Annual pasture quantity and quality in South West Western Australia

Page last updated: Monday, 18 May 2020 - 10:50am

The productivity of sheep is largely governed by the amount of pasture they eat and this is influenced by the quantity of the pasture on offer and its quality. Assessing pasture quantity and quality throughout the growing season gives managers the information they need to manage animal performance and pasture condition.

The Department of Primary Industries and Regional Devlopment provides this information for farmers to help improve the productivity and profitability of the livestock industry in Western Australia.

Pasture quantity

Feed on offer (FOO)

The quantity of pasture in a paddock is known as feed on offer (FOO). It is the amount of pasture in front of an animal at any one time, measured in kilograms of dry matter per hectare (kg DM/ha). Grazing pressure is the balance between how much pasture grows (pasture growth rate – PGR) each day and how much pasture is removed by grazing animals. Being able to accurately estimate FOO is an essential skill in managing animal production and balancing pasture grazing pressure.

There are different FOO targets for the production objectives of different classes of animal and for aspects of pasture management (such as manipulating pasture composition or reducing insect populations). The appropriate FOO target depends on the objectives for a particular class of animal or a particular pasture attribute and it may be that these are not compatible.

For instance, intensively grazing spring pastures to 1400kg DM/ha can have a marked effect on increasing clover content, reducing grasses and reducing pasture pests such as red-legged earth mite. However, 1400kg DM/ha is too low for lactating ewes and would significantly reduce milk production and lamb growth.

In the table below, lower and upper boundary FOO values are suggested for optimal growth of annual plants during different periods of the pasture growth cycle. The lower limits are usually based on the need of plants to have sufficient leaf area to capture energy for growth.

Table 1 Suggested FOO boundaries (kg DM/ha) for optimum growth of annual pastures (source: Doyle et al). Note: *maximum seed production may require spring FOO as high as 5000kg DM/ha
FOO targets Establishment (autumn) Vegetative (winter) Reproductive (spring)
Lower FOO 500 1000 1500
Upper FOO 800 2500 3500*

These boundaries are based on:

  • reducing grazing pressure on establishing pastures in autumn to maximise plant establishment
  • maintaining adequate leaf area during winter to maximise pasture growth rate
  • managing spring pastures to ensure adequate seed is produced for the following year’s pastures.

An efficient grazing system would aim to maintain FOO between 800–1400kg DM/ha in autumn/winter and 1400–3000kg DM/ha in spring. Within this range, animal intake requirements will be met, while pasture growth and utilisation will be maximised.

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Assessing FOO (watch the movie first)

 

Estimate the average green FOO, to account for the variation in FOO across a paddock, as sheep will selectively graze areas in a paddock. See Grazing annual pastures – using feed on offer (FOO) as a guide for how to measure FOO in the paddock.

The simplest method to assess FOO is to:

  • divide the paddock into sectors or proportions (say thirds or quarters)
  • assess the FOO in each sector
  • Divide the sum of these by the number of sectors to get the average FOO.

Example

  • Three estimates from three sectors of a paddock are 300, 450 and 600kg DM/ha
  • sum = 1350kg DM/ha
  • average FOO = 1350/3 = 450kg DM/ha

Remember that FOO in the establishment phase in autumn/early winter is very different to winter or spring FOO because plants contain more water and less structural material. For example, a FOO of 500kg DM/ha in autumn looks like a spring FOO of 1500-2000kg DM/ha. Thus, there is a tendency to overestimate autumn FOO.

Use the FOO boundary figures to decide if there is enough FOO to meet your production objective for your sheep.

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FOO targets for pasture growth and grazing management

Managing pastures to key FOO targets can result in improved production of grazing systems in mediterranean environments which are characterised by slow winter growth, peak spring growth and dry summers. These targets vary according to what production response is required.

Autumn

Figure 1 500kg DM/ha FOO in autumn. This grassy pasture is short and there are areas of bare ground

This is the minimum FOO that should be present before sheep are allowed to graze a paddock that has been deferred after the break. Overgrazing too soon after the break can lead to significant loss of plant density. The objective is to maximise FOO going into winter so that pasture growth is not reduced due to a decreased leaf area. A FOO of around 500kg DM/ha will maintain the condition of medium-framed single-bearing Merino ewes from day 100 of pregnancy.

