IMAGEIntegrated Model to Assess the Global Environment.

Livestock systems


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Livestock systems module in IMAGE 3.0
Flowchart Livestock systems. See also the Input/Output Table on the introduction page.

Key policy issues

  • What are the impacts of increasing livestock production on land use, greenhouse gases and other emissions to air and surface water?
  • How does the use of marginal lands for grazing increase the risk of degradation and loss of productivity, inducing more forest clearing?


Food production will have to increase in order to feed the world’s growing population. However, with increasing prosperity and falling production costs, dietary patterns are shifting to include a higher proportion of meat and milk. In the last few decades, traditional mixed farming systems have not been able to raise production levels sufficiently to meet increasing demand. Consequently, modern livestock production systems are expanding rapidly particularly for poultry and pork, creating growing demand for feed crops. This trend started in high-income countries and is now observed in emerging and developing countries (Alexandratos and Bruinsma, 2012).

Interactions between crop and livestock production are described in the livestock systems module of IMAGE, and also the consequences of changing practices in livestock farming for production of food crops and grass. For this purpose, IMAGE distinguishes pastoral livestock systems, and mixed and landless (industrial) production systems. Pastoral systems are based on grazing ruminants, while mixed and landless systems integrate crop and livestock production in which livestock are fed a mix of crops, crop by-products, grass, fodder and crop residues (Bouwman et al., 2005; Bouwman et al., 2006).

Livestock production is related to a wide range of the environmental issues, and the consequences of changes in the livestock system can be studied in the IMAGE framework:

  1. Expansion of grazing land and particularly arable land for feed crop production, is required to support increasing livestock numbers. According to Bouwman et al. (2005) most arable land expansion is to increase feed production;
  2. Large amounts of methane (CH4) emitted by ruminants during enteric fermentation are the second major source of greenhouse gas emissions after CO2;
  3. Excreta from all livestock categories is a source of ammonia, methane, nitrous oxide and nitric oxide;
  4. Odour nuisance and nitrate leaching to groundwater are major local-scale problems;
  5. A significant amount of land used for ruminants grazing is marginal, low productive grassland with low carrying capacity and high risk of degradation due to overgrazing, especially in arid and semi-arid regions (Seré and Steinfeld, 1996; Delgado et al., 1999). To compensate for productivity losses in these areas, forests may be cleared to expand agricultural land areas.

Input/Output Table

Input Livestock systems component

IMAGE model drivers and variablesDescriptionSource
Animal productivity Effective production of livestock commodities per animal per year. Drivers
Feed conversion Measure of an animal's efficiency in converting feed mass into the desired output such as meat and milk (for cattle, poultry, pigs, sheep and goats). Drivers
Livestock rations Determines the feed requirements per feed type (food crops; crop residues; grass and fodder; animal products; scavenging), specified per animal type and production system (extensive/intensive). Drivers
Production system mix Livestock production is distributed over two systems (intensive: mixed and industrial; extensive: pastoral grazing), with specific intensities, rations and feed conversion ratios. Drivers
Livestock production Production of livestock products (dairy, beef, sheep and goats, pigs, poultry). Agricultural economy
Management intensity livestock Management intensity of livestock, expressed at the regional level. This parameter is based on data and exogenous assumptions, i.e. current practice and technological change in livestock sectors, and is endogenously adapted within the Agricultural economy component. Agricultural economy

Output Livestock systems component

IMAGE model variablesDescriptionUse
Animal stocks Number of animals per category: non-dairy cattle; dairy cattle; pigs; sheep and goats; poultry.
Grass requirement Grass requirement; ruminants (nondairy cattle, dairy cattle, sheep and goats) are grazing animals, and part (in mixed systems) or most (pastoral systems) of their feed is grass, hay or other roughage; this grass requirement is calculated as a fraction of the total energy (feed) requirement.
Feed crop requirement Total amount of feed required for the production of animal products. Grass and fodder species are consumed by grazing animals only (dairy and non-dairy cattle, sheep and goats), while pigs and poultry are fed feed crops and other feedstuffs.