The Overseer model splits N losses between background (inter-urine) and urine patch sub-models.
Urine Patch losses
In most cases on pastoral blocks, leaching from the urine patch model is the largest source of N loss. N leaching losses from sheep and deer are less than that of cattle. Based on urinary patterns and urine patch characteristics (male cattle typically move when urinating and hence the patch size is narrower), it is also considered that male cattle had a lower risk of leaching than female cattle. The monthly N removal rates are driven by pasture or crop uptake, and when there is no uptake, there is potentially a high risk of N loss through leaching from the urine patch.
Background losses
When the inter-urine/background model is greater than urine patch losses, it is usually associated with high inputs of N such as fertiliser or effluent in which case it can be mitigated by reducing N inputs. However, 'leaching-other' can be particularly affected by the level of inorganic N in the soil, the level of drainage occurring in the block and soil type through Profile Available Water (PAW).
- Soil inorganic N depends on many inputs: block characteristics (e.g. slope), crop uptake, fertiliser applications, rate of mineralisation etc. In the model, a proportion of the soil inorganic N pool is assumed to leach each month.
- Drainage depends on climate and soil characteristics.
- PAW is soil specific
The higher the level of inorganic N, the higher the drainage, and the lower the PAW - the higher the Leaching-other will be.
On grazed cropping blocks, the amount of N leaching-other can be influenced by fertiliser rate and timing, effluent rate and timing, cultivation timing and method, crop yield, and harvesting of the crops.
Factors that could lead to higher N losses:
- High N fertiliser rate - It is best not to apply more than 50 kg N/ha at any one application. See your rates under each application in the Fertiliser tab
- Fertiliser/effluent applied during high rainfall months (winter months)
- High effluent rate/Effluent area too small - See Reports tab > Farm Details Report > Effluent Report
- The farm/block have well drained soils - See Soils tab
- The farm/block soils with low profile available water (PAW) - see Reports> Compare blocks> Soil and irrigation results
- High drainage caused by high rainfall and/or high irrigation
- High stocking rate - more urine and dung deposited on blocks
- Ground is left fallow between crops - with nothing growing to take up the nitrogen in the soil, more N is susceptible to be lost via leaching
- Low pasture production/stocking rate too low - OverseerFM subtracts the energy gained from eating supplements from the energy required by the animals (based on age, sex, weight, production etc.) to determine the amount of pasture eaten and hence pasture grown. Therefore, the more animals on a block, the greater the pasture intake and hence pasture grown. The more pasture grown, the greater the crop uptake which reduces soil inorganic N (N available to be leached).