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Phosphorus fertiliser for beef or sheep pastures - a decision-support package

J.W.D. Cayley1, G.R. Saul1 and P.M. Schroder2

1Agriculture Victoria, Hamilton, Vic.
2
Peter Schroder Rural Consulting, Hamilton, Vic.

Abstract

The decision support package shows how to estimate the amount of phosphorus (P) fertiliser required to increase the P status of a pasture, and maintenance applications of P required to maximise profits. The P required for maintenance is calculated by multiplying the stocking rate (expressed as dry sheep equivalents or DSE/ha) by the amount of P/DSE appropriate for various conditions. This ratio varies from 0.35 to 1.25 kg P/DSE depending on the rainfall, grazing management and soil type. The package concludes with a ranking system for assessing the need of individual paddocks for P fertiliser.

KEY WORDS

Decision support, phosphorus, fertiliser, pasture, soil tests.

Introduction

The profitability of pastoral agriculture in southern Australia is strongly linked with the efficient use of fertilisers that supply phosphorus (P). Attempts to provide advice about this issue have relied heavily on predicted responses in herbage production to applied P (1, 8). While soil tests give a good indication of the likelihood of a response to P (8, 12), recommendations based solely on likely responses in pasture growth may be conservative if soil tests are not low. Recent results from Hamilton show that there is a close relationship between inputs of P and profitability (5). This has led to recommending maintenance fertiliser applications on a P per dry sheep equivalent (P/DSE) basis (9, 13). This is a better approach because the amount of fertiliser recommended per hectare is proportional to the stocking rate. Widespread recommendation of a single P/DSE ratio will be wrong, however, because the amount of fertiliser required will depend on soil type (7,4), animal enterprise and the potential productivity of the site (4). This paper outlines a decision support package that attempts to tailor P/DSE recommendations to individual paddocks on the farm.

How much phosphorus is needed?

The use of more fertiliser per hectare as the stocking rate increases supports proposals made in New Zealand by Cornforth and Sinclair (6), who recommended replacing the P lost in grazing systems with P from fertiliser. They identified losses due to animals (transfer to camps and removed in product), and soils (fixed, sorbed or leached). There were only three categories of soil loss factor for the whole of New Zealand (6). These losses, expressed descriptively, are given in Table 1.

Table 1. Loss factors for sheep or beef cattle.

 

Soil type and rainfall

Loss factor

Soil

Recent alluvial soils, low rainfall loams

Low

loss

Podzols, clay-loams (rainfall less than 900 mm), rendzinas,

Medium

factors

Acid sands, kraznosems, organic soils

High



Animal
loss
factors

Grazing management

Intensive rotational grazing

Topography

Flat and rolling
Easy hill (mostly less than 25)
Steep (33% greater than 35)

Loss factor

Very low
Low
Medium

 

Set stocked or intermittent grazing

Flat and rolling
Easy hill (mostly less than 25)

Low
Medium

   

Steep (33% greater than 35)

High

Results from a long-term grazing experiment at Hamilton, Vic. (5) have been combined with the procedure developed in New Zealand (6) to predict the P/DSE required for maximum profitability for a range of conditions (4). A set of these predictions is given in Table 2. ‘Poor’ pastures, are those that are dominated by low-quality plants such as onion grass, silver grass, sweet vernal grass or native grasses. These have a lower ceiling yield than improved pasture (2). To use this approach, the stocking rate of each paddock must be known. For paddocks that are not continuously stocked, or where stock numbers vary from time to time, records of the type, physiological state and average number of animals present during each grazing event must be kept in order to calculate the overall stocking rate (see Appendix 1).

Table 2. Predicted kg P/DSE for maximum profit for a range of conditions.

