In the last decade, developed countries have changed the livestock production system from small family businesses with little intensification to intensive production, highly concentrated in certain areas. As a direct consequence of this change, the production of livestock waste has reached such a point that the land does not have sufficient capacity to absorb it naturally. This fact is further aggravated by the increase in the use of animal health products, derived both from the greater grouping of animals and from the improvement in the requirements and sanitary controls of these animals before they reach the consumer.
Livestock producers can suffer great economic losses due to the accumulation of manure in pastures. An adult cow produces 12 cows dung of almost 4 kilos each per day, which represents approximately 50 kilos of manure per day, 500 kilos in 10 days, 1,500 kilos in one month, 4,500 kilos in three months!
On a ranch with 100 cows, the production in three months would be 450,000 kilos of manure! If that amount of manure is not buried, the area of grassland covered is also large. If each manure deposited measures approximately 30 cm in diameter, 12 manures per day would cover an area of just over 1m2 of pasture. If there are 100 cows, their manure, in one day, would cover 100 m2, in 10 days 1000 m2, in one month 3, 000 m2 and in three months it would cover almost a hectare of pasture!!! In addition, the grass around each manure is also not eaten by the cattle. This additional area can be 6 to 12 times larger than the area covered by the manure, so the grass that is lost can be converted into a much larger amount. Considering the same ranch with 100 cows, the area of pasture lost could be a minimum of 6 hectares instead of one. A farm with only 2500 cows can produce the same amount of waste as a city with 411000 people (,EPA, 2004).
With this in mind, it is necessary to dimension the amount of waste that is generated worldwide when the populations of animals for consumption continue to increase, approximately: 1.7 billion cattle, 1 billion pigs, 2.4 billion poultry, and 2.2 billion sheep. And in the United States alone, 3 million kg of waste is produced every minute (,USDA, 1995; ,USDA, 2008).
The Negative Effects of Waste
Manure generated in livestock systems can cause negative environmental impacts if there is no control in storage, transport or application, due to the emission of pollutant gases into the atmosphere, and the accumulation of micro and macronutrients in soil and surface water bodies.
Although human diseases caused by animal excreta are not frequent, workers on poultry farms may suffer from asthma, pneumonia, and eye diseases (irritation) when ventilation on the farms is deficient. Another risk of disease for the human population is the consumption of water contaminated with 1) manure containing pathogenic bacteria, the most common being Escherichia coli, which causes diarrhea and abdominal gas; 2) high nitrate content that reduces the oxygen transport capacity of the blood, known as methemoglobinemia; 3) hormones, mainly estrogens, related to a reduction in the amount of sperm in humans. The environmental impact, such as the generation of greenhouse gases, eutrophication of water bodies, and nutrient overloading of crop soils caused by livestock excrement, will depend largely on the livestock species, the feeding system, and manure management.
Soil can be seriously affected by manure if it contains high concentrations of nutrients (nitrogen, phosphorus), pathogenic microorganisms (E. coli), antibiotics, and compounds that interact with the endocrine system (steroid hormones, phytoestrogens, pesticides, and herbicides).
Effects on Water
Water is contaminated by livestock excreta directly through runoff, seepage, and deep percolation on farms, and indirectly by runoff and surface flows from grazing areas and cropland. Nitrogen is abundant in manure and is related to groundwater contamination by nitrate leaching through the soil, while phosphorus in manure is related to surface water contamination.
Discharges to the atmosphere from manure include dust, odors, and gases from anaerobic digestion and aerobic decomposition, including ammonia, as well as other greenhouse gases including methane and nitrous oxide.
The intensification of livestock production increases the generation of manure, which results in a large amount of nutrients being disposed of and concentrated in a small area. A viable alternative to reduce the negative environmental impact of livestock excreta is to generate biogas, but the costs of equipment to capture and use this gas to generate electricity are still high. In the U.S. and Canada, the implementation and enforcement of specific regulatory frameworks on livestock excreta management and application offer multiple long-term environmental benefits, either through reduced environmental impact, energy generation, or participation in the carbon credit market. The regulations of this type in some countries still lack incentives for good practices in the management of livestock excreta; therefore, governments should be responsible for the periodic monitoring of livestock systems to control excess discharges of pollutants into the environment and natural resources, and should also promote compensation for carbon credits.