NativeForestManagement

Implications for grazing

By: Bill Schulke,

Extension Officer (sustainable beef production)

DPI&F, Bundaberg

Acknowledgement:

EDGENetwork Grazing Land Management

Introduction

Grazing lands as functioning ecosystems

Grazing ecosystem

Everything you manage on your property (and a fair bit of what you can’t manage) is part of an ecosystem. It includes: -

-Soils (different quality soils, soil microbes and invertebrates)

-Climate (Rainfall, variability, ENSO, drought)

-Plants (grasses, herbage, trees, weeds)

-Animals (livestock, native animals, feral animals)

-Community and society in general (resource management perceptions and expectations)

-And you (your need to make money and look after your land)

How efficiently this ecosystem converts sunlight and rainfall (through photosynthesis) into kilograms of beef, is partly due to how productive your land is (its inherent fertility), and partly due to three “gateways”; Land Condition, Pasture Utilisation and Feed Conversion Efficiency (see diagram).

Diagram 1. 3 Gateways model for grazing land management

Land condition

Land in poor condition will grow less than half the useful forage (given the same amount of rainfall) as when it is in good condition. Land is in good condition when it has a good coverage of 3P grasses (Palatable, Productive and Perennial), has few weeds, shows little sign of soil erosion or scalding and the woody weeds are in check.

Land that only grows half of its forage potential is not very efficient at converting sunlight and rainfall into forage, and hence, beef. It is probably not giving you a very good return on investment either.

Land condition can be maintained or improved through: -

-effective grazing management (safe utilisation rates, grazing systems, pasture spelling)

-strategic use of fire

-sown pasture development

-woody regrowth control

-weed management

Pasture Utilisation

The proportion of annual forage growth that is eaten by livestock is called the utilisation rate. A low utilisation rate means that the conversion from forage to beef is inefficient. A high utilisation rate may lead to short-term increases in animal production per hectare, but does so at the expense of individual animal performance. Continual high utilisation rates over several years will lead to a decrease in land condition and a consequent reduction in forage growth.

For native pastures in southeast Queensland, utilisation rates of between 20 and 30 % are considered to be sustainable. Sown pastures can be sustainably utilised at slightly higher rates of 30 to 40%.

The forage that is not eaten by livestock is not wasted. It plays a very important role in providing soil microbes (the unseen workers in a grazing ecosystem) with a source of energy and nutrients and provides ground cover that protects the soil surface from the damaging effects of sun, wind and rain.

The utilisation gateway is managed by matching forage availability to forage demand and by setting or adjusting stock numbers accordingly. Part of this equation includes allowing for other grazers (feral and native animals, termites), and ground cover. The aim is to always come out of a dry season (including a drought) with stubble on the ground.

Infrastructure (fencing and watering points), grazing systems and/or fire are all used to regulate when and where cattle graze.

Feed conversion efficiency

Feed conversion efficiency is a measure of how efficiently a beast converts forage into beef (or milk production). The class of animal and its genetics play a role but the import factor is the quality of the forage eaten.

Forage quality varies between forage species. Leafy grasses, such as the 3P grasses, are more digestible and higher in energy than stemmy grasses; legumes are higher in protein than grasses. Within a grass tussock there are differences too; leaf is better quality forage than stem.

The largest determining factor of forage quality is the stage of maturity. Fresh growth (such as just after the spring/summer break) is much higher in energy and protein than when it is mature (such as in July and August).

The grazing land ecosystem can be managed through this gateway by: -

-ensuring land is maintained in good condition (ie. the desirable plants are not grazed out),

-establishing high quality plants such as legumes in the pasture,

-setting appropriate stocking rates (high stocking rates force animals to eat the poor quality plants and the stemmy parts of the good quality ones)

-and by supplementing to overcome nutritional deficiencies when it’s economical to do so.

Grass / tree interactions

Why is the tree grass relationship important in the grazing industry?

All of the grazing land in the Burnett is or was woodland, the majority being eucalypt woodland.Woodland is a term used in grazing land ecology to describe vegetation having a mix of two distinct plant types: woody species (trees and shrubs) and herbaceous species (grasses and forbs). Where the woody species comprises commercial timber, these would usually be described as forest.

Most grazing properties in the Burnett derive the majority of their economic value from the herbaceous layer. Many properties with a mix of land types, some of which are quite productive in terms of timber production, derive significant income from the woody layer.

Despite the supplementary income derived from trees, most grazing enterprises have traditionally focused on enhancing the herbaceous layer through clearing and thinning. In the eucalypt forests, this has occurred on a cyclical basis through-out the last century.

Impacts of trees on grass

Trees can have both a positive and negative effect on the grassy layer, caused by: -

-rainfall interception

-shading

-root competition (for nutrients and moisture)

-microenvironment changes

-effects on soil condition (soil structure)

-nutrient cycling

Diagram 2. Impacts of trees on pasture

Trees intercept rainfall and store part of it in the canopy from where it evaporates. Some species can ‘funnel’ rainfall from the canopy, down the trunk onto the ground directly beneath the tree. The tree canopy can also alter the rain droplet size and the velocity at which it falls, which in turn can lessen the impact of heavy rain on soil surface condition.

