Better grazing
It is now generally accepted that rotational grazing systems, based upon grazing and rest periods determined by the rate and stage of plant growth and stocking rates related to available pasture, are essential to maximise pasture production over time. Increases in stocking rate and production of 100 to 200% are commonly reported (eg Norton 2002) from a range of continents and landscapes.
The strategies behind rotational grazing are successful because they work with the natural processes involved in grass growth, soil health and plant responses to defoliation. In a systems sense, they increase the magnitude and/or efficiency of flows of energy, water and nutrients. In a practical sense, the control required to enable stock to be moved at particular stages of pasture use and returned only after sufficient pasture recovery, can only be achieved by managed rotation of stock. A bio-economic analysis of this is provided by Marcus Sounness:
LINK
These natural processes in grass growth and soil health are difficult to describe because they involve complex interactions, feedback from one process to another, and variations between particular species, soils, grazing events and weather.
Some of the main processes and mechanisms are described below in terms of grass growth, followed by a brief discussion of some aspects of animal foraging behaviour.
The Ecology of Pasture Growth
Grasses as solar panels
The amount of energy from the sun which becomes available for plant growth, and to power the micro- and macro- animal food chains, depends upon the area of green leaf available to capture sunlight. In general, leaf area can increase capture of sunlight until the area of leaf reaches to about 4 times the area of the land on which it occurs (ie 4 square metres of leaf area per 1 square metre of land). After this, further growth leads to shading of existing leaf, and so no increase in total sunlight capture.
Grasses as carbon pumps
There is a mutually beneficial relationship between the roots of grass plants and the soil organisms which live in the narrow zone within 1 mm around the roots (called the rhizosphere). Plants release carbon compounds such as sugars which feed the rhizosphere organisms (eg bacteria, nematodes, fungi, protozoa and mites), and these organisms release nutrients (eg nitrogen and phosphorus) which can be absorbed by the grass roots.
The ratio of below-ground plant biomass to above-ground plant biomass may be as much as 2:1. So a pasture which is allowed to build up 10 tonnes/ha of above ground biomass (ie herbage) may have 20 tonnes/ha of below-ground biomass, while a pasture allowed to build up only 5 tonnes/ha above-ground biomass will have only 10 tonnes/ha of below-ground biomass. If this below-ground biomass is seen as the food store for soil biological activity, then there is twice as much potential for biological activity, and so twice the potential for nutrient availability and cycling.
Picture of grasses from Dr Christine Jones
The photo above demonstrates the relationship between above- and below-ground growth – more grass means more roots.
The importance of the biological release and cycling of nutrients from soil organic matter is considerable, since it is the primary source of nutrients to plants, including nitrogen, phosphorus and sulphur.
The microbial biomass in the soil is the driving force of most terrestrial ecosystems, including grasslands, because this biomass largely controls the release and cycling of nutrients (Killham 1994).
See Soil Foodweb Institute:
LINK
http://www.soilfoodweb.com.au/
See Soil Biology Movies:
LINK
www.agron.iastate.edu/~loynachan/mov/
See a good explanation of soil biology in agriculture from Adelaide University:
LINK
http://www.ento.csiro.au/pdfs/brochures/LifeinSoil.pdf#search=%22crcslm%2Cwaite%22
Grass grows sigmoidally
Pic of sigmoid growth
Grass growth increases exponentially with time, until the plant begins to approach maturity and flowering.
Image from www.newfarm.org
www.newfarm.org/grazingtall/images/graph.jpg
The relationship between above-gound and below ground growth is shown conceptually in the diagram below Root growth and carbon (organic matter) deposition increase with leaf growth.
Grass responds to grazing
The growth points of grass plants are away from the leaf tips and remain below the reach of grazing animals when the shoot is in the vegetative (ie non-flowering) stage. This growth structure makes grasses well adapted to defoliation by large herbivores such as sheep and cattle, since grazing animals can remove portions of a grass leaf without stopping the growth of the shoot.
In addition, grazing of grasses at the third-leaf stage and prior to the flowering stage can increase subsequent growth by reducing the production of hormones within the plant which inhibit leaf growth. Rather than progressing to the flowering stage there is an increase of vegetative growth. Grazing management which removes only up to 30% of the leaf material when grasses are between the third-leaf and flowering stages can provide an increase of 30-45% in total herbage production. (Manske, 2004)
Microbes interact with roots
Manske says:
The mutually beneficial relationship between grass plants and the soil organisms that live in the area around the roots of grass plants is described by Manske (2004). The area immediately around the roots of grass plants is called the rhizosphere and contains bacteria, protozoa, fungi, nematodes, mites and springtails. The roots of the grass plants release carbon compounds (such as sugars) to be used by these micro-organisms, and the micro-organisms release nutrients (eg mineral nitrogen and phosphorus) for direct use by the plants.