Winter

Figure 2 800kg DM/ha FOO in early winter. This pasture looks a lot more than 800kg DM/ha, but due to the higher water content in young plants, the actual FOO is lower than it might appear. In spring, a pasture like this might measure closer to 2000kg DM/ha. Care must be taken not to overestimate autumn/winter FOO

Around 800kg DM/ha is the minimum FOO required from day 100 of pregnant single-bearing ewes to ensure a gradual increase in condition. Twin-bearing medium-framed Merino ewes need around 900kg DM/ha FOO. If maintained through winter, this FOO will maximise branching in clovers, resulting in increased clover content in spring. It will also reduce insect populations such as red-legged earth mite (RLEM) and lucerne flea.

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Spring

Figure 3 1700kg DM/ha FOO in spring. This grassy pasture is short, dense and was heavily grazed through winter. The density due to winter grazing, and lower water content compared to autumn pastures, means there is more FOO than meets the eye

This FOO will maintain condition of adult medium-frame Merino ewes during lambing and lactation. If maintained through spring, it will result in increased clover and reduced grass content in the pasture. However, clover seed-set will be reduced unless the grazing pressure is released at 20% flowering. Redlegged earth mite and lucerne flea insect populations will be reduced in the current and following seasons if FOO is maintained at this amount through spring.

2000 FOO in Spring, if tightly grazing is only 2 to 3 cm high.
Figure 4 2000kg DM/ha FOO in spring. This pasture was heavily grazed through winter and is only 2-3 centimetres high

A minimum of 2000kg DM/ha FOO is required for near-maximum feed intake of grazing sheep. That is, sheep will grow at their near-maximum rate if provided with 2000kg DM/ha FOO and will not grow significantly faster if given access to more feed.

Don’t forget: FOO in autumn looks very different to FOO in spring. A general rule is that pasture height in autumn is double that in spring for the same FOO level.

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Pasture growth periods

Pasture growth drives productivity in grazing enterprises, so an understanding of the different pasture growth periods throughout a year is essential for producers wanting to maximise pasture utilisation and ensure pastures don’t deteriorate.

In mediterranean-type climates, the three pasture growth periods for annual-based pastures are: establishment, vegetative and reproductive. A fourth period, senescence, is a period where pasture dies or 'hays off’ with a rapid decrease in the amount and quality of the FOO.

Establishment

Pasture establishment occurs after the break of the season in autumn/early winter. It describes the period from germination and seedling emergence, to the point where the seedling has developed a root and leaf system that can sustain being grazed.

Successful pasture establishment is vital to ensure the persistence of annual pastures. Grazing a pasture too early after seedling emergence can dramatically affect pasture establishment due to:

  • uprooting of small seedlings resulting in reduced plant density which is the single biggest determinant of early growth rate
  • removal of leaf area (defoliation) resulting in reduced growth of seedlings, poor root development and poor drought tolerance.

Decreasing the grazing pressure, applying fertiliser (particularly nitrogen on grassy pastures), or sowing pasture seed can improve plant density and effective pasture establishment.

Grazing pressure during autumn/early winter can be reduced by supplementary feeding which reduces the amount of daily pasture intake as animals fill up on supplement. Deferred grazing, where animals are removed from the pasture, allows seedlings to establish better compared to grazing through the break which can lead to substantial loss of seedlings. Grazing establishing pastures also keeps FOO low, thereby reducing leaf area index resulting in poor root development and poor drought tolerance.

Figure 5 Establishment: clover cotyledons are still present

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Vegetative

Vegetative pasture growth describes the winter period where plants have become established and are mature enough to withstand being grazed and defoliated by animals.

During this stage, pastures are actively growing and stocking rates can be maximised. However, over-grazing by grazing animals can result in insufficient leaf area for maximum pasture growth. Small plants may have poorer root development and are more susceptible to cold stress in winter, further reducing growth.

General rule: higher FOO = higher leaf area = faster pasture growth.

A minimum FOO of 1000kg DM/ha is desirable going into winter but optimum pasture growth occurs at around 1400kg DM/ha.

Grazing during the vegetative pasture phase is important to:

  • encourage tillering in grasses and branching in clovers
  • assist in weed control
  • maintain palatability of the pasture by promoting young shoot growth.