Soil

Animal

Poor pasture

Improved pasture

loss

loss

Mean annual rainfall (mm)

factor

factor

300

500

700

300

500

700

Low

Very low

0.35

0.40

0.45

0.40

0.40

0.45

 

Low

0.50

0.50

0.55

0.50

0.55

0.60

 

Medium

0.60

0.60

0.65

0.60

0.65

0.70

 

High

0.70

0.75

0.80

0.70

0.75

0.85

Medium

Very low

0.55

0.60

0.60

0.55

0.60

0.65

 

Low

0.65

0.70

0.75

0.65

0.75

0.80

 

Medium

0.75

0.80

0.85

0.75

0.85

0.95

 

High

0.85

0.90

1.00

0.85

0.95

1.05

High

Very low

0.70

0.75

0.80

0.70

0.80

0.90

 

Low

0.80

0.85

0.95

0.80

0.90

1.00

 

Medium

0.90

1.00

1.05

0.90

1.05

1.15

 

High

1.00

1.10

1.15

1.05

1.15

1.25

               
               

Phosphorus status and soil tests

Soil tests for P are used to assess the ability of a soil to supply P to plants. The most commonly used test in Victoria and New Zealand is the Olsen P test (10). As soil tests also give a measure of the biologically active P in grazing systems (11), we suggest that they should be used to adjust the soil loss factor (SLF). For example, a steadily increasing soil test suggests that a lower SLF should be used.

Effects of applications of P fertiliser on changes in P status in the following year are illustrated in Figure 1. Our experience at Hamilton (3) indicates that it is necessary to apply about 10 kg of P in excess of maintenance to increase the Olsen P of basalt-derived soil by 1 unit (Figure 1a).

Figure 1. Predicted change in Olsen P in the year following applications of P (a) soil at Hamilton derived from basalt (3); (b) soil from Dundas Tableland derived from laterite

The thick horizontal line represents a state of maintenance. The amount of P required for this condition to be met corresponds to the labelled diagonal line that crosses the thick line at the required level of Olsen P. At Hamilton, the P required to maintain the Olsen P at 13 mg/kg is 20 kg/ha (Figure 1a). Less P is required to increase the Olsen P of a soil from the Dundas Tableland north west of Hamilton (Figure 1b). There, only 6 kg P in excess of maintenance will increase the Olsen P by 1 unit. We do not have data on the rate of decline in Olsen P for this soil, so have assumed it is similar to the soil at Hamilton.

Assessing needs for phosphorus fertiliser

For cases where anticipated fertiliser costs exceed the budget, we have provided an arbitrary guide to establish which paddocks have the greatest need for P (Table 3). The guide was developed from a system proposed by Keys (9), and is based on applying a score derived from research and field experience to the following factors that drive the need for P:

Anticipated stocking rate: There is no point in growing more pasture if utilisation is poor, but excessive (>20%) dead pasture in winter may be due to poor pasture quality.

Proposed use: As P fertiliser also increases the quality of pasture, areas that have the greatest need include those used for pregnant, lactating or fattening stock, or fodder conservation.

Pasture type: Classify paddocks after examining several areas of about 1 m2 in each paddock in spring:

Re-sown pasture: Applying fertiliser here protects the investment made.

Good pasture: Pasture consists mainly of sown species (perennial grasses and legume both > 25%).

Moderate pasture: Some perennial grass and clover is present wherever you look. Some onion grass, bent grass, silver grass, capeweed and barley grass may also be present.

Poor pasture: No sown grasses are present, and the sward is dominated by onion grass, bent grass, and other species that tolerate low fertility. Clover, if present, is stunted with small dark-green leaves (indicating P deficiency).

P status of paddock: If the soil level is not known, consider amount of the P applied recently.

Other soil-related factors: Excessive salinity, acidity or lack of water-holding capacity will limit the extent to which pastures will respond to P. Areas prone to flooding should not receive much fertiliser, as losses in run-off and problems in waterways or dams will be more likely.

Implications

The use of a structured process to assist in making decisions about fertiliser should result in a more effective use of resources. There is a need to establish exactly how New Zealand soil loss factors apply to Australian soils. Soil testing must be used to monitor the consequences of the decisions that are made. Initially, it will be necessary to test once a year in order to establish if appropriate amounts of P are being applied. The paddock-rating scheme is offered as a guide only. Perhaps ratings should be made on the capacity to produce income, and may vary according to soil type. The scores in Table 3 are also assumed to equivalent, and this may not be so. The 0.8 kg P/DSE for maximum profit established at Hamilton is a long-term average, which includes initial heavier doses. For areas with a low P status, heavier applications than those recommended here will be required during the establishment phase (see Figure 1).

