Below you will find a three-part article on grazing ecology. Part 1 talks about the vital role of grazing in the food chain and total grazing pressure. Part 2 discusses how to assess grazing pressure in a pasture and the effects of overgrazing. Part 3 considers the choice herbivores make when they graze. There are also some references at the end for those interested in extra reading .
Grazing (Part 1)
In wondering what to write about this month, I was surprised to note that there was not a single article on grazing. This is rather odd given that I have studied the effects of grazing on vegetation for many years. Maybe I needed a break. Anyway, here is the first instalment of an intricate topic which is of great relevance to the land manager…..
The vital role of plants in the biosphere is that they are able to capture energy from the sun using the chemical process of photosynthesis. The conversion of carbon dioxide and water into carbohydrates using the sun’s radiant energy powers all life on earth.
Virtually all non-plants (with the exception of a few bacteria) derive their existence either directly (e.g. herbivores) or indirectly (e.g. carnivores) from green plants. Herbivores obtain their energy by chewing, swallowing and digesting plants. As capturers of solar energy, plants are primary producers. As consumers of plants, herbivores are a first step in the distribution of the sun’s energy to other life forms.
While herbivores can be found amongst insects, molluscs, reptiles, birds and mammals, it is a subset of herbivorous mammals that are the most strongly associated with the term ‘grazing’. These animals only eat plant material and have special teeth to slice and grind large quantities, and special bacteria to digest it.
When someone asks whether you graze your property or not, they are generally asking whether you keep livestock – sheep, cattle, goats, horses. However, a range of other herbivores (and omnivores) may also be contributing a great deal to grazing activity such as:
1) A huge array of insects including beetles, caterpillars, bugs and grasshoppers.
2) Some lizards e.g. stumpy-tails (shinglebacks) and blue-tongue lizards. I have seen stumpy tails happily chomp their way through my precious yams daisies in past springs.
3) Some birds: wood ducks are dedicated grazers, while parrots and cockatoos are more selective in digging up tubers and shoots. Even magpies and choughs eat some plant material and seeds.
4) Mammals: rabbits, hares, wallabies and kangaroos are serious eaters of vegetation. Pigs consume plants above and below ground, and do much rooting in soil for tubers, bulbs etc.
Livestock, together with all the animals listed above can contribute to what we call total grazing pressure. As far as land management is concerned, total grazing pressure is more important than the effects of any one species, or one group of animals. Of course there are times when one or a few species eat the most e.g. when there is a locust plague, when a pasture is heavily stocked, or when kangaroo populations build up.
Total grazing pressure indicates how much of the plant material is left for the ecosystem to function properly. Enough plants need to remain uneaten to: i) ensure they can regrow, ii) protect the soil from erosion, iii) break down into the soil to recycle nutrients and return soil carbon. In other words it is necessary to control grazing pressure to maintain a healthy ecosystem. This means that rainfall will infiltrate, soil will have good structure and nutrients will be available for regrowth of plants.
If any one species eats most of the edible plants, there will not be sufficient to support the range of other species and the future productivity of the pasture may be reduced. From a farming perspective, this may not seem to matter much – the cattle or sheep are all that appears to be needed on the landscape, and nutrients can be trucked in for a price.
Nonetheless, overgrazing can make it impossible to maintain the pasture you want, the soil may not absorb the rainfall effectively, and the wildlife you enjoy may not stay around. Unhappy times.
Grazing (Part 2)
Lawns and patchiness
Last month I talked about total grazing pressure – the sum of impacts from all the grazers – caterpillars to camels. I also suggested that if total grazing pressure was too high, it would affect the soil and pasture productivity and the wellbeing of some grassland-dependent wildlife.
At some point you may have come across the term carrying capacity – the number of Dry Sheep Equivalents (DSEs) that a particular landscape can safely carry. One DSE represents the amount of feed required by a two-year-old, 45 kg Merino sheep (a wether or non-lactating, non-pregnant ewe) to maintain its weight. There are many aids to calculate stocking rates that take into account matters such as type of livestock, reproductive stage, pasture type and climatic conditions.