Clover seedlings with four to six leaves are generally able to tolerate a moderate grazing pressure. Clover leaves, flowers, burrs and seed all originate in the axils (branches) of leaves along the main stem. Since grazing stimulates branching until flower initiation, the aim up to the point of flowering is to graze clover pastures to maximise the number of branches per plant and, in turn, maximise the potential seed production.

Early growth of grasses can be increased with the application of nitrogen fertilisers to grass-dominant pastures, although the response tends to be site and season dependent. This boost to pasture growth may allow an overall increase in stocking capacity.

Grazing management through the vegetative period should aim to meet animal production objectives, while being sensitive to the productivity and sustainability of the pasture. This can be achieved through various grazing tactics, including strip grazing, grazing to target FOO and grazing to a paddock target.

Figure 6 Early vegetative period: stem thickening and branching is occurring
Figure 7 2000kg DM/ha FOO tightly grazed clover-dominant pasture with multiple branching in plants

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Reproductive

The reproductive period occurs in spring and is characterised by flowering and seed-set.

Grazing during this period can have an influence on seed production. Overgrazing during flowering will reduce seed production due to removal of flowering parts. This may affect annual grasses more than prostrate species such as sub-clover which buries its seed. Grazing management during spring can influence pasture composition in both the current year and following season.

Figure 8 Flowering or reproductive period: most species have started to flower

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Senescence

Senescence describes the period where pasture dies or ‘hays off’ in summer. Plant leaf material breaks down quickly after senescence which means that FOO decreases rapidly. Importantly, this occurs whether the pasture is grazed or not — hence the saying 'use it or lose it'. Research has shown that up to 80% of the peak spring pasture can be ‘lost’ during summer and not ingested or turned into animal product. The rate of decline is influenced by weather. Both quality and quantity declines faster after rain.

The quality (digestibility) of pasture becomes limiting to animal growth when it reaches about 50%. Animals grazing such pastures will lose weight/condition if not fed supplements.

Figure 9 Senescence: pasture has dried or 'hayed off'

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Pasture growth rates

Pasture growth rate is the daily growth in kilograms of green dry matter of pasture per hectare (kg DM/ha/day). Pasture growth rate information can assist farmers with management decisions such as grazing rotations, feed budgeting and fertiliser application. Satellite technology provides farmers with an accurate measure of feed on offer and pasture growth rates.

Estimating pasture growth is a key component of all feed budgets. The pasture growth rate is driven by a range of factors and can be highly variable. The major determinants of pasture growth rates are:

  • moisture
  • temperature
  • soil fertility
  • soil type
  • pasture species
  • plant density.
Table 2 Estimated typical daily pasture growth rates (kg DM/ha/day) by monthfor several regions in WA
Pasture – annual grass, sub clover and weeds J F M A M J J A S O N D
Dandaragan 0 0 0 2 15 26 28 36 57 51 10 0
Northam 0 0 0 2 8 13 15 29 48 24 9 0
Katanning 0 0 0 1 11 15 16 28 51 45 15 0
Esperance 0 0 0 7 12 16 18 29 47 35 23 0

Pastures from Space

Pasture growth rates are available on a shire basis from the Pastures from Space website. The program provides estimates of pasture production during the growing season by means of remote sensing. Satellite data is used to accurately and quantitatively estimate pasture biomass or FOO and is also combined with climate and soil data, to produce pasture growth rate estimates.

Map of pasture growth rates in Western Australia, which vary between 0-10 and 30-40 in the main sheep production zones
Figure 10 The Pastures from Space project generates large scale maps that show different pasture growth rates represented by varying shades of colour

This map can be scaled down to show variation across local areas.

A paddock-level subscription service is also available from Landgate.

Example map at paddock scale showing different pasture growth rates on different land management units.

This information can then be used for creating simple feed budgets.

Item Paddock 1 Paddock 2
Pasture growth rate 55kg/ha/day 70kg/ha/day
Number of animals that could be maintained at current pasture growth rate if each eats 1.2kg per day 45 58

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Stocking rate versus grazing pressure

Stocking rate and grazing pressure are important aspects of grazing management. Stocking rate is the number of animals per unit area (for example five wethers per hectare, two steers per hectare). Grazing pressure is the balance between how much the animals are eating from a paddock each day, and how fast the pasture is growing in that paddock each day.

That is, grazing pressure (GP) = rate of removal of pasture/rate of supply of pasture.

