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Ver la versión completa : Patricia Muir



sujal
03-dic-2008, 09:08
Patricia S. Muir
Professor,
Ph.D., 1984, University of Wisconsin, Madison
muirp@science.oregonstate.edu

http://www.science.oregonstate.edu/bpp/faculty/muir/index.html

TROPHIC ISSUES

Trophic issues address questions about who eats whom. ("Trophic" refers to feeding.) That is, trophic issues deal with information about food chains and food webs. (The grass is eaten by the insect, which is eaten by the bird, which is eaten by the cat, etc.)

Have you ever wondered why there aren't animals that eat the highest order carnivores -- like grizzley bears or wolves? That is, why is there a ceiling to food chains, with most chains containing only three to four links, as in the example given above?

It isn't just because these top carnivores are so big and fierce!

There is a ceiling to food chains, basically because there isn't enough energy in the population of these top carnivores to sustain a viably-sized population of the animals that would prey on them.

This ceiling derives from the following laws of thermodynamics:

1) Energy can neither be created nor destroyed; only converted from one form to another

2) Energy is degraded when it changes from one form to another (entropy is always increasing). No energy transfer is 100% efficient, and most are considerably less efficient than that, such that energy is lost every time it is transferred.

Eating transfers energy from one place and form to another; the energy in the apple you eat is transferred to you, but some is lost along the way.

In any ecosystem, the primary producers (plants) "fix" only so much energy from the sun through photosynthesis. This amount of fixed energy is then available to feed the rest of the living members of the ecosystem. Each level in the food chain (or web) depends for its energy on the level(s) below it. That is, the herbivores (plant eaters) depend for their energy on the plants, and so forth. Remembering that no transfer of energy is 100% efficient, you can see why there is less and less productivity (kcal/m2/year) at each step up a food chain.

This concept about energy limitations in food chains can be visualized using a pyramid, which diminishes in size as it goes up. The bottom level of the pyramid would include the primary producers, which fix a certain amount of the sun's energy into useable form each year (kcal/m2/year). This level also, of course, sustains losses of energy as the plants respire. The herbivore level that depends on the primary producers will contain less energy than the primary producer level, because energy is lost in the process of it being transferred from the primary producers to the herbivores, and so on up the food chain.

Thus, there are not fiercer dragons on Earth simply because the energy supply will not stretch to support super dragons!


What does this have to do with sustainable agriculture?

We could support more people on Earth for a given area of land farmed if we ate lower on the food chain -- if we ate primary producers instead of eating herbivores (corn instead of beef). OR, we could support the same number of people as at present, but with less land degradation because we wouldn't need to have so much land in production. These consequences of a change in our diets result from the basic thermodynamic principles outlined above.

What follows is a general example – the numbers quoted are very approximate, and are given only for the purposes of illustrating the point:

Let's say we have 20,000 kcal of corn. Assume that we feed it to cattle (as we do with about 70% of the grain produced in the U.S. and 37% of world grain production; poorer nations such as India feed only about 2% of their grain production to animals, as doing so is such a luxury – as you'll see!

The cow will produce about 2,000 kcal of useable energy from that 20,000 kcal of corn (assuming 10% efficiency; the efficiency is actually somewhat higher than that, but 10% is easy to work with and illustrates the point reasonably).

That 2,000 kcal of beef would support one person for a day, assuming a 2000 kcal per day diet, which is common in the US.

If instead people ate the 20,000 kcal of corn directly, instead of passing it through the cow, we would be able to support more people for that given unit of land being farmed; not necessarily 10 times more, because people are not as efficient as cattle at using corn energy, but considerably more than the one that could be supported if the corn were passed through the cow first!

(Beef cattle produce about 19 kg of protein per acre per yr while soybeans produce 200 kg/ac/yr. Recognizing that beef protein is a "complete" protein from the human perspective, while soy protein requires complementing with proteins from grain to acquire all necessary amino acids, this is still a huge difference in production!)

Most citizens of the US are big meat eaters. In the US, consumption of meat and poultry averages in the range of 178 - 250 pounds per year, or between 0.5 and 0.68 pounds per day (depending on whose data you use!). Data from the National Cattlemen's Beef Association indicate that, in 1998, the average US family serves beef eight times per week, with highest rates for lower middle income households with children, where the rate is about 14 times per week. (Ten percent of US households do not serve any beef.)

It is estimated that if everyone in world ate as US citizens do (deriving about 25% of our calories from meat or animal products), less than half of the present world population could be fed even on the tremendous grain harvests of 1985 and 1986. If, by contrast, everyone in the world ate a typical South American diet, in which approximately 15% of calories come from meat or animal products, then about 75% of the world's population (about 3.9 billion persons) could be fed. And, finally, if everyone was well-fed but on a strict vegetarian diet, we could feed about 1 billion persons more than the present world population, based on 1986 harvest levels.

