Why use Fertilisers?
Good pasture production requires good soil fertility. To achieve good soil fertility and vigorous pasture growth, generally means applying fertilisers. Actual rates of fertilisers applied will be influenced by a number of factors such as soil fertility, stocking rate, pasture utilisation, pasture composition, growing extra feed and economics.
These factors are discussed in greater detail below. The timing and frequency of application as well as other pasture management practices such as sowing rates and grazing management will also affect the response to fertilisation and ultimate profitablity.
Factors which Influence Fertiliser Rates
1. Soil fertility - A soil test is a good guide in assessing nutrient status and fertiliser requirements. A common approach where soil nutrient reserves are very high, is not to apply that nutrient, but to utilise the soil reserves until a pasture response to applying that nutrient is more likely. If soil tests show extreme acidity or salinity, then responses to fertiliser will be limited until soil is limed or the salinity level decreased. Sometimes, soil structure or poor drainage will need to be improved before worthwhile fertiliser responses will occur. Fertiliser test strips are another tool to help identify whether it is worthwhile applying a particular nutrient over a whole paddock. Your local Department of Primary Industries office will be able to advise on how to set up test strips on your paddock or refer to appropriate Landcare note.
2. Stocking rate - Fertilisers are a cheap way to provide extra feed when an increase in stocking rate is planned. Phosphorus in particular has been shown to increase pasture production if other factors are not limiting pasture growth. Where a decision has been made to increase stocking rates well above the typical stocking rate for a given district (e.g. high producing dairy farms, heavily stocked lambing paddocks), then increased fertiliser use above the typically recommended rates is recommended. Increasing stocking rates is not an option for some farmers as it exposes them to extra risk and may involve changes in management (e.g. employing extra labour, change to lambing season, increasing fodder reserves). Some farmers may stock well below the district average by choice or might be forced to stock lightly because of limitations to pasture growth such as high salinity or poor weed control. In these cases, rates below the typically recommended rates would be appropriate.
3. Pasture utilisation - Stocking rate should be matched as closely as possible to the amount of feed produced. Pasture that is grown and not eaten or cut for hay or silage is wasted pasture. It is therefore pointless and costly to increase fertiliser use to increase pasture production if existing pasture is already poorly utilised. Pasture utilisation can be estimated by assessing the quantity of pasture residues from the previous spring that are still present in the following late winter/early spring period. Improved pasture species generally have higher protein content and digestibility than unimproved species and therefore should be encouraged to increase pasture utilisation.
4. Pasture composition - Often, native grasses (e.g. spear grass), introduced annual grasses (e.g. silver grass, annual poa), weedy perennial grasses (e.g. bent) and weeds (e.g. sorrel, rushes) do not respond well to fertiliser. Improved pasture species such as white clover, sub clover, Phalaris, perennial and annual ryegrass do respond well to higher fertility. Therefore, fertiliser priority should be given to newly sown pastures or pasture with a reasonable content of clover (e.g. 30%) and improved pasture grasses. Because of the time and money invested in sowing new pastures it is crucial that they receive adequate fertiliser. Note that a pasture of good composition for a low rainfall sheep enterprise may consist of a good sub clover component and some perennial grass only, while a good pasture in a high rainfall dairy situation would consist of an almost pure sward of white clover and perennial ryegrass. Pasture composition can often improve markedly with increased fertiliser and/or grazing pressure. Some paddocks, however, will require renovation with improved pasture species before a worthwhile response to fertiliser can be achieved.
5. Growing extra feed - If hay or silage is to be cut or extra feed is required at certain times of the year, extra fertiliser may need to be considered. It is generally more profitable to utilise extra feed by grazing than by cutting hay or silage. As a rough guide, a good hay crop (250 bales/ha equivalent to about 3 t/ha) removes the equivalent of 8-10 kg/ha of phosphorus and 50-60 kg/ha of potassium. These nutrients may need to be replaced depending on soil reserves and whether the fodder is fed back onto the paddock from which it was cut. When favourable seasonal conditions result in an early autumn break, the early production of feed may influence some farmers to reduce fertiliser rate for that year. However, long-term soil fertility needs to be considered. Late breaks may mean a shortage of winter feed and encourage the use of fertilisers to promote pasture growth. Where the growth period over summer has been extended by regular rains and cool weather, extra fertiliser could also be considered.
6. Economic considerations - A common response by farmers is to only apply fertiliser when they can afford it. However, other things also need to be considered.
- Optimum economic rate - If a particular soil nutrient is limiting pasture growth, then each extra kilogram of tht nutrient applied will increase pasture production until a point is reached where at a particular rate of that nutrient no extra pasture production is gained. This rate is the point of maximum production, but it is not an economic fertiliser rate to aim for. The economic optimum rate of fertiliser is below the point of maximum production and occurs when the cost of the extra fertiliser used starts to be higher than the value of the extra pasture grown. Some value has to be placed on the pasture produced and the fertiliser used, to establish this optimal fertiliser rate.
- Cost of fertiliser - The cost of fertiliser is the cost ex works plus transport and spreading costs. Before spending money on fertiliser, consideration should be given to altemative investments and retums. (For example, what interest would you eam if you leave the money in the bank? whats the cost of buying extra feed as hay/silage or grain?). Borrowed money obviously is more costly and retums would need to be higher to warrant increased cost. Seek advice before borrowing money to buy fertiliser. Consideration should also be given to the long term costs of not putting fertiliser out (nutrient decline, invasive weeds, future resowing costs etc).
- Value of pasture grown - It is difficult to put a dollar value on extra pasture produced. Extra feed is likely to be worth more on a dairy farm than on a sheep farm. Extra feed is worth more on a property stocked at 30 dse/ha than a property stocked at 5 dse/ha. The time of the year the feed is produced and how much feed is actually utilised (eaten) and the feed quality will also affect the value of extra feed. As the aim is to convert pasture grown into a saleable commodity such as wool, milk meat or hay, commodity prices will have a big impact on the value given to extra pasture grown.
Fertiliser programs should be reviewed regularly by taking into account the factors discussed above.
Fertilisers for Sowing Pastures
The rate of phosphorus that can be applied at sowing may be limited by the sowing equipment used but typically pastures are sown with between 10-25 kg/ha of phosphorus. If more phosphorus is required it can be applied prior to sowing, immediately after sowing or, in higher rainfall areas, in the spring.
If potassium is required it can be applied prior to sowing, immediately after sowing or in the first spring. Potash must not be sown with the seed due to the risk of affecting germination.
Applying fertiliser rates above the suggested rates may be necessary in the first 2-3 years to enable the pasture to establish well.
Nitrogen at sowing is sometimes recommended in the form of D.A.P, M.A.P or Pasture Sowing Mix. No more than 12 kg/ha of actual nitrogen should be applied at sowing as excessive nitrogen may affect nitrogen fixation by the clovers and may also affect germination.
Where soil pH (measured in water) is strongly acidic, the use of a product containing lime (e.g. Super Lime) may need to be considered at sowing. Such products when sown in contact with the seed increase the soil pH immediately around the seed providing more favourable conditions for the rhizobium bacteria and the germinating seedling.
Your local Department of Primary Industries office should be contacted regarding local trials comparing the usefulness of such products. Regardless of what fertiliser is used, clover seed should be inoculated and lime coated prior to sowing. Always read labels and follow instructions carefully for any products used.
Further information is available from your local DPI office or customer service centre on 136186.