Fertilisers and topdressing for pastures

Cost / benefit analysis of fertiliser and paddock productivity

One way to assess the impact of fertiliser price is to construct a gross margin and adjust the fertiliser price to see what impact it has on the bottom line. 

It is important to remember that you don’t need to apply fertiliser to your whole farm. Being strategic about application rates and what paddocks you target can go a long way to reducing your fertiliser bill, especially when prices are high.

If the decision is to reduce input costs, it would be worth targeting key paddocks that you will be relying on this season.

Before you fertilise pastures

Things to consider before deciding if you need to apply fertiliser.

• Is the pasture actively growing? Fertiliser won’t help dormant plants.
• Is there enough groundcover to prevent runoff?
• Are you targeting legumes or grasses? Nutrient needs differ.
• Is follow-up rain likely? Moisture is essential.
• Have you done a soil test recently? It guides product choice and fertiliser rates.
• Fertiliser can increase risks like grass tetany or nitrate poisoning in livestock.

Current soil nutrient status 

Soil quality can be checked with a soil test to determine if there are any key nutrient deficiencies and the extent to which these deficiencies may be impacting potential pasture growth. 

Identification of paddocks where nutrient replacement is more important (lambing / weaning paddocks, pasture improved or hay cutting paddocks) may also help prioritise where nutrients are most essential if funds are limited.

Past fertiliser history

Having a good fertiliser history is it gives you greater capacity to skip applications when encountered with price hikes, supply constraints, wet seasons etc.

Producers with a good fertiliser history also have the option of cutting back fertiliser rates to reduce cost and keep the system maintained. The maintenance rate will vary between properties and can only be calculated with confidence by doing annual soil testing to track nutrient trends over time.

Current stocking rate

The key with any fertiliser program is to make sure the nutrient status of your soil is in balance with your conditions and stocking rate. 

If you have a lot of surplus feed and your stocking rates are below optimal (for example, if still rebuilding following drought conditions) then it may be prudent not to fertilise.

Are paddocks in a responsive state? 

Paddocks must be well grazed/open so light can get down to ground level and stimulate new growth. If paddocks have a large amount of carry-over feed that will not be under control before winter, you would be better to skip fertilising these paddocks for now and target them further down the track.

How to fertilise pastures

Most pasture fertilisation is done in the form of topdressing.

Topdressing for legumes

Autumn suits legume-based pastures, especially sub-clover. Benefits include:

• stronger root growth.
• better nitrogen fixation.
• improved seed set and persistence.

If adding clover seed to fertiliser, do it early. After mid-June, seed set may fail. Some varieties have hard seed that persists, but relying on this is risky.

Topdressing for grasses

Spring suits grass-dominant pastures, especially if autumn growth was poor or legumes are absent. But spring fertilising has risks which include:

• low soil moisture may limit uptake
• storms can cause nutrient loss
• fertilising vulpia-dominant pastures may encourage weeds.

Making a fertiliser decision

The choice of fertiliser depends on price and the need for additional nutrients.

Nitrogen (N)

The most common nutrient that pastures respond to is nitrogen. Urea and ammonium fertilisers need to
be broken down to nitrate, the plant available form of nitrogen.

• Effective on small quantities of green pasture.
• Best applied early in winter - soil temperature is still slightly warmer than normal.

General guidelines:

• 1 kg of N produces around 10 kg DM/ha of green pasture
• Maximum response occurs when applying 20–60 kg N/ha (equivalent to 43–130 kg Urea/ha)
• Common application rates: 60–100 kg Urea/ha.

Expected outcomes:

• 60 kg/ha Urea = ~300 kg/ha of additional green DM
• Lock paddocks up for ~3 weeks to maximise response and manage nitrate risks.

Approximate costs: (based on buying around 5 tonnes of urea)

• 60 kg/ha Urea = $81/ha
• 100 kg/ha Urea = $120/ha.

Phosphorus

Soils in Australia are naturally depleted in phosphorus due to weathering and erosion. Agriculture removes phosphorus from the soil in farm products. 

Phosphorus costs are high and a decision on the rate required to reach the desired test result needs careful consideration. Soil test first with attention to the Phosphorus Buffer Index (PBI) which estimates the amount and strength of phosphorus binding to the soil particles.

Gibberellic Acid (GA)

Gibberellic acid is a naturally occurring plant hormone that promotes growth.

Cold weather reduces production of gibberellic acid. Applying GA can “trick” plants into growing more dry matter. The additional green dry matter produced will have cost benefits against expensive hay/silage fodder alternatives. 

Pastures do well with gibberellic acid application when soil moisture is not limiting and green herbage mass is limited by the low temperatures.

Considerations for use:

• Requires minimum 800 kg DM/ha of pasture before application
• Works best on introduced pasture species, especially Phalaris
• No withholding period
• Lock paddocks up for ~3 weeks post-application.

Costs and effectiveness:

• $29–$33/ha
• Higher rate (80 mL/ha GALA) may yield ~300 kg DM/ha
• Response can be variable. 

To apply gibberellic acid:

1. choose a perennial grass that will be responsive to the application, for example phalaris or perennial ryegrass
2. choose a paddock with an open canopy (recently grazed, minimal dead standing stalks), approx. < 1,500kgs/Ha total herbage mass, to ensure droplets can easily contact the growing points at the base of the pasture plants
3. read and comply with the label recommendations of the product, especially the minimum 100 litres/Ha water rate application for ground rig applications
4. ensure the extra pasture growth is utilised/grazed 3-6 weeks after application. Gibberellic acid treated pasture should increase the utilisation rate, due to the increase in height of green
5. do not apply to new pastures in their first year of establishment. 

Practical Tips

• Use calibrated spreaders for even application.
• Avoid fertilising before heavy rain or on wet soils.
• Consider split applications if conditions vary.
• Monitor pasture response and adjust future plans.

If pastures don’t respond or show nutrient deficiency, revisit your soil test. Other issues like compaction or poor drainage may be limiting growth.

Read more about:

• Seasonal pasture management
• Managing agricultural soils
• Fertilisers for pastures in high rainfall areas