Pasture production in Overseer is "back-calculated" from production.
The Overseer science model first estimates the metabolizable energy (ME) requirements of animals on the farm (based on type, numbers, productivity etc.), it then allocates them all the ME available from crops and/or supplements.
The remaining ME requirements from the animals are then assumed to be supplied by pasture (Pasture ME intake):
Pasture ME intake = animal ME requirement - ME supplied as crops - ME supplied as supplements.
Pasture DM production is then estimated based on Pasture ME intake, Utilisation, and ME content of pasture.
When calculating the default ME content within the Overseer model there are a number of other inputs that may impact the ME that have not been included within the calculation including: clover content, irrigation (less dead matter in an irrigated system and therefore improved quality), and grazing method.
As a result, the pasture production (yield) can be under or over-estimated for an area depending on the farms location, and the system operated.
It is recognised that the default ME values within the Overseer science model under-estimate the ME value of irrigated dairy pastures in Canterbury. The lower the feed quality, the more dry matter required to achieve the same production.
Figure below provides the Overseer ME values for the South Island, with the highest value being 11.17 MJ ME/ Kg DM in Spring (October). The data shown in Table 1 derives from the most recent comprehensive dataset available, namely that published in the report Litherland & Lambert (2007) Factors affecting the quality of pastures and supplements produced on farm. Chapter 6. Editors Rattray P V, Brookes I M and Nicols A M. New Zealand Society of Animal Production, Occasional Publication No. 14.
Source: Overseer Technical Manual - Characteristics of Pasture.
This means, that in practical terms, if the modelling is reasonably estimating the ME required from an irrigated farm system in Canterbury, but that the ME values available to it are likely lower than what is actually being achieved in the farm system; Overseer will over-estimate pasture production (yield) in order to ensure that sufficient ME is available for animal requirements.
Hence, pasture yield should be used as a representation of ME yield NOT pasture production.