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Effects of Grazing Method and Soil Fertility on Pasture and Wool Production

L.K. Warn and G.R. McLarty.

Department of Natural Resources and Environment, PO Box 879, Seymour, Vic.

ABSTRACT

An experiment at Broadford, Victoria, comparing 2 rotational grazing systems (intensive rotation and a simple time based rotation) to continuous grazing (set stocking) has shown that there are large economic and environmental benefits to be gained from a rotation. At least a 20% increase in stocking rate (4DSE/ha) is possible from switching from continuous grazing to some form of rotation. Wool cut per head was similar across grazing treatments. Under the rotations there were higher perennial grass (phalaris) contents, more ground cover over summer/autumn and reduction of sheep camps. The most successful treatment was the combination of higher soil fertility (25 kg P/ha/year) and the intensive rotation where the rest period is based on phalaris recovery, rather than a fixed time. This treatment allowed 23 DSE/ha to be carried and produced 115 kg greasy wool/ha.

KEY WORDS

Grazing management, stocking rates, fertiliser responses, phalaris, rotation.

INTRODUCTION

Under continuous grazing (set stocking) wool and meat producers have demonstrated that large increases in stocking rate are possible through improved fertiliser use and pasture utilisation (Grassland Society of Victoria, 1998). However, with the combination of high soil fertility, high utilisation and continuous grazing, pastures tend to become clover dominant. This in turn leads to excessive bare ground by mid summer, causing concern by producers about the sustainability of their grazing system. Rotational grazing was seen as way of maintaining a higher grass content in the pasture (particularly improving the persistence of any sown perennial grasses). However producers were not confident that they would get an animal production benefit from adopting rotational grazing and they were not confident there was any economic benefit in going beyond a relatively simple time based rotation. This project was initiated in response to these producer concerns.

MATERIALS AND METHODS

This field experiment commenced in 1998 at Broadford, Victoria. (annual rainfall 625 mm, clay loam soil). The pasture was sown in 1988 to Sirosa phalaris, Porto cocksfoot and Trikkala subclover. Two levels of fertiliser, LOW P (6 kg P/ha) and HIGH P (25 kg P/ha), have been applied since 1994, when the initial Olsen P was 4 ug/g. Since 1998, the following grazing systems have been applied: Continuous Grazing (set stocking), Simple Rotation (4 paddocks, rotation based on fixed time. During autumn, winter and summer the rotation consists of 2 weeks grazing/ 6 weeks rest, and in spring it is sped up to 1 week grazing/3 weeks rest), and Intensive Rotation (20 paddocks, rotation interval flexible and based on phalaris recovery to at least the 3 leaf stage. A paddock is grazed for 3 days, and the rest period varies from 20 days in spring to 70 days in summer, depending on plant growth rates).

Stocking rates are adjusted based on live-weights (merino wethers) and pasture availabilities, with the aim to keep average live-weights similar across all treatments. Pasture composition, growth, phalaris persistence, wool quality, and soil moisture is being monitored.

RESULTS AND DISCUSSION

Increases in stocking rate were achieved by improving soil fertility and additional increases were achieved by changing from Continuous Grazing to a rotation (table 1). The Intensive Rotation allowed 20% more stock to be carried than Continuous Grazing under both Low P and High P conditions, without compromising per head production (Table 2). By the third year of the experiment, the Simple Rotation showed a 10% benefit in stocking rate compared to Continuous Grazing, but only under High P. The results highlight that to obtain optimum pasture growth benefits from a rotation, the rotation interval or "rest" should be flexible and based on the plant recovery model, not time-based recipes.

Table 1. Stocking rates achieved under different combinations of fertiliser and grazing system.

TREATMENT

DSE/Ha

1998

1999

2000

Low P/ Continuous

13.0

13.7

16.3

Low P/ Simple

13.0

14.3

16.9

Low P/Intensive

13.7

15.6

19.2

       

High P/Continuous

18.4

17.9

19.6

High P/Simple

19.6

19.6

21.5

High P/Intensive

19.7

19.8

23.0

The increases in stocking rate under the rotations were attributed to more grass growth. At the farm scale, additional benefits could be obtained from also improving pasture utilisation, particularly in hill country. The Intensive Rotation was superior in stocking rate to the Simple Rotation due to the substantial increase in grass (mainly phalaris) contribution to the sward (figures 1 and 2).

Table 2. Wool production per head and fibre diameter for September 1998 and 1999 shearings.

TREATMENT

Wool
Kg/head

Fibre diameter
(u)

1998

1999

1998

1999

Low P/ Continuous

6.6

5.9

21.4

22.0

Low P/ Simple

6.0

6.1

21.5

22.1

Low P/Intensive

6.4

6.1

21.2

22.5

         

High P/Continuous

6.1

5.7

21.4

21.6

High P/Simple

6.1

5.9

21.0

21.5

High P/Intensive

6.2

6.0

20.7

22.1

         

l.s.d. (p=0.05)

0.3

0.5

0.7

1.1

Inadvertently, through 1998 and 1999, Low P plots were slightly understocked relative to High P plots (with respect to target sheep weights). Consequently, there was a significant (p= 0.05) fertiliser effect on wool cut/head and fibre diameter. There was no grazing system effect on wool cut per head or fibre diameter in 1998 or 1999. Wool cuts for the LowP/ Continuous and Low P/Intensive were significantly higher than the LowP/Simple indicating that there was an interaction between fertiliser and grazing system for 1998 only (table 2).

Figure 1. Changes in perennial grass (mainly phalaris) content of pastures from spring 1998 to 1999.

Figure 2. Changes in subclover content of pastures from spring 1998 to 1999.

ACKNOWLEDGMENTS

This project is funded by The Woolmark Company and NRE’s Wool Strategy.

REFERENCES

Grassland Society of Victoria.1998. Grasslands Productivity Program, Final Report to Members.

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