, leaving rich channels down to the water and nutrients leached from the surface.
If weeds were eliminated all together, our genetic material to breed new crops would be that much less. Yes, what we call weeds have been used to strengthen our delicate crop plants to withstand harsh environments, nasty bugs, and weakening diseases. Our gene pool would suffer.
Weeds definitely have their place in the environment of your crops or agroecosystem. But of course we must keep them in their place.
By allowing unlimited weed growth in a field your yields will be reduced considerably. This is a biological fact that has been studied and observed by farmers for many years. In fact for many commercial crops too many weeds will lower the market value, sometimes to the extent that the cost of harvesting will be more than the income the crop will bring when sold.
When we look at the benefits derived from weeds, we can see the need for weed management as opposed to weed control. Once we can view weeds as vital parts of the crop environment, we can develop guidelines for total agroecosystem management.
Weeds have traditionally been considered to be totally unwanted plants that reduce yields by directly competing with crops or by harboring insect pests, plant diseases, and looking unsightly. Many papers have been written on the role of weeds as reservoirs of pest insects and plant diseases. But competition is probably the most mentioned reason to control weeds.
Now that we have decided to let some weeds grow, what proportion is good and at what percentage do we need to start controlling them. With increasing weed density in a field, yields are going to be reduced depending on the weed species involved and the time the competition occurs. There are many factors affecting the optimum crop to weed balance in an agroecosystem.
Some of the factors are the density of the crop and the weed, and weed species and distribution, crop type and variety, soil moisture, soil fertility, weed/crop interaction, weed physiology, and duration and time of the competition.
There is a critical period of weed competition that every crop experiences. This has been determined experimentally by planting plots with varying weed free periods starting from the beginning and then increasing to the end of the growing season. You find a point where the production doesn't increase with longer weed free periods. After that the expense of weeding doesn't pay off in terms of yield.
Generally the critical competition occurs in the first third of the crop cycle. For sugar beets it is 9 weeks, for beans it is 5 weeks, corn 3 weeks, collards about 6 weeks. Of course this will vary some depending on the area of the country, crop variety, and weed species present. Specific interactions must be considered.
After this period of time you can let the weeds grow and there will be no effect on the yields, biologically speaking. At harvest time there are some considerations with certain weeds in a crop that will make it harder to pick or decrease its value by contamination.
When you consider that in this country today we use a tremendous lot of equipment to do the harvesting, and that weeds often interfere, this can be very important. If a carrot field has many weeds you just can't harvest it with a machine, the machine plugs up. And who wants to dig carrots?
We need to deal with some agricultural engineering problems as well to be able to harvest weedy fields. Just as those who are growing using minimum tillage have to work on modifications of machinery to deal with litter on the soil surface.
Crop type and variety come into play, as crops differ in their ability to compete with weeds. Plants that quickly form a canopy and tall crops with dense leaves suffer less from competition. Barley is more tolerant to weed interference than wheat, and wheat is more tolerant than oats.
Increased weed density will reduce the growth of crops and lower the yield. Generally. But some considerations change this.
Weed species have great differences is the competition they give to crops. Annual broad-leaved weeds are generally more competitive than are annual grass weeds at the same populations or densities. In addition to just competing for resources there may be chemical warfare (allelopathy) influencing the effects a weed will have on a crop and visa-versa.
In highly fertile soils, there may be little differences in crop yields between weedy and weed-free crops. However, if the crop and weed have to fight for nutrients, weedy crops will yield less than weed-free crops.
Soil moisture will affect competition between weed and crop depending upon which two are involved. Lowest Soybean and foxtail compete least when soil water was either adequate or limiting for the entire season.
Weed physiology can make a difference too. Plants take in carbon dioxide in different ways, some much more efficient under certain conditions. Don't worry what it means but they are refereed to as C3, C4, and CAM. Cacti and succulents use the CAM route (purslane is an example). Otherwise the main crop plants use C3 or C4 route to capture (fix) this vital ingredient to make sugars and starches.
When the light and temperatures are high and carbon dioxide is getting scarce (middle of a corn field on a hot sunny muggy day) C4 weeds fix carbon dioxide much faster than C3 crops such as soybean and cotton. The C4 plants can still fix CO2 when the concentration of CO2 is very low where C3 plants can't. Weeds that use the C4 mechanism include the tropical grasses, like foxtail, and broad-leafed weeds like amaranth, and etc. The rate of plant growth depends on energy captured from the sun and stored as sugar. C4 plants can grow faster than C3 plants in hot crowded situations.