Table 3. Assessing the need of paddocks for P fertiliser.

References

1. Bennett, D. and Bowden, W. 1976. In: Reviews in Rural Science 3. (Ed. G.J. Blair) (University of New England). pp. 77-81.

2. Cayley, J., Cameron, F. and Saul, G. 1999. Proceedings of the Grassland Society of Victoria 40, p 138.

3. Cayley, J.W.D. and Kearney, G.A. 1999. Aust. J. Agric. Res. 50, 547-554.

4. Cayley, J.W.D. and Kearney, G.A. 2000. Proceedings Australian Society of Animal Production 23, Vol B, 191-194.

5. Cayley, J.W.D., Kearney, G.A., Saul, G.R. and Lescun, C.L. 1999. Aus. J. Agric. Res. 50, 1179-1190.

6. Cornforth, I.S. and Sinclair, A.G. 1982. NZ J. Exp. Agric. 10, 53-61.

7. Fleming, N.K. 1997. SARDI Research Report Series No 14. (SARDI, Adelaide). p. 28.

8. Jones, H.R., Maling, I.R. and Curnow, B.C. 1984. Aust. J. Exp. Agric. Anim. Husb. 24, 579-585.

9. Keys M.J. 1996. Management of profitable and sustainable pastures: A field guide. NSW Agriculture. pp. 45-46.

10. Olsen, S.R., Cole, C.V, Watanabe, F.S. and Dean, L.A. 1954. USDA Circular No. 939.

11. Roberts, A.H.C., Sinclair, A.G., Johnstone, P.D., Risk, W.H., Smith, L.C., O'Connor, M.B., Nguyen, L. Morton, J.D. and Shannon, P.W. 1994. NZ J. Agric. Res. 37, 229-237.

12. Sinclair, A.G., Johnstone, P.D., Smith, L.C., Roberts, A.H.C., O’Connor, M.B. and Morton, J.D. 1997. NZ J. Agric. Res. 40, 559-567.

13. Watson, R. c.2000. How to fertilise high profit pastures: Part 1 phosphorus. (Holmes Sackett and Associates, Wagga Wagga, NSW). pp. 27-31.

Appendix 1 (Procedure for calculating the stocking rate of an individual paddock) Conversion of stock classes to dry sheep equivalents (DSEs)

Sheep

Live weight (kg)

 
 

20

30

40

50

60

Dry ewes or wethers (maintaining weight)

   

0.9

1.1

1.3

Dry ewes or wethers (growth rate 50 g/day)

   

1.1

1.4

1.6

Last month of pregnancy (singles or twins)

   

1.6, 2.2

2.0, 2.6

2.4, 2.9

Lactation (singles or twins)

   

1.9, 2.9

2.4, 3.3

2.9, 3.7

Weaners (growth rate 100 g/day)

1.2

1.5

       

Beef cattle

Live weight (kg)

 

200

300

400

500

600

Dry cows or store steers (maintaining weight)

   

6

7

8

Dry cows or store steers (growth rate 0.25 g/day)

   

7

9

11

Dry cows or store steers (growth rate 0.75 g/day)

   

11

13

15

Last 3 months of pregnancy

   

8

9

10

Cows with 0-3 month calves

   

13

15

17

Cows with 4-6 month calves

   

16

17

18

Yearlings (growth rate 0.25 or 0.75 kg/day)

4, 7

6, 9

8, 11

   

If the area is a ha, and the average number of animals of a given type and physiological state is n, with a DSE-equivalence of e, and they are present for d days, then the stocking rate (SR, in DSE/ha) for that grazing event is given by:

If there are g grazing events in a year: annual total SR = SR1 + SR2 +…SRg. Similar calculations are performed in many computerised farm management packages.

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