Soil micro-organisms convert organic nitrogen to mineral nitrogen i.e. the form that plants can use. The activity of the soil micro-organisms increases with the availability of carbon compounds in the rhizosphere, and increased micro-organism activity results in increased nutrient availability to the grass plant.
Grazing of the lead tillers between the third-leaf stage and the flowering stage can increase the amount of carbon compounds the defoliated plant releases into the rhizosphere. The increase in mineral nitrogen made available by increased rates of micro-organism activity allows the plant to accelerate growth and recover more quickly from defoliation.
LINK
http://www.ag.ndsu.nodak.edu/dickinso/research/2003/range03a.htm
In Australia the Soil Foodweb Institute has played a role in providing knowledge about the micro-organism activities in local soils.
LINK
Grass growth improves soil health
Grass growth improves soil health in a number of inter-related ways. Firstly, grass growth provides a source of humus through deposition of litter on the soil surface and the deposition of organic matter within the soil from root growth and decay.
Dr Robert Pettit describes how humus functions to improve the soil’s water holding capacity. The most important function of humic substances within the soil is their ability to hold water. From a quantitative standpoint water is the most important substance derived by plants from the soil.
Humic substances help create a desirable soil structure that facilitates water infiltration and helps hold water within the root zone. Because of their large surface area and internal electrical charges, humic substances function as water sponges. These sponge like substances have the ability to hold seven times their volume in water, a greater water holding capacity than clay. Water stored within the top-soil, when needed, provides a medium for nutrients required by soil organisms and plant roots.
LINK
http://humusandcarbon.blogspot.com
Water is commonly a limiting factor in plant growth. The amount of extra water that can be captured and held for plant use in a soil through increasing levels of organic carbon is significant. Christine Jones has provided some calculations:
Assuming a soil bulk density of 1.2g/cm3, a topsoil depth of 30 cms, and that organic carbon holds 4 times its weight in water, Christine concludes that an increase from a soil organic carbon level of 1% to a soil organic carbon level of 2% would make available an extra 144,000 litres of water per hectare.
Once captured in the soil this water would be used for plant growth; so there is the potential for this amount of additional water to be available for plant growth several times per year if there is follow-up rainfall.
LINK
http://soilcarbonwater.blogspot.com
The desirable soil structure required for improved water infiltration in pasture soils can be enhanced through rotational grazing techniques, as shown in the graphs and explanatory notes below (which have been sourced from Neil Southorn and Stephen Cattle: The dynamics of soil quality in livestock grazing systems)
Figure 1. Average topsoil (0-100 mm depth) macroporosity under alternative grazing tactics with sheep over 3 years. Data points not shown for clarity; lines are those of best fit. Least significant difference is within any year at 5% (SS ≡ set stocked, HI-SD ≡ high intensity short duration rotational grazing, C≡ ungrazed control, CA ≡ grazed over pasture cages).
Under the same treatments shown in Figure 1, larger macroporosity was measured not only at the soil surface, but to a depth of at least 100 mm (Figure 2). At this depth, significantly larger macropores were observed under rotational grazing and pasture cages compared to set stocked grazing. This indicates that the mechanisms generating macroporosity operate throughout the potential depth of influence of compaction.
Figure 2. Average soil macroporosity at 100 mm depth under alternative grazing tactics with sheep. Presentation and treatment details as for Figure 1.
Grass plants grow best when not too much leaf material is removed by grazing
Grass plants are damaged when too much leaf material is removed by grazing. And long grass is easier for cattle to eat.
The quantity of carbohydrates stored in the plant during the winter ‘hardening’ process is related to the amount of active leaf material at the end of summer and early autumn. Removing too much leaf during late summer and autumn will reduce the potential for plant growth in the following spring.
Soil ecology
“Nitrogen is the most abundant element in our atmosphere. It is a vital element since compounds essential to living systems are nitrogen-containing compounds (a necessary element in the composition of proteins, nucleic acids and other major cellular components). Nitrogen is a primary nutrient for all green plants, but it must be modified before it can be readily utilized by most living systems.”
“It is one of nature's great ironies, however, that most life forms, including all plants and animals, are unable to enlist dinitrogen gas (N2), which comprises 80 percent of the atmosphere, in their life-sustaining biochemical processes. Plants are able to use nitrogen in the form of nitrate (NO3-) or ammonia (NH4+), but these compounds are present in limited supply in the soil and are easily lost by leaching and by biological reduction of NO3-(denitrification). Because crop plants generally require relatively large amounts of nitrogen for growth, it frequently becomes the limiting soil nutrient for plant growth.”
