Edible shrubs supply more reliable carbon flows
When carbon moves around a grazing paddock, above and below ground, nitrogen goes for the ride. The amount of nitrogen partnering carbon at any point in time is known as the carbon:nitrogen ratio. The issue for producers is that nitrogen sometimes chooses to part company with carbon before it moves into sheep and cattle. This happens as grass matures and dries out.
Pasture connoiseurs know what they are looking for in a good balanced meal.
However, this is not a problem with edible shrubs such as old man saltbush or fodder trees like leucaena. Their nitrogen (protein) content over summer is much more constant because they have the ability to remain green. In the case of saltbush, it grows in both summer and winter and is not affected by frosts, so maintains its nitrogen/protein year round.
Edible shrubs and fodder trees have the ability to draw on deep moisture not available to the grasses.
Increasing the pathways by which carbon can flow into the paddock
The four plant types livestock rely on for energy and protein -annuals, perennial grasses, perennial edible shrubs and fodder trees.
There are different pathways by which carbon enters the paddock.
At one extreme, we have the fast growing annuals with shallow roots that utilise the surface moisture.
At the other extreme, are perennial edible shrubs and fodder trees that transfer the use of rain further into the future. They can grow under adverse conditions. They maintain carbon flows over time because of their deep roots sourcing moisture deeper in the landscape.
Leucaena has the added advantage of directly introducing some nitrogen into the surrounding soil.
The importance of “green pick” (green leaf)
Animal production is very sensitive to small increases in green pick from herbs, grasses, palatable shrubs and fodder trees. A little goes along way. Small increases of “green pick”, when it is in short supply, can “double production”. (Source: CSIRO Rangelands Series Sheet No 7.)
The graph above demonstrates how wool production increases from one to three kilogram per head with a very small increase in “average supply of green leaf”. At first glance, the production curve appears to go straight up then to the right. In fact, it is leaning a little to the right as it rises quickly. This indicates that production is increasing quickly while supply of green leaf is only increasing marginally.
Perennial edible shrubs and fodder trees are both important sources of “green pick” which is critical for production in dry times.
Sheep and cattle select to maximise “green pick”
Proportions of green grass, dry grass, forbs and shrubs in the diet of steers grazing between November 1977 and May 1978, in Alice Springs, as seasonal conditions varied.(Source: Squires and Siebert, 1983.)
In the graph, the consumption of shrubs (shown at the top of the graph) is going up and down depending on the availability of green grass.
When cattle couldn’t source “green pick” from green grasses, they sourced it from shrubs.
Note how the shrub consumption decreases briefly in April when there is a short-term increase in green grasses.
In dry times, the rumen microbes in sheep and cattle rely on shrubs and fodder trees to supply protein/nitrogen for them to build their little bodies. Then they can break down poor quality (low protein/nitrogen) grass and empty the rumen quicker.
The grass is drying off while the planted old man saltbush is still full of protein/nitrogen and producing carbon flows as it keeps growing.
Using old man saltbush plantations for resting pastures after rain
In the mid 1990’s, the Federal Department of Agriculture became aware that I was suggesting that old man saltbush (OMSB) plantations could be used as somewhere to put livestock, to allow resting of pastures for a short period after rain – the time when the bulk of the carbon flows into the paddock. I was subsequently funded to conduct a $272,000 Drought Regional Initiative project to perfect the use of OMSB for this role.
The drought resistance of OMSB means it is always available for this role, especially important when an isolated fall of rain arrives during dry times or when rain arrives at the end of a drought, when pastures are bare.
Seeing OMSB plantations as a management tool for resting pastures after rain is a paradigm shift for those who see it solely as a drought reserve. A case of using it in the mud and not the dust.
Nitrogen content drives digestibility and unless the digestibility of the feed can be maintained above about 50%, then methane emissions skyrocket.
Edible shrubs and fodder trees speed up the flow rate of pasture (carbon) through the rumen of livestock in dry times, when the nitrogen content of standing grass is low. They increase the flow rate by changing the carbon:nitrogen of the total diet, which lowers methane emissions per kg of production.
With the cattle at Alice Springs, if the shrubs had not been available during the times when they were eating them, methane emissions per kg of production would have really risen.
Australia has the most variable climate in the world which appears to be becoming even more variable. Reducing the effect of this variability is the best way to reduce methane emissions. This is where edible shrubs and fodder trees fit in.
Resting pastures after rain, increases pasture resilience. Resilient pastures are greener over time, which is another way to reduce the effect of a variable climate.
Edible shrubs and fodder trees hold nitrogen with carbon longer. If present, they are the only spot where adequate nitrogen is held in the pasture when grass has lost its nitrogen.
They also transfer the use of rain further into the future i.e. they generate carbon flows when it is not raining.
This discussion is a bit like the relay runner passing the baton i.e. passing the baton to the next species responsible for running nitrogen through your sheep and cattle. Your race is over if there is nowhere to pass the baton.