Agroecology professors, Steve Gleissman and Miguel Altieri, have an idea that they think will help modern agriculture. They have been working to rescue the knowledge from the traditional small farmers in developing countries. This will be a very valuable source of information and concepts that will help us design better systems for here and in developing countries.
We can't come up with one farm design model that we would promote as best, because it should be site specific, the model will change where ever we go and will differ with soil, climate, resources, and so on. These observations should help to develop our perceptions of integrating a farm into an environment.
A set of important underlying principals, unstated by the farmer but behind his actions, has evolved out of this study.
In most of the developing countries poor farmers have to deal with some incredible handicaps. Imagine having a small farm, low capital, no credit, low quality of natural resources, and marginal soils (through political maneuvering) and trying to feed a growing family. These farmers have adapted to these conditions, and formed the best system to deal with these problems using their resources. The results of adaptation look very different. In all of these, diversity is a manifestation or elaboration of these principals.
Steve Gleissman, when he was in Tobasco, Mexico, found that the natives had a non-weed concept. That means they left certain weeds in the field and eliminated others depending on their value to the farmer. This fascinated him that a wide variety of plants were seen as useful and nurtured along with the planted crops.
In Plascata, Mexico, Gleissman ran across this example of a field where a farmer removed all of the weeds that were 'mal planta' (bad plants). But he let chicle grow. He considers chicle a good plant because he makes chewing gum out of it, markets it, and gets some much-needed cash. So to him it is another economic plant. It is now obvious that depending on the ecological setting and also the needs of the farmer certain weeds will become an economic element in the system.
Now what are the consequences of allowing this weed growth in the crop system? This involves insect populations, diseases, allelopathy, and etc.
Again in Plascata Mexico, Gleissman found several farmers that allowed the growth of other plants that did not have an economic value but have an ecological value. They would allow the growth of beetles in lupine. These legumes just came up they didn't plant them. They let them stay in the field and they serve as a trap crop, for macrolactigus beetles, a scarab beetle that feeds on the female flowers.
This farmer did not go through systematic experimentation with this relationship. This was a trial and error observation that was then passed from generation to generation. His ancestors found that this kind of interaction works. They found that when they allow the lupine to flower in their fields the lupine serves as a trap crop for these beetles. The beetles concentrate on this plant and then the farmers cut the lupines and put them in a big tank with oil to kill the beetles.
In another area farmers manage weeds like arnica in the borders surrounding their fields. When this weed is flowering the macrolactigus beetles move toward the arnica and this allows the corn to grow relatively free of these beetles.
Weeds can also serve as nectar sources for bees. Those plants that have value as honey plants are noted in the weed descriptions. In some areas of the U.S., plants considered weeds are the source of large amounts of honey, much sought by connoisseurs of a fine jar of honey. Fireweed, that springs up after a fire in the Pacific northwest, gives a light delicately flavored honey that is highly valued. Occasionally weed honey is very bitter or foul tasting and only fit for feeding back to the bees in winter.
If you go into some areas of Southeast Asia, you will find that there is no plant that does not have a function. In the nava farming system in Java, Altieri found this woman weeding out the plants she considered weeds in her cropping system. She took them to where her animals were nearby to serve as feed.
In some cases farmers allow animals to eat the weeds directly in the field. In a carrot field in southern Chile where the carrots were not up yet the sheep were allowed to eat the weeds. In another farm nearby, pigs were allowed to feed on many weeds in a squash field. The farmers had the empirical knowledge that the pigs preferred the weeds over the crop.
Later when the weeds were tougher the pigs would rather eat the flowers of the squash, and could not be let into the field. This is an example of a selective suppression or management of aggressive pests with animals, and with the bonus of supplying the animals with fodder at the same time.
The annual weeds hold nutrients till they are eaten and then the animal feces return the nutrients to the soil, after getting some benefit from them. Weeds are returned to the soil in a form that is more easily used by the crop plants. Organic mater is being recycled in a very efficient way.