Similar data come from an article in Science (5 Dec. 2005): A balanced Chinese diet of the early 1990's containing 20 kg of meat per person per year was produced from an average land area of ~ 1,000 m2/person, which a typical western diet took up four times that land area per person. China's meat consumption is increasing rapidly -- it more than doubled in the past generation. If the world's entire populaton was to converge on a typical western diet of 80 kg of meat per person per year, the global agricultural land required would be about 2.5 billion hectares, or about 2/3 more than we use now (assuming that productivity remained at today's level, rather than either decreasing or increasing!)

So, it is apparently erroneous to argue that every one could eat as we do in the US if only we could improve equitability of food distribution; production is limiting as well!

There is a great deal of variation animal to animal in the efficiency with which they convert grain into food products. In the US (also considerable variation from source to source in these efficiency estimates, I'm sorry to say!):

-- Cattle consume 4.8 pounds of grain + soy for each pound of meat produced (this excludes the grass that most of them eat for at least part of their lives)

-- Pigs consume 7 pounds of corn and soy per pound of meat produced (more than cattle, because cattle eat grass for part of their lives)

--Chickens consume 2.8 pounds of feed per pound of meat produced (this doesn't include their egg production)

--Dairy cows consume 0.1 pound of grain + soy per pound of milk produced (efficient in terms of grain consumption because they are largely grass fed)


So, if we ate lower on the food chain, one or more of the following benefits would be likely:

(1) We wouldn't have to use as much land and other resources raising grain to feed to animals. We could decrease the intensity of agricultural production and all the impacts associated with that production (impacts discussed previously in this course).

(2) Overgrazing on public and private range lands could decrease.

(3) We wouldn't have to farm or graze marginal lands as intensively, and could even leave them alone! Of course, much of the land that is grazed is not suitable for crop production; I am not suggesting that we could farm it instead, but we could certainly decrease pressure on it. In particular, we could decrease the rate at which we convert tropical rainforest (or other natural ecosystems) to crop production, thus decreasing losses of biodiversity on Earth. (As one example, cultivated soy acreage in Brazil doubled over the past decade, with most new acreage resulting from conversion of cerrado (grassland) and rain forest lands to soy production. The soy is grown largely to feed livestock in Brazil, China, India, and elsewhere. [Science 9 Dec. '05]. (I've read that McDonald's announced that it will not buy chicken fed on soy that was raised on former tropical rain forest lands...)

(4) More people in the world could receive an adequate diet now and even somewhat into the future (assuming that inequities in food distribution could be rectified).

(5) Less fossil fuel energy (and associated emissions of CO2) would be required to produce our food. At present, food production accounts for about 10% of US energy use. The typical US diet that is 70% plant-based and 30% based on meat, eggs, dairy and fish generated about 1.5 metric tons of CO2 per person per year more than would a plant-based diet that provided the same number of calories. The emissions difference is analogous to the difference between driving a SUV versus a compact car. (WorldWatch July/Aug '06)

sujal
03-dic-2008, 09:08
OTHER PROBLEMS ASSOCIATED WITH MANY FORMS OF LIVESTOCK PRODUCTION

There are other problems associated with the methods by which most (but not all!) livestock in the US in raised; problems which call the sustainability of these methods into question:

Raising animals in the typical US style is very energy intensive. Approximately half of the energy used in US agriculture is devoted to the livestock sector. ( Pimentel estimates that it takes about 30,000 kcal of fossil fuel (about 4 litres of gas) to produce 1 kg of pork in the US). A calorie of beef takes about 33% more fossil fuel energy to produce than would a calorie of potatoes (State of the World 2004)

Raising animals in the US requires abundant water. In California, livestock agriculture uses nearly a third of the irrigation water. It is estimated that half the grain and hay fed to US livestock grows on irrigated land, and that it takes on the order of 400 gallons of water [perhaps considerably more, depending on who is doing the estimating and on the practices being used -- see State of the World 2004 or the book, "Six Arguments for a Greener Diet," by M.F. Jackson [2006] for even larger estimates!] to produce one pound of meat. At typical US meat consumption rates of about 0.5 pounds of meat and poultry per person per day, that means almost 200 galls of water per person per day are required to supply that meat; this is twice what typcial US citizens use at home for all other purposes combined!! The typical high-meat diet in the US takes at least twice as much water to produce as would an equally nutritious vegetarian diet (State of the World 2004).

The U.S. EPA estimates that animal wastes are responsible for 5-10% of US water pollution. Nitrate levels over the safety standards are commonly detected in groundwater (wells) in livestock producing states, originating both from fields (many raising grain for livestock) and from feeding facilities for animals.