The tree canopy captures sunlight and prevents it from reaching the grass layer. The proportion intercepted varies between species, canopy health, season and time of day (especially for the eucalypts and acacias). In the closed forest types (scrubs, rain forest, wet sclerophyll) grass is generally absent, but most land types used for grazing have a grassy understorey.

Trees can act as nutrient pumps where they draw nutrients from deeper in the soil profile (from where grasses can’t access them) and cycle them through leaf fall into the top soil. Tree canopies can also trap dust which is washed into the soil beneath them with rain. Often the soil biological activity is higher beneath trees and the subsequent soil structure benefits.

The net result is that pasture quality is often higher under trees compared with pasture growing in the absence of trees.

Competition

Despite the benefits of trees in terms of pasture quality the net effect of trees is to suppress pasture growth. This is due primarily to competition for soil moisture. The impact of this competition on pasture growth is related to tree density. A useful measure of tree density is tree basal area (TBA) which is the sum total of the cross sectional area of all tree stems measure at 30 cm from the ground. Foresters often use the term ‘stand basal area’ to describe the same measurement.

Generally pasture growth decreases with an increase in TBA. In drier forests, this tends to be a curvilinear response; in moister forests the response in more linear.

Graph 1. Impact of tree density on grass production

In the coastal Burnett, the major land types used for timber production are those with a high proportion of spotted gum. The best ‘dual purpose’ country is the ironbark and spotted gum on duplexes and loams (derived from granodiorites). Bluegum flats are usually the most productive in terms of cattle production, but also have good potential for timber production.

Graph 1 shows the impact of TBA on grass production for these three land types. The impact tends to be linear on the productive bluegum flats and curvilinear for the generally low producing spotted gum and wattle country.

Generally, fully clearing heavily timbered country leads to a three to four fold increase in pasture production. However, in many of the eucalypt forests, initial clearing costs and ongoing regrowth management renders such clearing as uneconomical. This is especially the case when the eucalypt forest has potential to produce millable hardwood and fencing timber.

NoteBroadscale clearing of vegetation mapped as remnant under the Vegetation Management Act, is now illegal in the absence of appropriate approval.

Other benefits of trees

Forests provide vital habitat for faunal biodiversity and are an important source of floristic biodiversity. In the coastal Burnett, as in the rest of the south east Queensland bioregion, the vast majority of remnant vegetation is eucalypt forest (much of which is on freehold land).

Trees also play an important role in the hydrological cycle within a landscape (diagram 3). Retaining deep rooted perennial plants, such as trees, reduces the risk of dryland salinity. Hazardous land types, such as those with saline sub-soils (such as those derived from marine sediments), or land types down slope of highly permeable soil types (such as deep red soils), can be protected by retaining trees in the recharge areas.

Diagram 3. Role of forests in preventing dryland salinity

Impact of thinning for forest production and the grazing enterprise

Optimal tree stocking densities

As mentioned already in the workshop, trees need space to grow. Cyclical harvesting and thinning of a stand is critical in optimising the productive potential of that stand. The optimal stocking densities for a range of tree size classes are given in table 1. However, in a native eucalypt forest all size classes are generally present. Depending on the relative fertility of the site and climate constraints, optimal timber production for a mixed stand with all tree diameter classes present, usually occurs when the tree stocking density is around 150 stems per ha with a total tree basal area of 12 to 15 m2/ha (table 2).

Table 1. Optimal tree stocking densities for individual tree size classes.

Tree diameter range (cm) / Spacing (m) / Stocking density (trees/ha) / Tree basal area (m2/ha)
10 - 20 / 5 – 7 / 200 / 3.5
20 - 30 / 7 – 10 / 100 / 5.0
30 - 40 / 10 – 12 / 70 / 7.0
>40 / 12 - 15 / 45 / 9.0

Table 2. Impact of tree stocking density on tree basal area for an optimally stocked forest and an overstocked forest; both with a mix of tree size classes.

Tree diameter range (cm) / Forest with optimal stocking / Overstocked forest
Stocking density (trees/ha) / Tree basal area (m2/ha) / Stocking density (trees/ha) / Tree basal area (m2/ha)
10 - 20 / 40 / 0.8 / 100 / 2.0
20 - 30 / 40 / 2.1 / 75 / 4.0
30 - 40 / 35 / 3.6 / 60 / 6.1
>40 / 35 / 5.8 / 45 / 9.2
150 / 12.3 / 280 / 21.3

For the main timber producing land types in the coastal Burnett (ie. those with spotted gum forest), a well managed stand with optimal stocking density, should yield about 1 m3/ha/year. In this situation, individual trees in the smaller diameter classes would grow at a rate of about 1cm DBH/year. The gross value would be between $80 and $100 /ha/year.

Post thinning pasture production

Following a harvest or thinning operation, the stand TBA will be reduced. This will lead to long term productivity gains in the forest, but will also result in a short term increase in grass production. Table 3 shows the impact of tree density (as measured by tree basal area) on grass production.