Some weeds are used for local consumption by humans. Some of the aquatic weeds that grow in rice patties are collected and eaten in Bali. Arrowhead and others are an important element of the diet of these people. Rural people commonly eat many weeds as greens or vegetables all over the US, especially in Appalachia, the Blue Ridge Mountains and the Deep South.
Another function that weeds and other plant materials have is in the recycling of organic mater and nutrients in farming systems. In an agroforestry system in Mexico, all the weeds are collected into a compost pile for a while. Then the material is used as a feeding mulch.
In Mexico City farmers grow crops on artificial islands. Here leaching of organic mater from the soil is serious. The water gets over fertilized or eutrophies and this promotes the rapid growth of water plants, like water hyacinth. The hyacinths are then collected by the farmers for use as mulch to conserve moisture, control weeds and enrich the soil.
Other weeds like azola, the water fern, in rice patties are not considered pests by the small farmers in Java. They play a role in the fixation of nitrogen thereby reducing the need for fertilizer in this type of cropping systems. Azola has a symbiotic relationship between a blue green alga and a fern. The algae (Anabaena azollae) lives in pockets in the Azola leaf and fixes nitrogen. Azola occurs naturally in the patties and farmers encourage it and don't remove it. Today there is a lot of research going on in terms of exploiting this type of interaction.
For a number of years Altieri has been interested in the role of weeds on the dynamics of insect populations. If you look at a crop monoculture, certain types of interactions usually happen.
An insect pest enters and colonizes the field. The pests attack crop plants and attract natural enemies. The enemies will get established, and start controlling the crop pests. This control takes time, during which, the crop suffers.
But when you allow the presence of another kind of crop or non-crop plant, and increase the plant diversity within the system, you also increase the type and level of insect interactions that you have.
This is just a simplified diagram of these kind interactions.
Altieri did some research in Columbia to understand the consequence of leaving weeds in a crop system. In his first experiment he grew beans in a monoculture, beans with broadleaf weeds and beans with grass weeds.
This was uncontrolled weed growth so we were not concerned with bean growth; he was just concerned with the reaction between the crop and insects and weeds and insects. In the monoculture the beans were eaten by leafhoppers, the main bean pest of the tropics. Not so in the bean plot with broadleaf weeds, and even less in the plot with grass weeds. He had his first indication that the presence of grass weeds had something to do with the reduction of leafhoppers.
Altieri wanted to keep this effect but reduce the grasses impact on the yield. So grass weeds were grown around the beans in such a way that bean yields were not reduced yet he had the presence of the weeds. He found that the weed borders reduced the density of leafhoppers in the adjacent 20 rows, where in the field with no weeds there was an incredible outbreak of leafhoppers, and the yields were considerably reduced.
He sampled the borders to see if there were home to beneficial insects. Few beneficial insects were found. The weedy border was checked to see if it was serving as a trap crop. But it wasn't. Then he started thinking that there was some sort of chemical reaction taking place.
He made a solution by taking 100 g of grass leaves to a liter of water in a blender then filtering it. This solution was sprayed on the bean plants. And sure enough he found out that when he sprayed the solutions the leaf hopper populations were low compared to a monoculture sprayed with water. There was some sort of chemical reaction or chemical dependency occurring that allowed the beans to minimize the colonization by leafhopper.
Altieri found that allowing a certain selected weed growth among corn plants we could reduce dramatically the corn earworms (Heliocoverpa zea [=Heliothus zea]). The presence of certain weeds can be specific repellents to certain insects and not to others. There is also some kind of site specific type of reactions that occur. Also we found out that the abundance of beneficials on maize was significantly higher in the diversified corn as compared to the monoculture.
In soybeans a similar situation occurs. The velvet bean caterpillar is a very severe leaf eater that defoliates the plants. Altieri found that 99 days after planting in the weed free systems there was an outbreak of this pest in the southern US. A reduction of pests was discovered near the weedy borders. Again, due to the increase in natural enemies, general predators were significantly more abundant in the weedy systems as opposed to the weed free systems.
Leslie Linn studied the role of the weed Spurgula arvensis in promoting ecological pest control in Brussels sprouts for her master's thesis at UCSC. Here is a good example of the benefits of one weed in one crop and the considerations that should be explored in making weed management decisions.