90% of all Nitrogen within a soil is to be found in the soil’s organic matter.
For further explanation of the Nitrogen cycle and various aspects of soil biology go to:
LINK
http://www.soils.umn.edu/academics/classes/soil2125/doc/s9chap1.htm
The number of microbes growing in the root area varies with the stage of growth of the plant. The maximal number of bacteria is observed during the period of the most active growth of the plant. The more intense are the life processes, the more organic substances are excreted by the root, and the more intense the multiplication of microbes in the rhizosphere.
Observations show that an abundant growth of microorganisms takes place in the early stages of the plant's growth; however, the most vigorous growth of microbes ensues during the period of flowering and in the period directly preceding it. Sometimes one also observes three elevations in the growth curve of microbes: the first small elevation is seen in the early stage; a second great elevation ensues before and during flowering; the third elevation occurs before ripening. The last is usually barely noticeable.
LINK
http://www.soilandhealth.org/01aglibrary/010112Krasil/010112krasil.ptIV.2.html
Implications for grazing management
We tend to think of plants as growing by extracting the water and nutrients that they need from the soil. While this is true, it is only half the story. Plants don’t simply take what they need: they trade energy and carbon for water and nutrients.
The amount of sunlight captured by plants represents the total amount of energy available for use by animals and micro-organisms within the agro-ecosystem. Primary production (plant growth/area/time) determines the limits to secondary production (animal growth/area/time).
In a pasture between 60-90% of primary production occurs below ground in root systems, and up to 15% of above-ground production may be consumed by insects and small mammals.
He relationship between above-ground growth and below-ground growth is shown conceptually in the diagram below. Since grass growth is exponential until maturity, root growth (or the production of soil organic matter) is also exponential.
Not all of the remaining above-ground production will be consumed by stock, as some will inevitably remain as litter.
“The fundamental ecological dilemma encountered in grazing systems is the inability to simultaneously optimise the interception and conversion of solar energy into primary production and the efficient harvest of primary production by stock. Severe grazing ensures that available production is efficiently harvested, but eventually reduces production by minimising the capture of solar energy. Alternatively, lenient grazing maximises primary production, but a large percentage of the production is incorporated into the decomposer compartment without being consumed by stock.”
“The grazing intensity which maximises animal production per unit area over a long time, is that which optimises the processes of solar energy capture, harvest efficiency and conversion efficiency”. So management strategies must maximise both the magnitude and efficiency of energy capture and flow if they are to result in maximum stock production. Such management strategies must be very flexible. (Briske & Heitschmidt)
So an understanding of how grass grows and how its growth is affected by grazing (sometimes called herbivory) is crucial to designing management strategies.
Properly timed grazing can enhance the mutually beneficial relationship between rhizosphere soil organisms and the roots of grasses. Biologically Effective Grazing Management (Manske) involves timed grazing that removes only a small portion of the leaf material (i.e. 10-33%) on about 80% of the plants in a pasture where grasses are between the third leaf and flowering stages, and activates the beneficial biological processes that result in improved health of the pasture and a 30-45% increase in herbage production.
These improvements in herbage production from grazing management which follows plant growth requirements have been demonstrated by researchers in Australia (Norton 2002) and America (Manske 2004) and the experience of graziers in many areas (eg Armidale, NSW: Wright; Western Australia: Sounness 2005).
The Biologically Effective Grazing Management described above requires a measure of control, planning and timing of grazing by stock which can only be achieved by planned rotational grazing based upon the critical factors:
- Number of stock (and hence feed demand)
- Amount of feed available per unit area
- The growth stage and rate of growth of the grasses in the pasture
Explanations of the methods and procedures required to maximise the benefits of pasture management are provided in Meat & Livestock Australia’s More Beef from Pastures program. The MLA’s The Producers Manual is available:
Call MLA on 1800 675 717 or email publications@mla.com.au to order a hard copy or CD-ROM version of the MLA More Beef from Pastures - The producer's manual.
- Hard copy folder plus CD-ROM - $65 MLA members, $130 non-members
- CD-ROM only - $10 MLA members, $20 non-members
- Download PDFs - free
see also for tips on grazing management Manske’s Grazing Handbook:
LINK
http://www.chaps2000.com/grazing.asp
Several grazing management training courses are also available:
ProGraze:
LINK
http://www.dpi.nsw.gov.au/agriculture/profarm/courses/pastures-and-rangelands/prograze-plus
Holistic Management:
LINK
http://www.holisticmanagement.org/new_site_05/CE/CE3_ce_listings.html
Grazing for Profit:
LINK
Monitoring soil and pasture condition
There are a range of tools available for monitoring soil and pasture condition and health.
