Reviewing farm systems in OverseerFM: focus on pasture energy supply

When reviewing an OverseerFM analysis, ask:

Can this farm realistically supply the pasture energy needed to support the animal numbers and production entered?

A high modelled pasture yield is a reason to investigate. It should not, by itself, determine whether a farm system is realistic. It is recommended  that users review the metabolisable energy (ME) supplied by pasture, alongside farm records, feed quality and management.

Why start with ME?

ME is the energy in feed that animals can use, it is expressed as megajoules of ME per kilogram of dry matter, written as MJ ME/kg DM.

OverseerFM estimates animal energy requirements from the animal characteristics, management and production entered (e.g. total milk solids). 
It then back-calculates the pasture energy needed after allowing for energy supplied by crops and supplements. The pasture requirement is therefore calculated from the farm system described; it is not an independent measurement of what the farm can grow. Overseer Knowledge base

The model converts pasture energy intake into dry matter intake using its pasture ME assumptions. It then estimates pasture production by allowing for utilisation. 
If assumed pasture ME is lower than the feed quality achieved on the farm, more dry matter is needed to supply the same energy. This can contribute to a high modelled pasture yield. Overseer Knowledge base. 

The pasture quality data used by the Overseer science model to set its default monthly pasture ME values is relatively old and limited in geographic coverage. The model currently uses pasture ME data for three broad regions - Upper North Island, Lower North Island and South Island - with a separate default ME value for each month in each region. 

Overseer Ltd are actively working with the science community to identify any more recent and comprehensive data which is available, and which may be useful for the model. This is an ongoing process. 

Pasture quantity still matters. Review it together with feed quality and utilisation, rather than using a tonnage threshold alone.

Start with the Animal Reports

Review these reports together:

The ME source report shows energy allocated to animals from each feed source. Its pasture total should not be described as gross pasture energy grown without accounting for utilisation and other relevant feed flows. MJ ME/Month – Overseer Knowledge base

Check monthly patterns as well as annual totals. An annual feed budget can appear reasonable while relying on unrealistic pasture supply during particular months.

Check whether the pasture energy requirement is achievable

Use evidence appropriate to the farm and the purpose of the review. Useful evidence includes:

• Pasture measurement records, with the measurement method explained.
• Herbage tests showing feed quality across relevant seasons.
• An independent feed budget that accounts for pasture, crops, supplements and conserved feed.
• Records of animal numbers, liveweights, production and time away from the farm.
• Irrigation coverage, system performance and water availability.
• Pasture renewal, grazing management and utilisation records.

Assess different parts of the farm separately where necessary. Evidence from a well-managed irrigated block may not support the same assumptions on dryland or less productive blocks.

For a proposed system, identify what must change to supply the additional energy. More animals or higher production need a credible feed and management plan. Consider whether the proposed performance can be maintained over time, including during water restrictions or changes in farm management.

A simple energy check

For illustration, suppose pasture supplies animals with 165,000 MJ ME/ha/year.

These are illustrative calculations, not recommended pasture quality values or performance benchmarks.

They describe pasture eaten. At 80% utilisation, eating 15 t DM/ha would require approximately 18.8 t DM/ha grown, before considering other relevant feed flows.

When comparing measurements with model outputs, use the same area, period and definition: pasture grown, harvested or eaten.

Avoid changing inputs simply to make a yield look acceptable

Enter supplements when they are part of the actual or proposed system. It is not recommended that additional supplements, or supplement tonnages be added solely to bring modelled pasture production below a preferred threshold.

Changing supplements changes the diet and may change nitrogen intake, excretion and modelled losses. The effect depends on the feed and the wider system. A revised analysis must still represent how the farm operates. Overseer Knowledge base

If the evidence does not support the pasture energy requirement, identify the unsupported assumptions and revise the system accordingly.

What does “years in pasture” mean?

“Years in pasture” describes land-use history. It is the number of qualifying pasture years within the 10-year period covering years 3 to 12, where the reporting year is year 1 and the previous year is year 2. A qualifying pasture year contains at least eight months of continuous pasture. 

Only the total number of qualifying pasture years within that 10-year period is counted; the timing of the cropping phases is ignored. For example, five years in pasture followed by five years in crop gives the same “years in pasture” value as five years in crop followed by five years in pasture. For cropping blocks, “years in pasture” is entered by the user, with a maximum value of 10.

The input affects nitrogen mineralisation and nutrient cycling. Overseer Knowledge base

How should consultants review it?

Check paddock history, crop dates and how rotations are represented in the analysis.

A long rotation does not mean every area has the same history. Some paddocks may have remained in pasture throughout the relevant period, while others have recently been cropped. Do not automatically change every entry from 10 to 9 because cropping occurs somewhere on the farm.

Equally, do not assume that all entries can remain at 10 without checking the crop duration, qualifying pasture years and block setup.

For fodder crops rotating through pasture blocks, check that the selected pastoral blocks and crop entries describe the rotation correctly. The full crop sequence must be represented where required. Overseer Knowledge base

Record the history and the reasoning behind the entry. Do not select a value because it produces a preferred nitrogen result.

Practical review checklist

• Confirm the analysis period, model version and system being reviewed.
• Check animal numbers, liveweights, production and grazing-off periods.
• Check crop and supplement amounts, units, timing and feeding locations.
• Review monthly ME requirements and energy supplied by each feed source.
• Identify the energy that pasture must supply.
• Check how pasture demand is distributed across blocks.
• Compare that requirement with pasture measurements, herbage tests and a feed budget.
• Distinguish pasture grown from pasture eaten and document utilisation assumptions.
• Check irrigation coverage, performance and water reliability.
• For proposed changes, identify how additional feed demand will be met.
• Check “years in pasture” against paddock history and crop duration.
• Record corrections, remaining uncertainties and the evidence supporting the conclusion.

A useful review conclusion explains what is supported, what requires correction and what remains unproven.