Spurgula arvensis or corn spurry is a low growing naturalized weed, which often occurs in crop fields. Its flowers produce nectar, which attracts beneficial insects; the most important of which are flies of the family Syrphidae. Syrphid fly larvae are voracious feeders on aphids the major pest of Brussels sprouts.
Spurgula interplanted with cabbage and Brussels sprouts resulted in crops with less pest damage than those same crops without Spurgula. The presence of Spurgula near the crop attracted a beneficial insect complex of flies and parasitic wasps.
These insects rely upon the extra nutrients provided by the Spurgula nectar. The predators migrated into the crop to feed on their favorite foods, the plant pests there. In this way Spurgula increased the diversity of insects associated with a crop in a way which enhanced the natural control mechanisms. The role of added or incidental beneficial insects without this additional food source would be insignificant.
Spurgula was planted in a yard wide strip along the edges of a 30 by 40 meter field of Brussels sprouts at the UCSC farm. The Brussels sprouts were monitored weekly for the presence of aphids and Syrphid flies, and percent parasitization of the aphids recorded. The rate of parasitization was about 20% around the Spergula, which is a great deal of activity, but is the market place going to accept Brussels sprouts that have any aphids on them.
Considering a frozen market requirement of less than 2% by count with insects present. This was without any soap sprays or anything in addition because I wanted to monitor the insects. So it is a possibility that with organic methods a very clean crop could be grown. I think there is a great deal of potential but not quite enough to meet the 2% market for processing.
Ms Linn was very happy with Spergula. It is non invasive and easy to control. There are other plants that may be able to attract more beneficial insects. But some questions to ask are: are these hard to grow, do they have thorns, are the seeds expensive, are they tall, do they take a lot of sun, and do they take much water.
I just threw the Spurgula seeds on the ground, they grew, and they keep coming up and flowering all through the summer. They took care of themselves. With their multitudes of flowers, they are a pleasant plant to work with.
If you attract aphids to an area with a trap crop the aphids may begin to multiply and continue if natural controls don't come into play. In this case you will have to deal with them on the trap crop, or they will spread to your plants. There are many things like that to consider with a trap crop. (compare with insectary plant)
Aphids will move from a host plant to the immediate surroundings but they don't cue in on particular plants, or fly upwind, just with the wind. Cabbage aphids prefer members of the cabbage family but other than that there hasn't been much work in the area. Other people at UCSC have done work on the effect of wild members of the cabbage family, or Brassica weeds, on the populations of aphids.
As far as pests being harbored by the Spurgula is concerned, occasionally imported cabbageworm were found. They weren't seen gathering nectar so Spurgula wasn't a preferred or significant food source for them. No difference was found in the sampling. The pests were seen much more frequently on the Brassica weeds. I've also heard that the cabbage (miner) can be found on (a weed).
You are providing a tantalizing nectar source that the adults of pests as well as the adults of beneficial insects can use. But that didn't prove to be a significant pest reservoir in this study either.
Since Santa Cruz County grows over 70% of the nation's Brussels sprouts, they are a very important crop in the county. One of the primary limiting factors in successful production of Brussels sprouts is pest control, and pesticides constitute a large percentage of the farmer's required inputs. The predominant pest of Brussels sprouts is an aphid of the species Brevicoryne brassica and Myzus persicae.
Leslie A Linn, Research assistant, Agroecology Program, UC, Santa Cruz
Yield vs. Insects
Now, again the question of yields. How can we work out this compromise between weed science and entomology? We want yields that are not affected but at the same time we want these biological mechanisms functioning that will help the system.
Soybeans were grown with four different weed free periods: weed free all season, two weeks, four weeks, and weedy all season. Weed free for four weeks had the same yield as weed free all season. In the weed free for four weeks the canopy of the soybeans was about to close. There were few weeds so the yields were not reduced, but at the same time the presence of the weeds was going to have an impact on the pests.
Lets look at what happens. For instance (sympa ageredo) ??? is a penta formate that will feed on the pods, and inflict serious economic damage on the soybean. In the weedy systems there were very low numbers, compared to the two and four weeks, and the highest numbers occurred in the weed free all season. Natural enemies behaved the other way. Highest in the weedy all season and lowest in the weed free.
