Managing pasture

Pasture growing on a farm, whether comprised of native or improved species, is a valuable plant resource that can be fed to animals either directly by grazing or after being cut and carried to stores. CLEM offers a range of resources and activities to include pasture in your simulation. Two approaches are offered with a simple approach as used by the Integrated Analysis Tool (IAT) and a more advanced approach similar to that used by the Northern Australia Beef Systems Analyser (NABSA). We have attempted to replicate the functionality of both precursor models and you should be able to produce similar results, but as NABSA provided improved pasture modelling capabilities, we have concentrated on these improvements throughout CLEM. When running simulation comparable to IAT, it is expected that you will take advantage of the new functionality and you will need to understand what settings to turn off to purely replicate IAT.

The same methods used to grow a crop (see Managing crops) can be used to grow pasture using a manage crop activity to place the grass production provided by a crop file reader into either an animal food store for manual feeding (replicating cut and carry and feed) or into a graze food store to allow grazing (see Graze ruminants) or can be cut and carried (Cut and carry) to an animal food store.

This approach uses harvest dates and biomass of pasture from input files. On each harvest date the biomass harvested (with associated Nitrogen content) is placed into the Graze food store representing the pasture. This biomass is then available for animals to graze.

  • This approach allows you to use simple forage input files (same as for crops) for pasture growth and does not require access to the other pasture models.
  • Can include pasture detachment and quality decay rates to maintain pasture and prevent runaway pasture accumulation (see advanced approach)
  • Ruminants can be moved to the pasture to allow grazing
  • Initial pasture biomass can be provided at start up
  • Ruminants can immediately graze the associated Graze food store type
  • Can access a limited number of the ecological indicators such as utilisation rate.
  • Does not include any feedback into pasture production based on utilisation rates and grass basal area
  • Requires a Cut and carry activity to move from pasture store, capable of tracking N content, to an Animal food store type from which you can use manual Feed ruminants activities.

The advanced approach includes a range of improvements over the simple approach in order to better simulate grazing on native pastures. This starts with the use of the manage pasture activity which allocates the land available for the pasture and manages the new pasture processes.

The biggest difference from the simple approach is the inclusion of the calculation of a land condition score which is influenced by grazing pressure (for full details see Tracking land condition and pasture state). This index, along with grass basal area and stocking rate is used in the lookup database to determine pasture growth. This means that, while coming from an external data source, the pasture production will respond to herd management and grazing levels. In order to obtain the pasture production data, CLEM links to output from other pasture models which have previously simulated pasture production over the range of land condition and grass basal area values for the time period of the simulation. This data access is handled by a Pasture data reader SQLite. There is quite a bit of background experience needed to create the pasture growth database. Versions are available from the developers for collaborative projects or contact us for more information. We are developing methods to assist with creating the pasture growth database using another grass production model, but the link between pasture production and the Land Condition Index would need to be defined by the user.

A range of ecological indicators relating to the pasture can be tracked and these include the land condition index and grass basal area required to determine pasture growth. The Relationships between these measures with respect to utilisation rate are provided with the Manage pasture activity.

The graze food store also adds additional functionality to manage the detachment and decay of pasture through time. As the external pasture data only provides the new growth each time-step, CLEM implements decay in pasture quantity and quality as used by the NABSA model. This approach places all new growth with given nutritional quality (from the datafile or user parameters) into a pasture pool. This pool then decays over time via detachment of material and loses quality through the loss of nitrogen and dry matter digestibility as set by the user. This process allows for the modelling changing pasture quality over time and especially simulates the seasonal changes in pasture biomass and quality in the northern Australian rangelands. Detachment and decay can be set to zero to turn this feature off. As the simple approach can also use the graze food store, it is also able to take advantage of this improved functionality.

If you are not undertaking a grazing activity, any pasture placed in a graze food store must be cut and carried to an animal food store before it can be fed to animals, but will undergo the growth and decay.

  • Pasture responds to herd size and management through pasture utilisation rates.
  • Includes pasture detachment and quality decay rates to maintain pasture and prevent runaway pasture accumulation
  • Ruminants can be moved to pasture to allow grazing
  • Allows calculation of other ecological indicators such as run-off, tree cover, sediment loads etc.
  • Requires a look-up data-cube of grass production using a pasture simulation model. Creating this database will take some expertise and requires access to other pasture models.

 

The common land shared by any number of neighbouring farmers can be included in your simulation. Common land is either placed in the animal food store or the graze food store (if grazing is to occur) and linked to an on-farm pasture to determine its quality relative to the farm. While common land is an infinite pool, you can add additional labour requirements to collect it when levels are low.

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