Manure management is a big problem. Here's an amazing statistic: when the two large feedlots outside Greeley, CO operate at full capacity, they produce more excrement than the combined populations of Atlanta, Boston, Denver and St. Louis (Sierra Nov/Dec. 06)! Approximately 1/4 of livestock manure is produced in stockyards and feedlots, and about 80% of the manure produced in those places isn't used to full potential. Present beef feedlot systems result in losses of about 50% of the nitrogen in the manure before the manure is removed (losses to the atmosphere and to water), with another 25% being lost in handling and storage. These wasted nutrients: (1) pollute water and (2) mean that more nutrients are applied inorganically to fields than would be the case if nutrients in the manure were more efficiently retained and returned to soils over a large area. (There is so much water in manure that it isn't profitable to haul it long distances (e.g., from livestock facilities to far away fields), hence nearby fields get overloaded.) In fact, it is estimated that wasted nutrients from manure could replace 12% of the nitrogen, 32% of the phosphorus, and 30% of the potassium that farmers now apply using inorganic fertilizers.

Methane (CH4) produced by US livestock digestive processes as of the year 2000 was estimated to contribute about the same amount of global warming potential as do the emissions of 33 million cars each year ["Six Arguments for a Greener Diet," by M.F. Jackson [2006]; reviewed in Science 3 Nov. '06].

An interesting prospect in manure management involves rearing industrial quantities of insects in the waste and then feeding the insects back to the livestock. It is estimated that manure from a standard 100,000 bird chicken house would produce 66 tons of edible larvae in 5 months. The larvae use up half of the manure and can be cooked, dried and mixed in with conventional feeds. Because the insects are 42% protein (by weight) they are nutritionally comparable to soy or corn, and chickens, pigs, cattle, and fish are reported to love them. The larvae are worth approximately $350 per ton.

Now, it is clear that beef is not equal to corn in protein value for humans. Nor am I trying to tell anyone how they should eat. I am simply arguing that if we decreased our focus on meat, we could improve the sustainability of agricultural production, based on first principles of energy transfer. We in the US eat much more meat than do citizens of most other nations, and we could benefit the planet -- and our health -- by decreasing the emphasis on meat in our diets.

ANIMALS AS PART OF A SUSTAINABLE AGRICULTURAL SYSTEM:

Many aspects of livestock production are quite different when livestock are raised as a side line, rather than being the focus of the agricultural system. When livestock is integrated with other types of farming, smaller quantities of livestock can be produced than in a pure livestock operation, but mixed livestock and grain can be part of a sustainable agricultural system. Such systems typified the older style of family farm, in which agriculture was diversified; livestock and various crops were all produced on the same farms.

(1) Animals can utilize low quality feed from marginal lands that aren't suited for cultivation. This gives the farmer a high quality saleable product from these marginal lands, and diminishes pressure to plow up that marginal land; the farmer can graze it lightly instead. Use of marginal lands for light grazing also decreases feed costs for the farmer, and provides some financial security in that the animals can be sold.

(2) Animals can also use crop residues and damaged grains that can't be readily marketed. That is, they convert farm "waste" into meat and fertilizer (manure).

(3) Production of livestock in an integrated system can encourage use of crop rotation, because livestock can use forage that is grown as part of the rotation. When meat prices are high, this can make rotation with less valuable crops (such as alfalfa) more economically feasible. Such rotation could be encouraged by government farm price support system changes, so that forages (alfalfa, for example) are covered, not just feed grains. Current crop subsidies encourage feeding livestock with corn and soy, but not with forages, which are "nonprogram crops."

(4) Small scale production of livestock as part of integrated agricultural systems may also encourage more efficient use of manure. (The National organic standards in the US (2007 info) indicate re. manure use that raw manure must either be composted before being applied to fields, be applied to land used for a crop that is not intended for human consumption, or incorporated into the soil at least 90 days before harvesting an edible product that does not come into contact with soil or soil particles, or at least 120 days before harvesting a prooduct that does have such contact.)

SUMMARY: SUGGESTIONS FOR SUSTAINABILITY REGARDING LIVESTOCK

EAT LESS MEAT
INCREASE GRAZING FEES ON WESTERN LANDS
INCREASE WATER CHARGES TO ENCOURAGE CONSERVATION
ALLOW FORAGES TO RECEIVE GOVERNMENT PRICE SUPPORTS AS GRAINS DO
INTEGRATE LIVESTOCK IN DIVERSIFIED AGRICULTURE PROGRAMS
To move to the next section in this discussion of sustainable agriculture (on one method to decrease soil erosion), click ">>" at the bottom of this page. To move to the master Table of Contents for the BI 301 Home Page, click on "CONTENTS" or click here to jump to the Topic Outline for sustainable agriculture.

Page maintained by Patricia Muir at Oregon State University. Page last updated Dec. 27, 2007.

http://oregonstate.edu/~muirp/trophic.htm