The performance of Trichogramma, an important egg parasite, was also affected by the presence of weeds. In weedy systems a higher level of parasitization of eggs was found compared to the weed free soybeans. Why was this? This question was examined and a test was devised to find if there was a specific association with that kind of weed and the Trichogramma. It was found that putting pest eggs artificially on soybeans with and without weeds, the same number of eggs on each, that the parasitization rate was significantly higher on the soybean with weeds.
Why is it that the presence of the weed allows higher parasitization on the soybean? Is it that the weed is providing a certain chemical queue or signal to Trichogramma? To remove that doubt, we did some experiments again with water extracts of amaranthus. The extract was sprayed on the soybean, cowpea, tomato, and cotton fields.
Then we compared the parasitization rate of Heliocoverpa zea eggs. We saw in all cases that the parasitization rate was significantly higher on the system sprayed with amaranthus than the one sprayed with water. Further tests showed that pigweed has karomones. These are substances that are aiding the natural enemies to locate their host.
All this work is interesting because Joe Lewis in Tipton GA is finding out that the scales of the wings of the Heliocoverpa zea have traposine. Heliocoverpa zea leaves a few scales when they lay their eggs. Traposine also functions as karomone to attract parasites. If we could synthesize this compound we could apply this to a field. Someday we will get into a situation where we will be able to modify the behavior of natural enemies through these chemicals that are naturally occurring in the environment. Without putting things that don't have anything to do with the natural system and at very low rates.
At UC, Santa Cruz the Agroecology students looked at the presence of weeds in collards. Weed control was relaxed at different times or kept at some level. In the weed free collards there were more flea beetles than in the systems where weeds were present. The mustard weeds had more flea beetles than the collards, probably because they have 32 times higher concentration in their tissue of mustard oil (glucosynolates) than the collards or kohl crop, so they would attract pests more than the crop.
At the Gill track in Albany Brassica kaber (wild mustard) was intercroped with Brussels sprouts and collards, also fava bean was intercroped with Brussels sprouts. It was found that population densities of the cabbage aphid (Brevicoryne brassicae) were considerably higher in the monoculture than in the intercroped Brussels sprouts. Parasitization of aphids by -------- lapi, a kind of bartend, is significantly higher in the double density system, where the density was increased to twice normal, and the weedy system where Brassica kaber was intercroped. And paridow parasitization rates in the intercroped with fava beans. Doubling the density of Brussels sprouts or intercroping with fava beans increased parasitization dramatically.
Weeds not only within the field but also outside the field can also have an affect on insect populations. Altieri did a study in Florida where corn surrounded by weeds was compared with corn surrounded by other crops and no weeds. The numbers of all predators were found to be significantly more abundant in the complex system as opposed the single corn crop surrounded by corn or soybean fields.
In Oregon some studies of the effect of weed borders in terms of how far are they effective in aiding the parasitization by natural enemies. From near the weedy border to the center of the field there was a clear decline of natural enemies. Very low numbers were found in the center, and very high numbers near the border. This shows the presence of certain weed associations can have an important impact on the parasitization by natural enemies. This reservoir of beneficial insects helps reduce the pest densities, and increase the soybean yield.
Weeds, are they friends or foe? It depends. I'm not arguing with anyone that weeds will not compete with the crop. This has been shown, in fact it depends on the degree of weed control and the management of the cropping system. This style of weed management of course is not something that you can follow in any type of crop system because it might interfere with yields. But there are ways in which perhaps we can allow certain weed growth in crops and at the same time get all the fine ecological effects that have been described.
One way to do that is to polycrop. In a Corn-bean association compared to corn monoculture, there is very little light that reaches the soil. There is a lot more light intercepted by the polyculture. The effects on weed growth are very dramatic. The total biomas is greater than for single crop systems and for weedy-crop systems. That means it is more productive. It is also more efficient as far as using resources. In Java corn and sweet potatos are planted together and there is no chance for weeds to grow. This is a good managed cover. Sweet potato is also used as a ground cover in orchards.
There is a lot of potential here for this kind of management in orchards here in the U.S. in cover crops, with legumes. There are other cropping systems, like row crops, where we can grow a living mulch, like clovers, vetch and such, where we can arrange the cropping science in such a manner that we are suppressing all the niches available.
Another example of course in which you can manipulate tolerable levels of weeds within a crop field is by just crop spacing. If you have a very wide spacing like in some soybeans you will have a lot of weed growth, but if the plants are spaced so that the leaves of one plant touch those of another, you won't have a lot of weed growth.
