Grazing is specified by a management event which specifies the intensity of grazing, the amount of material returned to the system, and the effect of grazing on the root:shoot ratio.
Grazing parameters for crops and grasses are specified in the text file graz.100. Multiple parameter sets are stored in this file. The order of the parameter sets within the file does not matter.
Grazing parameters specify fractions of aboveground live (grazing parameter FLGREM) and standing dead (FDGREM) plant material removed each month. The fractional returns of C (GFCRET), N, P, and S (GRET(*)) are specified, having allowed for losses in animal carcasses and milk, transfer of dung and urine off the area being simulated, volatile losses of N from dung and urine patches, and leaching of N and S under urine patches. The proportion of N, P, and S returned in organic forms (FECF(*)) is also specified as is the lignin content of the feces (FECLIG). As discussed in Grassland/Crop Submodel, grazing can have variable effects on plant production through the parameter GRZEFF.
C returned to the system from feces and urine is accumulated annually in the crop/grass C output variable CRETGA . The amount of C, N, P, and S removed from the system is recorded in annual accumulators. For crops and grassland, C removals are reported as annual totals in the crop/grass C output variables SDRMGA (standing dead and litter) and SHRMGA (shoots); N, P, S are returned in ERETGA(*), SDRGAE(*) (standing dead and litter), SHRGAE(*) (shoots).
The model has three primary options for estimating the impact of grazing upon actual production (grazing parameter GRZEFF = 0, 1, 2). For GRZEFF = 0 there are no direct impacts of grazing on plant production except for the removal of vegetation and return of nutrients by the animals. GRZEFF = 1 is referred to as the lightly grazed effect (Holland et al., 1992 ) and includes a constant root:shoot ratio (not changing with grazing) and a linear decrease in plant production with increasing grazing intensity. GRZEFF = 2 is referred to as the heavy grazed (Holland et al., 1992 ) option and includes a quadratic curve where aboveground plant production is increased for moderate grazing and decreasing sharply for heavy grazing levels (greater than 40% removed per month). The root:shoot ratio is constant for low to moderate grazing levels and decreases rapidly for heavy grazing levels. In all three options the nutrient content of new shoot will increase in relation to the residual biomass ( BIOMAX, PRAMN(*,*), PRAMX(*,*)). The remaining options (GRZEFF = 3, 4, 5, 6) are various combinations of the the first 3 options.
An example parameter set for crop/grass grazing is:
GM Graze low intensity moderate (linear) effect on production 0.100 FLGREM 0.010 FDGREM 0.300 GFCRET 0.900 GRET(1) 0.950 GRET(2) 0.950 GRET(3) 1 GRZEFF 0.500 FECF(1) 0.900 FECF(2) 0.500 FECF(3) 0.250 FECLIG
In this example, in the first line, the first 5 characters "GM " are the name the of the parameter set, which will be used in a management GRAZ event. The remaining part of the first line is a readable description of the set. The remaining lines are value-name pairs; the amount of white-space between them does not matter. See the table above for descriptions of the names. The parameters must be ordered as shown.
Currently tree/shrub browsing is not implemented directly. A tree-removal event (TREM) event can be used to remove aboveground biomass, but no C or N returns to the soil are made for urine or feces, but an organic matter addition event (OMAD) might be used to approximate these returns.
The C/N ratio of grazed biomass can be calculated from the following output variables.
|Aboveground live biomass C/N ratio.||AGLCN|
|C/N in standing dead material.||STDEDC / STDEDE(1)|
|Annual C/N in shoots removed during grazing.||SHRMGA / SHRGAE(1)|
Annual C/N in structural and metabolic litter and
standing dead biomass removed during grazing.
|SDRMGA / SDRGAE(1)|
|C/N in forest/shrub system leaf component.||RLEAVC / RLEAVE(1)|
|C/N in forest/shrub system fine branch component.||FBRCHC / FBRCHE(1)|
Crop and Grassland Submodel
Crop/Grassland C Output Variables
N Output Variables