jeudi 6 août 2015

50- Plant protection -3- Herbicides, what for?

HERBICIDES, WHAT FOR?

Since I first explained you what the pesticides are, and what is their historical justification, I will now address what probably seems to you not compatible with the explanations of the first two chapters.
This is what the public understands and accepts the least, especially lately, with all what is said about glyphosate.
Why do we use so many herbicides?

Herbicides don't enter exactly a similar category to other pesticides. Why? Because they are not medicines for plants in the strict sense. They do not cure attacks of insects, fungi or bacteria. Their goal is to remove or reduce the weeds competing with crops. The only crime of weeds is to seek to grow in the middle of crops.


There are certainly many reasons for that:
1-  To limit competition with crops
- Competition in water. All plants need water. If weeds are growing in the middle of the crop, they will consume water at the expense of the culture. The consequence will be, or a reduction of food production, as a crop that lack of water will always produce less, or an increase of water consumption by irrigation to compensate for that taken by weeds.
- Competition in nutrients. According to the same process, nutrients mineralized by the soil, or brought by farmers as fertilizer, will be shared between crop and weeds. We will therefore have the same consequences, namely a reduction of production or an increase of nutrient requirements, therefore an increase in fertilizer consumption.
- Competition in light. Plants don't all have the same development, both in size and speed. Some crops are very low, very close to the ground. These include for example the potato, peanut, strawberry, carrot, some cereals, etc. If the weeds that grow are high, such as mallow or Johnson grass, they will represent a significant competition to light. Photosynthesis of crop will not be sufficient, and food production will be affected. It was the same case with invasive plants such as clematis or morning glory that will develop by taking the crop, whatever it is, as a support. In some severe cases, fruit trees can be almost suffocated by creepers.

2-    Allow seeds to germinate. This is the same situation that the competition in light. Weeds grow on a sown field, so foliage causes a strong shade the ground, preventing the seeds of crop from germinating.
Any field contains millions of seeds of all kinds, carried by the wind, by bird droppings, or from a previous fallow. Germination conditions are different from one plant to another. Imagine you sow an autumn cereal (a narrow leaf plant with slow start at that time). At the same time occurs the germination of mallow (broadleaf plant of rapid growth both height and width). If the presence of mallow is not controlled, the cereal will probably not survive or will lose much of its ability to produce. The use of a herbicide selected for the situation, allows the cereal to germinate and grow in good conditions. Once it's out of trouble, the competition of a late mallow germination will be much less problematic, since the cereal dominates in light and root system, so in supply possibilities.

3-    Avoid maintaining a humid atmosphere under the canopy. An active presence of vegetation under the tree canopy maintains a high level of humidity. This is the case, particularly in orchards or vineyards. This almost permanent moisture promotes the development of fungi that need a humid environment. This is particularly the case with many storage diseases (Monilia or Botrytis, for example) or a common disease in many cultures the Phytophthora, but also many bacterial diseases. The risk may cause a sharp increase in fungicide needs. In this case, the grass control allows limitation, at least partial, fungicides applications.

4-    Avoid or reduce the risk of attack by certain pests like snails, slugs, spider mites or voles, which always maintain in wet and grassy areas. It also allows the application of baits or repellents against these problems, in a localized way for a reduced environmental impact. The effectiveness of the protection is always better if the ground is clean of weeds in the application moment.

5-    Control weeds while avoiding tillage, which degrades soils structure and promotes erosion. It is a fundamental principle that modern agronomy highlighted. Plowing, one of the most traditional actions in agriculture, severely promotes soil erosion, greatly reduces its capacity to retain rainwater and significantly reduced its fertility by accelerated oxidation of organic matter. That is so true that more and more farmers are converting to no-till, ie the elimination of plowing between crops. In South America, this phenomenon has taken on a large magnitude, as in some countries, over 80% of cultivated areas have adopted this technique. But weed control remains a problem. Under these conditions, the use of herbicides allows a combination of advantages. You can go further on this topic by reading the following article (in Spanish), about no-till in South America and herbicides http://www.croplifela.org/es/menu-default-actualidad.html?id=593

6-    Avoid the presence of toxic plants in crops. These plants, weeds also, could be mixed and present, when harvested, severe food safety problems. This is the case of plants like datura, black nightshade or hemlock, which are strong poisons and when they are in the middle of crops at harvest time, can make some crops unfit for consumption, for risk of poisoning.