The United States Department of Agriculture has a Soil Health Test Kit which is currently being modified for Australian conditions by Peter Grace at Queensland University of Technology.
LINK
http://soils.usda.gov/sqi/assessment/test_kit.html
Land & Water Australia are conducting a Healthy Soils for Australian Farms Knowledge Base Project. The project began in 2006 and will provide resources and information on soil health to farmers and advisors.
LINK
http://www.lwa.gov.au/Research/Research_Programmes/Healthy_Soils/index.aspx
The Farm Sustainability Indicators Kit (FSIK) provides some useful and simple assessments of soil and pasture condition. The FSIK is available from Hunter Region Landcare Network for people wishing to undertake baseline assessments of the condition of their properties, or establish long term monitoring sites within their property to track changes over time as a result of management .
(LINK)
Useful monitoring exercises (from the FSIK) can be done quickly and simply:
Rainfall infiltration assessment:
Infiltration rate is the rate at which soil can accept water from rainfall. It is important because it determines how much rain which falls on your land can be captured for use by plants.
The rate at which water enters the soil is a function of soil texture, structure and pore spaces. Water enters the soil from rainfall through cracks, channels created by organisms and pore spaces between soil mineral particles.
The best time to do this assessment is around 2 to 3 days after good rains, when the soil has some moisture in it. (A dry soil may cause water to bead, while a saturated soil will not accept any more water)
Clear the ground of long grass and hammer a 200 mm piece of plastic sewer pipe or a steel cylinder (from 150 mm to 300 mm diameter) about 40 mm into the soil.
Place a texta mark near the top of the cylinder, and measure the distance (in mm) from the texta mark to the ground.
Pour water steadily into the cylinder, and when up to the texta mark, begin timing with a watch.
Note the time it takes for the water level to reach the ground.
The infiltration rate is the number of millimetres per unit of time. For example, if the texta mark is 160 mm above the ground, and it takes 45 minutes for the water level to reach the ground; then the infiltration rate is 160 mm per 45 minutes. (to convert this to mm/hr, divide by 45 then multiply by 60 : the answer in this example is 213 mm/hour).
Height to the texta mark X 60 = infiltration rate (mm/hr)
Number of minutes
Soil biological activity assessment
Soil biological activity refers to the role of macro-organisms (ie ones you can see, such as worms, insects and ants) and micro-organisms (ie ones you cannot see, such as bacteria, fungi, algae and protozoa) in the soil. The level of biological activity reflects the extent to which all the conditions for healthy plant growth are present in the soil.
The cotton assessment for soil biological activity gives an indication of the number of micro-organisms active in the soil. Soil biological activity changes with soil temperature and soil moisture. There is more biological activity when soils are warm and moist, than when soils are dry and/or cold.
The cotton strip assessment is therefore most useful for comparisons when soils temperature and moisture are similar. This may be either soil from the same site taken when conditions are similar (eg between years, with samples taken at the same time of year and after some rainfall) or between soils from different sites under the same conditions (eg same time of year and similar rainfall).
- Take 10 pieces of cotton strip about 400mm long and 25 to 50 mm wide They could be white cotton from a roll used in dressmaking, or strips of unbleached calico – it is just important that they are all the same.
- Select 2 sites that you want to compare.
- Push a square-nosed spade into the ground to a depth of 100 mm. Wiggle the spade to loosen it so that you can pull it out without disturbing the ground too much, and remove the spade leaving a vertical slot in the ground.
1. Fold a cotton strip in half equally over the end of the spade.
2. Push the spade and cotton strip into the slot in the ground, to the depth of 100 mm.
Remove the spade to leave the cotton strip in the ground, with some cotton strip protruding above ground, and tamp the soil around the cotton strip with your boots to ensure good contact between cotton strip and soil.
Place another 4 cotton strips nearby, within 1 metre. Repeat this procedure at the second site.
After 4 days remove one cotton strip from each site and note the extent of discolouration of the cotton strip. In a soil with lots of biological activity you may find that the cotton strip has rotted below ground.
If there was significant discolouration, then return after another 4 days to remove another cotton strip to see if it has rotted.
If you had very little discolouration after the first 4 days, then return weekly to remove the second and later strips.
The time taken for the cotton strips to rot is the measure of biological activity. Local experience suggests that rotting after 4 days is common in warm, moist soils with lots of biological activity, while in soils with low biological activity the strips may be largely unaffected after even 40 days under similar soil temperature and moisture conditions.
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