Model description - Structure and modules
The structure of GLEAM consists of five main modules -herd, manure, feed, system and allocation and two additional modules for the calculation of direct and indirect on-farm energy use and postfarm impacts.
Herd module
The herd module is about the animals themselves and describes: the herd structure and the characteristics of the animals.
To allow an accurate accounting of production, natural resource use and GHG emissions (including the use of IPCC (2019) Tier 2 methodology), GLEAM considers different on animal types, weights, phases of production and feeding situations. The national herd is disaggregated into six cohorts: adult females, adult males, replacement females, replacement males and male and female fattening animals (or surplus animals). Key data for herd modeling are mortality, fertility, growth and replacement rates. Other parameters used to define the herd structure are the age or weight at which animals transfer between cohorts, the duration of key periods such as gestation, lactation or molting and the proportion of adult males to adult females.
Manure module
The manure module describes how manure is managed and simulates the rate at which excreted nutrients are applied to feed crops. The calculation is based on the total amount of excreted nutrients in each cell (using Tier 2 excretion rates), the proportion of nutrients lost during manure management and the area of arable/grass land in the cell to determine the rate of application per hectare.
Feed module
The feed module calculates the composition of the ration for each species, production system and location, the nutritional values of the ration and the impact associated with it such as land-use and GHG emissions.
The determination of the ration is done separately for ruminants (cattle, buffalo, sheep and goats) and for monogastrics (pigs and chickens).
Ruminants feed ration
In all ruminant production systems, the composition of the feed ration depends on the availability of pasture, fodder, crops and their respective yields. The fraction of concentrates varies widely, accordingly to the need to complement locally available feed, the purchasing power of farmers and access to markets. The major feed ingredients for ruminants include:
- Grass: comprises natural pasture, improved and cultivated grasslands, rangelands and marginal areas such as roadsides.
- Feed crops: crops used to feed livestock such as maize silage or grains.
- Tree leaves: browsed in rangelands or forests or collected and fed to livestock.
- Crop residues: plant material not harvested such as straw or stover.
- Agro-industrial by-products: by-products from processing non-food crops such as oilseeds cakes, molasses or cereals brans.
- Concentrates: high energy or protein mixtures of by-products and grains are processed at specialized feed mills into compound feed.
The proportion of each feed component is determined differently for industrialized and developing countries. In industrialized countries, the composition and proportion of feed ration materials are taken from national inventory reports, literature and targeted surveys. For developing countries, a feed allocation scheme was developed. It is based on literature and expert knowledge, and relies on a close relationship between land use, feed availability and the feed ration. It was also crossed referenced with a survey at national level.
Monogastrics feed ration
The feed materials used for pigs and chickens are grouped into three main categories:
- Swill and scavenging: domestic and commercial food waste and scavenged feed;
- Non-local feed materials: concentrated feed materials sourced from various locations, included international trade;
- Locally-produced feed materials: feed materials produced locally and used extensively in intermediate and backyard systems. This is a more varied and complex group of feed materials that can include local by-products, grade crops unfit for human consumption, crop residues and forages.
The proportion of non-local (or imported) feed was defined, where possible, country by country after an extensive literature research and expert consultation. The total amount of local feed available is compared with the estimated local feed requirement within the cell. If the availability is below a defined threshold, small amounts of grass and leaves are added to supplement the ration.
System module
The system module calculates:
- the average energy requirement of each animal cohort and the necessary feed intake.
- the total production of meat, milk and eggs.
- the use of natural resources and the environmental impacts of production, which currently include feed use, land use, nitrogen use and GHG emissions.
For ruminants, the system module calculates the energy requirements of each animal as described in the IPCC (2006) Tier 2 guidelines. The gross energy requirement of an animal is the sum of the energy needed for maintenance, pregnancy, animal activity weight gain and lactation. IPCC (2019) does not include the necessary equations for calculating the energy requirements for pigs and poultry. Therefore, GLEAM uses slight modifications of models found in the literature.
The feed intake per animal in each cohort is calculated by dividing the animal’s energy requirement by the average energy content of the ration calculated in the feed module. Feed emissions and land use associated with feed production are calculated based on the total herd or flock feed intake and the emissions or land use per kg of feed dry matter calculated in the feed module.
The CH4 and N2O emissions arising from manure management are calculated following the IPCC (2019) Tier 2 methodology. The proportion of manure managed in each system is based on official statistics, such as the Annex I countries’ National Inventory Reports to the UNFCCC, literature reviews and expert judgment.
Enteric CH4 emissions from ruminants and pigs are calculated from a slight modification of IPCC (2019) equations relating the amount of feed gross energy consumed by the animal and the proportion of feed converted to CH4.
Allocation module
The allocation module is the last step in GLEAM calculations. Its functions are to sum up the total impacts for each animal cohort, allocate the impacts to each commodity and calculate the total aggregated impact per commodity and their intensity.
The impacts from direct and indirect energy use and post-farm activities, which are calculated separately, are incorporated into the allocation module at this point.
Impacts are first allocated between edible commodities, which are meat milk and eggs.