As you can see, none of the points made any reference to the profitability of the farm. Yet it is an aspect that should be added, but I consider it implied in any logic of production and business.
In other words, and without wishing to make an exhaustive list of benefits of using them, I want again you to understand that the use of herbicides responds, also to a logic that fits perfectly in sustainable agriculture: ensuring sufficient production that the farmer can live in dignity, limiting the necessary area to ensure sufficient global production and ensuring a good level of food safety.
Never forget that the search of productivity is not a defect or an affront to Nature, on the contrary. The search of productivity is above all a good way to optimize the available natural resources. The farmer must seek productivity for the profitability of his business. But sustainable agriculture has to seek productivity to limit its own environmental impact. Why? Simply because the environmental impact of a crop, whatever it is, is practically the same producing 100 tons, or producing 200. The difference is that at the global level, it will be necessary only half surface to get the same amount of food, so the impact on water resources, consumption of fertilizers or pesticides will be greatly reduced, while enhancing food security. At the same time, there will need less fertile land to feed the same number of people, and the production of greenhouse gases will be less.


Can we replace herbicides with something else?
The traditional method, mechanical weeding, consists of ploughing the soil superficially, to destroy harmful plants. This is done either manually or with a tractor or a draft animal, using a moldboard type tool, which can take many shapes and sizes depending on the case. But the disadvantages are significant due to the risk of erosion, direct damage to the crop, and disruption caused to soil life. The manual uprooting of weeds, a very ecological and social way of doing it, is virtually out of reach of farmers in industrialized countries because of the cost of labor. This technique is however used in addition of herbicide sprays under certain conditions.
Other types of tools are available, such as those based on rotary whisks that destroy present weeds. The soil is hardly disturbed, but some localized irrigation types don't allow it.
Thermal weed control is a new method of burning or causing a thermal shock on the weeds, so as to cause its death, without damaging the crop. There are several types of tools with gas burners for this type of work. However, there is a high risk of fire if the climate is too dry. There may also be damage to the localized irrigation systems.
The technique of mulching or permanent soil cover, on perennial crops like orchards o vineyards, is an alternative under study and/or development. This consists in keeping the soil covered by inert materials such as straw, pine bark, or even a plastic tarp. These techniques prevent the germination of weeds, but require maintenance, can mask potentially dangerous attacks of voles, and consume a huge quantity of material.
The total permanent vegetal cover with carefully selected plant species is also being studied. Very low plants such as for example clover consume little water, extract nitrogen from the air, and therefore are very little concurrent. However, their presence does not solve the problem of moisture in the canopy, or the risk of development of voles.
Organic herbicides. Some researchers are working this line, from toxins extracted from plants, and that could represent, in the future, an interesting alternative. But there is still a long way to go. Incidentally, you can read the following article (in French) http://www.lefigaro.fr/jardin/2015/05/08/30008-20150508ARTFIG00228-la-revolution-des-nouveaux-desherbants-bio.php


None of these techniques is currently a totally satisfactory alternative, each one having significant defects. So either we produce less (more hectares are required to produce the same overall quantity, with therefore a greater environmental impact), or more water and more fertilisers are consumed, with the problems that it implies, to compensate for the presence of grass. But in all cases, we end up with sanitary problems.

The weed control is currently a major problem in all organic production methods. You can read the following article (in Spanish) on the grass control in organic gardening, ie in a short scale. Imagine the problem in a large scale. http://www.agrohuerto.com/controlar-malas-hierbas-en-el-huerto/
This simply shows that it is today difficult to eliminate herbicides, since production protocols that refuse its use, have difficulties to dominate the problem with the available alternatives.

I believe it's absolutely necessary to advance the works on the search for alternatives to herbicides, as their use poses obvious environmental problems. Environmental movements deliberately exaggerate these problems, but it would be absurd to deny this fact.

The use of herbicides keeps the soil in an optimal state of natural fertility, thus increasing crop yields while controlling consumption of water and fertilizers.

Good use of herbicides significantly brings more advantages than disadvantages.
The overall environmental impact of their use is very positive if the application is carried out with the right products, the necessary and sufficient dose at the right time and with equipment in good condition and properly adjusted.

You could say that, currently, herbicides are a necessary evil in agriculture. It is likely that this situation continues long. Which probably change is the type of products that will be used in the future and their environmental impact.

Herbicides are necessary for humanity. They are one of the tools available to agriculture to increase food production to meet the needs of a global population steadily increasing.
Let us develop them and use them in a precise and reasoned manner. We get all the benefits they can bring us, minimizing their disadvantages.



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