Thinning an overstocked stand to optimal stocking density may reduce the tree basal area from 24 m2/ha to 15 m2/ha. This would lead to an increase in pasture production of between 40% and 50%. A heavy thin and harvest that reduces TBA to 10 m2/ha could lead to and increase of pasture production of between 85% and 120%, the proportional increase being greater on the more fertile land types (such as the bluegum flats).

A significant regeneration of the forest usually follows a disturbance, and if left unmanaged can lead to a rapid increase in TBA. In these situations the increase in grass production following a harvest or thinning would be short lived.

Table 3. Impact of Tree basal area on grass production for three coastal Burnett landtypes.

Average annual grass production
(kg DM /ha)
TBA (m2/ha) / Ironbark and spotted gum - duplexes and loams / Spotted gum and wattles - sandy duplexes / Bluegum flats
0 / 4300 / 3300 / 5500
1 / 4100 / 3000 / 5300
2 / 3800 / 2700 / 5000
4 / 3200 / 2300 / 4500
6 / 2600 / 1900 / 4000
8 / 2200 / 1700 / 3600
10 / 1900 / 1500 / 3200
12 / 1600 / 1300 / 2800
15 / 1400 / 1200 / 2300
20 / 1100 / 900 / 1700
24 / 1000 / 800 / 1500

Changes in carrying capacity

An increase in pasture production leads directly to an increase in carrying capacity. The carrying capacity of a land type is based on a safe utilisation rate of between 30% and 35% of the average annual pasture growth, depending on the relative fertility of that land type. This means that a long term stocking rate will be set at a level such that only the desired proportion of annual grass growth is grazed. Generally stocking rates are based on an adult equivalent (AE = 450 kg dry beast) and expressed in hectares per adult equivalent (ha/AE). Table 4 and graph 2 show the impact tree density has on carrying capacity.

Table 4. Impact of Tree basal area on carrying capacity for three coastal Burnett land types.

* where :- IB&SG_dup,loam = ironbarks and spotted gum on duplexes and loams
- SG&W-duplexes= spotted gum and wattles on duplex soils

Reducing tree basal area as part of a harvest and/or thinning operation will lead to a short term increase in grass production. Although this increase in grass production may lift carrying capacities significantly, the effect will be short lived.

Graph 2. Impact of TBA on carrying capacity

Generally, the retained trees in the stand grow relatively quickly which leads to an increase in tree basal area. Further more, there is usually a significant regeneration event following disturbance in a forest (this is one of the aims of sustainable silvicultural practice). This regeneration event also increases tree basal area.

Where a forest is retained primarily for silvicultural purposes, the aim is to convert nutrients, water and sunlight into wood, not beef.

Roles for fire

Grazing

Fire is an important management tool for graziers especially in the grazed woodlands. Graziers can responsibly use fire to: -

-remove moribund growth (improve feed quality)

-even out patch grazing

-alter species composition (increase the proportion of desirable pasture species)

-reduce wildfire risks

-assist with low key establishment of legumes

-manage the tree – grass balance

In the coastal Burnett most native pastures are burnt on a regular basis. The frequency will vary with seasonal conditions but averages out at 2 to 3 years. Most graziers will burn at the end of the winter following 30 mm to 50 mm of rainfall (usually any time from the end of August). Compared with unburnt pastures, burnt pastures tend to be higher in protein and have more accessible green leaf during the growing season. Animal production is subsequently enhanced during this period.

Where the aimis to improve species composition (eg.remove wiregrass), fire should be used annually. Pastures should be spelled following the fire to enable the desirable species to establish and /or replenish stored root reserves.

Patch grazing is a desirable behaviour for grazing animals as it allows them to optimise nutrient intake; restricting it will reduce individual animal performance. However, persistent patch grazing over several years will reduce land condition in the grazed patches. Fire is a useful tool for evening out the patch grazing from one year to the next. The patches that are relatively ungrazed in one year will tend to burn well at the end of that dry season and become the grazed patches in the following growing season and vice versa.

Graph 3. Impact of tree height on susceptibility to fire

Fire is a useful tool in reducing eucalypt seedlings and a shrubby understorey. Many acacias are susceptible to frequent ‘cool’ fires. In some woodlands, a fire frequency of 3 years or less has been shown to break the life cycle of wattles. Eucalypt seedlings less than 1 m high are susceptible to fire. Larger saplings, though not killed by fire, can be damaged enough to reduce their competitive advantage over grass.

An effective fire to reduce tree and shrub competition may require fuel loads of greater than 2000 kg/ha. If the aim of the fire is to even out patch grazing or change species composition, fuels loads as low as 1000 kg/ha may be effective. Grazing management needs to be consistent with the planned fire regime to ensure sufficient fuel and post fire pasture growth.

Native forest management

Where woodlands are retained primarily for timber production, fire is still a useful tool but the fire regimes and management programs will differ slightly to those used primarily for grazing. Fire is an important tool for seed germination following a harvest, and for managing a shrubby understorey. A post harvest fire also removes residue (such as tree heads, log dumps etc.) left over from the harvest operation itself.

The frequency of fire in the dry forests (of which spotted gum forest is one type), is a little longer than that for grazing and averages out at 2 to 5 years.