Download Grazing Management Part 1

Growing More Grass
By Col Freeman
Everyone knows that rain makes the grass grow. I drove through the countryside in south east
Queensland about two weeks after some recent drought-breaking rain and it was great to see how the
country had greened up. But not all the grass had grown equally: on the roadside verge it was tall and
showed new growth, some paddocks had new growth of about 15 to 20 centimetres, but in some
paddocks the grass had not grown at all but had simply changed colour from yellow to green.
These differences are often noticed and remarked upon by graziers, and the explanation for the
different responses to the same amount of rainfall lies in the way that grass grows and how it is
managed as a pasture.
In the first of 2 articles on this topic we will look at some fundamental aspects of how grass grows, and
in the next edition a second article will discuss how you can manage your pastures to be sure that your
grasses can make the most of any useful rain that falls.
4 factors in how grass grows.
1
Grasses are solar panels:
The energy from the sun which drives all life on earth is captured primarily by green plants, such as
grass. Solar energy is converted by plants into chemical energy which is used for plant growth and
reproduction and becomes available to animals through eating the plants. During this process, called
photosynthesis, plants use carbon and oxygen from carbon dioxide in the atmosphere and hydrogen
from water in the soil to produce carbon compounds with stored chemical energy (carbohydrates) and
oxygen.
You may remember the equation from high school science classes, but the important fact for graziers is
that the “solar panels” which capture the energy from the sun are the leaves of the grasses. A bigger
area of leaves means more energy can be captured to grow more grass and feed more stock. At least up
to a point: once the leaves begin to overshadow each other the advantage is lost.
It is common for people to assess the level of groundcover in a paddock, with 70% cover often being
recommended as a minimum level to reduce the risk of soil erosion. But when groundcover is looked at
as “solar panel surface area” then there can be scope to capture more solar energy by increasing
groundcover.
3 x groundcover
pics
2
Grasses are Carbon Pumps and Carbon Traders:
We tend to think of grasses as growing by extracting the water and nutrients they need from the soil to
produce their growth. In fact, there is more of the plant (its biomass) under the ground than there is in
the green shoots on top. The roots of a grass plant typically constitute the majority (up to 80 percent has
been estimated) of its total mass, and there is currently a lot of interest in the ability of grasses to store
carbon in the soil as a means of reducing the climate change impacts of the greenhouse effect.
Rather than extracting nutrients from the soil, plants actually provide carbon (and the energy within the
bonds of the carbon compounds) through their root systems to feed a complex web of micro-organisms
such as bacteria, nematodes and protozoa which live around the plant root system. These microorganisms trade nitrogen and other nutrients to the plant for the carbon they receive.
The grass plants provide carbon to the micro-organisms in several ways. As the grasses die or are
trampled their litter provides organic matter to the soil. But the roots themselves provide carbon
directly to the organisms though shedding of the outer layers of the roots and abrasion as the roots
penetrate the soil.
Most importantly, grass plants also exude a range of sugary carbon-based substances to attract and feed
a specific range of micro-organisms that live in close association with the roots. The carbon substances
stimulate the growth of bacteria and these are in turn a food source for nematodes and protozoa so that
the cycling and availability of nutrients is rapidly increased.
This trading of carbon for nutrients is limited, like the growth of leaves, by the rate of amount of carbon
and energy which can be captured by the leaves from the atmosphere and sunlight.
Roots and Shoots: the below-ground
mass of the grass plant is greater than
its above-ground mass.
3
Grasses are driven to Reproduce:
Grasses can reproduce by tillering (for example by growth of rhizomes or stolons as occurs with creeping
or sod-forming grasses) and by producing seed. In the case of seed production, grass stems which had
been producing leaves change their nature to become the flower stalk. Tillering and the onset of flower
formation each occur at about the 3 and 3.5 leaf stage of growth.
Grazing by animals is a threat to the grass plant’s ability to set seed and reproduce, and grasses have
developed mechanisms to reduce this threat. Some grasses make grazing unattractive to animals
through having spines or being poisonous or unpalatable – but these are not popular pasture species for
obvious reasons.
The common pasture species have developed mechanisms to tolerate and compensate for the loss of
leaf through grazing which interrupts plant growth and photosynthesis. Carbon from remaining leaves
and shoots is re-allocated by the plant to leaves which have been eaten to allow for maximum regrowth. This continues until there is sufficient leaf growth for the new leaf to act as its own solar panel
and find its own carbon and energy through photosynthesis.
Grasses also compensate for leaf loss through the death of some of the roots, and this organic matter
provides the food for a burst of micro-organism activity which results in an increase in nitrogen available
for plant growth. Small increases also occur in the rate of growth of new leaves compared to old leaves,
and leaves grow longer on grazed plants than un-grazed plants.
3 to 3.5 leaf stage
4
Grasses foster the conditions they require for growth:
Grass growth requires sunlight to provide the energy for growth, and is encouraged or limited by the
availability of water and nutrients. The activity of plant growth and decay provides soil organic matter
which directly affects the availability of water and nutrients in linked and interacting ways.
The growth and decay of the root systems of grass determine the availability of water and nutrients in
the soil. Root systems enhance water penetration and the capacity of the soil to hold water through 1)
creating channels for rainfall to enter and move within the soil 2) providing the ‘gums’ which allow soil
particles to join together and maintain spaces between the particles for water to move, and 3) providing
organic matter which has a high water holding capacity. The leafy part of grasses traps rain which falls,
and slows the movement of water allowing more to enter the soil.
The organic matter provided by grass root systems also provides the food source for all of the activity of
micro-organisms which provide (or make available) the nutrients for plant growth. The pastures which
respond to rain are those which have sufficient soil organic matter to allow rainfall to penetrate the soil
where it can be a trigger for the increased activity of soil micro-organisms to provide a flush of nutrients
for plant growth.
You can get a feel for how well your pastures are likely to respond to rainfall by simply digging a few
holes in different places around your property: in different paddocks; under some trees and out in the
open in the same paddock; in the roadside verge; and under a fenceline. Look for ease of digging (easy is
better than hard), changes of colour (darker is better than light); smell (more like compost is better than
less) and the colour of roots (white is better than brown).
Pic of good soil and pasture roots
Paradoxically, the way to grow more grass is to let more grass grow. If grasses are allowed to grow they
will foster the conditions required to maximize their growth. Grazing management can enhance or
restrict this ability of grasses to maximize growth. The second article will look at how graziers are
managing to maximize grass growth.