For example, comparing the grazing pressure of a paddock in winter stocked at 20 sheep/ha to the same paddock in spring also stocked at 20 sheep/ha, it is clear that the grazing pressure in winter is higher because the pasture is growing slower, while in spring the grazing pressure is lower because the pasture is growing faster. In each case, the stocking rate is the same (20 sheep/ha) but the grazing pressure varies.

In winter, 20 sheep/ha will eat pasture faster than it is growing, so there is an overall decrease in FOO. In spring, pasture can grow faster than 20 sheep/ha can eat, so there is an overall increase in paddock feed.

Using animals to maintain a target FOO requires an understanding of grazing pressure and knowledge of the rate of pasture growth and the rate of pasture removal by grazing animals.

Grazing pressure is a better yardstick than stocking rate for grazing management because it takes into account how well a pasture is growing. FOO is the simplest indicator of grazing pressure.

  • GP <1 (light grazing pressure, FOO increases)
  • GP = 1 (neutral grazing pressure, FOO doesn’t change)
  • GP >1 (heavy grazing pressure, FOO decreases)

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Pasture quality

The quality of a pasture for sheep production is largely influenced by two main characteristics:

  • the digestibility of pasture present
  • the proportion of clover in the pasture sward.

Digestibility

Digestibility refers to the proportion of plant matter that is retained in the animal's body after eating. For example, a high quality pasture in its ‘green' state might have a digestibility of 80%. This would mean that 80% of the pasture eaten by the animal is utilised for body maintenance and growth, with the remaining 20% of pasture passed out as faeces. All digestibility measurements are expressed in terms of dry matter (DM).

Example: for 1kg dry matter intake, 0.2kg is passed as faeces so 0.8kg is retained in the animal's body. This equals 80% digestibility
Figure 11 A pasture of 80% digestibility means that 80% of the pasture eaten by the animal is utilised for body maintenance and growth, with the remaining 20% of pasture passed out as faeces.

Digestibility and metabolisable energy (ME) are directly related by a linear equation. Either measurement can be used to describe pasture quality and both measurements are usually reported by feed testing laboratories.

The relationship between Metabolisable Energy (ME) and digestibility in forages shows a linear response from 40% at 5.2MJ to 80% at 12MJ
Figure 12 The relationship between Metabolisable Energy (ME) and digestibility in forages (source FEEDTEST)

Impact of digestibility on feed intake

Digestibility of pasture not only determines the amount of nutrients that the animal can extract but also influences the speed at which the plant material is passed through the gut. Feed sources higher in digestibility are able to pass through the digestive system faster and this allows the animal to increase its daily intake. Conversely, low quality feed ‘clogs’ the system and slows the passage of feed through the animal. This explains why the availability of highly digestible feed is so important to animal production.

Energy intake for a 50kg dry ewe grazing a 1000 kg DM/ha FOO pasture varies from 2kg/day at 40% digestibility and 14MJ/day at 80% digestibility (Grazfeed®).
Figure 13 The energy intake for a 50kg dry ewe grazing a 1000kg DM/ha FOO pasture (source: Grazfeed®)

Digestibility of green feed versus dead feed

Green pasture will always be higher quality (60-85% digestibility) than dead herbage (35-60%) of the same species.

Pasture digestibility is also largely influenced by stage of growth. Pasture is of highest quality in the vegetative stage but declines rapidly after flowering.

Digestibility of different species

During the growing season, there are relatively small differences in digestibility between annual species, and as a general rule, green growing annual plants are highly digestible (70-80%).

Digestibility declines rapidly after senescence and plateaus at around 50%. The hotter the day-temperature the quicker the decline in quality, with many pastures only taking 30 days to reach 50%. In cooler areas, it may take up to 60 days to reach 50% digestibility.

Perennial species typically maintain higher quality into summer, depending on soil moisture and/or rain, with varying amounts of green plant matter present. Once dead however, the quality of the perennial sward can be as low as 35%.

Growing pasture has a digestibility of close to 80% which equates to 12MJ/kg of energy. By the time it is dry and stalks remain, it has a digestibility of less than 50% and an energy of 7MJ/kg.
Figure 14 Digestibility decline of annual grasses and clovers in a mediterranean climate

Contact information

Danny Roberts
+61 (0)8 9892 8535
Perry Dolling
+61 (0)8 9821 3261