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Pasture Improvement Initiative

Filling the winter feed gap as pasture growth declines

Frosty cattle _PII newsletter

With plummeting temperatures across much of southern Australia, pasture growth in many regions has all but come to a standstill.

Options to fill the winter feed gap in the short-term are limited. In the longer term producers can look to limit the gap by:

  • building a feed wedge, or surplus feed, leading into winter,
  • introducing other pasture species into the farming system.

In the short term the options are limited to supplementary feeding or improving winter growth rates of existing pastures by applying nitrogen fertiliser or gibberellic acid.

Nitrogen fertiliser

Urea is the most favoured nitrogen fertiliser option during winter, as it is less prone to losses (leaching and de-nitrification) during cold wet weather. However, slow pasture growth rates during winter, often in the order of 5–10 kgDM/ha/day), means the fertiliser will also be slow to release during this period (about 5kgDM/kgN).

Soil temperature, moisture, fertility and pasture species composition all impact on the pasture response to applied nitrogen. For example, a north-facing slope may be as much as 2°C warmer than a south-facing slope in winter. Applying nitrogen to the north-facing slopes during July and south-facing slopes during August as temperature start to increase can give a greater growth response than applying nitrogen to both areas at the same time.

Apply urea immediately after removing stock from a paddock. Delaying the application of nitrogen fertiliser reduces the potential pasture response by about 1% per day after stock have been removed.

Moisture stress (either too dry or too wet) will also impact on the pasture response to nitrogen, so review the mid to long seasonal rainfall forecasts as part of the decision-making process.

Annual grasses respond most efficiently to nitrogen fertiliser, followed by short-rotation grasses, then perennial grasses. Match pasture availability with animal requirements when prioritising which pasture species to fertilise.

If other nutrients (e.g. phosphorus, potassium) are limiting growth the response to nitrogen could also be reduced.

Gibberellic acid

Gibberellic acid is a naturally-occurring plant hormone that stimulates growth through cell expansion. In grasses, this results in stem and leaf elongation.

Gibberellic acid is produced in higher quantities during warmer months, and at low levels during colder months. Applying gibberellic acid during colder winter months stimulates the plant and can improve the quantity of feed on offer (FOO).

The rapid growth may cause the plants to ‘yellow off’ in colour for the first couple of weeks after application, however, the quality of the feed on offer is not affected.

Results can be obtained using rates of 2.5 to 20g of gibberellic acid/100L water. Apply gibberellic acid in a minimum volume of 100L/ha. Phalaris is highly responsive to gibberellic acid. Rates of 2.5 to 10g/100L are appropriate for phalaris-dominant pastures. If applying to perennial ryegrass-dominated pastures, annual ryegrass or cocksfoot, 10–20g/100L is appropriate.

Multiple applications can be used on a paddock in conjunction with a rotational grazing program. Growth stimulation is usually seen within seven days of application and ceases around 3–4 weeks after application.

Cease applying gibberellic acid from mid-August, when soil temperatures are rising. Natural levels become sufficient in the plant and untreated areas will perform at similar levels to treated areas. Applications during late winter (post mid-August) or early spring may in fact lead to a suppression of spring growth.

Soil nutrient levels need to be adequate to support the additional growth applications of gibberellic acid may induce.

The key to maximising returns from applying either nitrogen fertiliser or gibberellic acid is to match pature availability with livestock needs. Ensure any additional feed is fully utilised.

The EverGraze Nitrogen and Gibberellic Acid Calculator can be used to make your own estimations of likely responses and cost effectiveness to applying nitrogen and/or gibberellic acid. The calculator incorporates the cost of application, the quality (digestibility) of the feed, expected responses to applications and utilisation of feed to calculate cost on a cents per megajoule (c/MJ) comparison. This can be used to consider the costs of supplementary feeding. This information has been adapted from the EverGraze website

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