The time of emergence of the weed is important in relation to that of the crop. If you plant an earlier crop, it can have an advantage.
Allelopathy, which is the quality using plants inhibiting the growth of other plants, can also be used as a strategy to control weeds. The trick is to find a plant that will have allopathic effects against the weed but not against the crop. This is often tested in potted crop plants with extracts of suspect plants. Field observations often suggest chemical interactions between plants, so this test separates many of the physical and competition affects of the one plant. An example is that Johnson grass will inhibit the growth of Brassica campestris (rutabaga), but will not inhibit the growth of corn because they are in the same family. So a possible use of this would be to use a Johnson grass mulch to suppress Brassica campestris (a serious weed in Chile) in corn.
Biological control can also be used as a weed management tool. Definitely in annual crop systems not much research has been done on BC because a complex of weeds occurs so a complex of insects would be needed. Also the impact of the insect on the weed is delayed in relation to the crop cycle. But Altieri has described situations where (again very site specific) where this can occur. In Columbia the complex of crysometes???, especially Vesonica can wipe out completely Amaranthus from a corn-bean association. It completely destroyed the weed so that the weed did not reduce the yield of the corn.
Integrated Pest Management
These methods taken together are considered IPM. Many approaches are "integrated", or put together, to set up a system to greatly discourage the pest, with least damage to the environment and pocket book. Taken by themselves, they are ineffective but combined have significant impact. This is in contrast to a chart that states for this weed spray this herbicide.
One of the important ideas of IPM is that of "economic injury level." This is finding or estimating the level of weed infestation that will reduce the yield or marketability by the cost needed to treat the weeds. If the projected injury to the crop is $10/acre but the cost to plow is $20/acre then it would not pay to plow.
How do we pin down these kinds of figures? In part it is like nailing Jello to the wall. You are combining numbers that aren't firm with some guesses and experience. Some factors to consider are the crop characteristics, weed competition, weed aggressiveness, and allelopathic interactions. These are characteristics that are included in the database. Others will need to be obtained from local elders.
We are talking about altering the crop weed balance in favor of the crop. To do that we have selected a bunch of factors that we can work with. These include cultural, physical, and biological factors. We actually have a lot of room to do these kind of manipulations but in order to do them we have to understand the cropping systems first. Therefore it is very important to have a multidisciplinary approach in which the weed scientist, the entomologist, plant pathologist and the economist, together are looking at the same thing, and are talking the same language.
One day in Florida, this IPM group got together to develop an IPM package for soybeans. The first day the entomologist said I have the biological control cure for the larvae that is eating the soybean. He came and sprayed with an insect pathogen, a fungi. The next day the plant pathologist came and sprayed Benylate, a fungicide, to control a rust, killing the fungi that should have been killing the insect. The third day the weed scientist came and sprayed with an herbicide to kill certain weeds. The herbicide had certain physiological effects on the soybeans that promoted another disease on the crop. At the same time the herbicide eliminated many weeds that could have served as sources of beneficial insects. These approaches have some value, but we have to put them together so that they are consonate or function together in harmony.
As many were taught in agricultural school, the best system is a completely weed free system all season, and the worst system is one that is very weedy. If we start being a little more open minded, we might find that our agriculture teachers were kind of wrong (a little wrong?). It depends on what your objectives are in agriculture. Our primary objective is still making a living; our secondary objectives have shifted with a view of a deteriorating environment. Farm management depends on what you want to get out in the long and short term. Chemicals can mean short-term profits and long term problems.
Many times people have argued that the kinds of approaches that are advocated here are not consistent within the economic goals of our society today. We need to view the situation in a wider perspective and longer term.
Our present agricultural system depends on a non-renewable resource, petroleum, whose supply can't be assured or may be priced out of reach. When these inputs disappear or become very expensive, we will have no more chemical intensive agriculture. Let us all hope that we can develop a sustainable agriculture that we can depend on to feed us.
Many of the chemical and energy intensive technologies that we are using today are causing a lot of systemic degradation. We are loosing a lot of soil to erosion or salinization; we are acquiring more and more pesticide resistance in insect pests, and a host of other problems. So the consideration of living interactions, an ecological approach, will give us clues to solving our problems in agriculture.