While an estimate of carrying capacity is a useful starting point for guiding stock numbers, the complexities of weather and many other variables means that you cannot simply set and forget a stocking level. More than once I have heard pasture agronomists being very disparaging about using carrying capacity as gospel for fixing stocking rates, as it can lead to overgrazing.
Perhaps the condition of the livestock is a good guide? Certainly if animals are losing weight, alarm bells should be ringing. Unfortunately this is also risky. Grazing trials have shown that soil and plants suffer degradation well before the livestock start to lose weight. The conclusion from extensive research is that the best way to manage grazing, is to monitor the pasture itself. Regardless of what is eaten, it is the plants left behind that determine pasture health.
The upside of this, is that there is a simple rule that can be used to decide whether set-stocked pastures need some rest. So simple in fact, that a brief look at a pasture can give a reasonable assessment of its status.
The higher the grazing pressure, the hungrier the grazers and the less selective they are. Starving animals will eat pretty much anything. So non-selective grazing produces a lawn-like sward structure – short and even.
If there is only moderate grazing pressure, livestock can become choosier; the less tasty grasses are left to grow, and the delicious ones are nibbled constantly. They also like the new growth on areas that have been heavily grazed before, thus creating more or less permanent ‘grazing lawns’. This makes a pasture look patchy, with irregular areas of short, medium and tall grass.
If total grazing pressure is reduced even more, the sward becomes tall and more even, and it may not even look grazed – the grazers can now afford to be very fussy and eat only the best (an exception to this is when long-established grazing lawns are completely rested; they will not grow very tall as the only surviving plants will tend to be short-growing, grazing tolerant species).
Short grazing-lawns have been shown to have reduced water infiltration and nutrient cycling, due to the trampling and lack of deep roots and this leads to a loss of pasture condition. My own research with David Tongway recommended that a native pasture in good condition should have no more that 30% of short-grazed patches. This strikes a balance between overgrazing and underutilization.
Note that while livestock do not change their eating behaviour, these patterns may be different under rotational grazing, where heavy, non-selective grazing is alternated with long periods of resting. In this case, the short duration of the grazing is the key in allowing palatable plants to persist in the pasture. Rotational grazing aims for pasture evenness, with short and tall swards varying in time rather than in space.
 Well, anything except perhaps dead Joycea. In the 2004-5 drought, the kangaroos on Gang Gang ate hessian in preference to Joycea hay, thus proving that Joycea is the all-time best plant for erosion prevention.
Grazing (Part 3)
On palatability and preference
There is a tendency to regard cattle, and sheep even more, as rather dense creatures, especially when they do not want to go through that particular gate. But when it comes to choosing food, there is nothing cleverer than an ungulate. As I suggested in an earlier instalment of Indumentum, a starving herbivore will eat almost any plant material, even hessian. But given choice, these creatures are highly discriminating and only behave like a lawn mower when they are starving.
In the first instance, herbivores seek out the plants that are highest in protein and sugar, and lowest in fibre. While they are superbly adapted to digesting fibre, doing so has an energetic cost. Fresh growth of perennial plants and winter-growing annuals are attractively digestible. Some species are more nutritious than others. Forbs tend to be have more protein and minerals, and less fibre than perennial grasses. Some perennial grasses (e.g. wallaby grasses) are less fibrous than others (e.g. wire grass).
Regardless of the species variation, the nutrient content of all pasture species declines rapidly when the plants flower and set seed. At flowering time the plants need to withdraw their valuable nutrients from the leaves, and use them to build a nutritional package for the next generation – that is, seeds.
Unlike humans, herbivores are quite good at regulating their diet so that they eat enough fibre – which they need. Too much rich food is not good for herbivores – think of horses foundering in lush green pastures. In richly grassed fertilized pastures, trees can be killed by livestock chewing the bark to satisfy their fibre needs.
Plants go some way to protecting their precious leaves from grazing with special compounds called secondary metabolites. These can make leaves taste awful or even be toxic. Plant breeders have striven to remove secondary metabolites from the forage varieties they promote, but in doing so, they may affect the ability of plants to do a range of other things. Secondary metabolites can also assist in attracting pollinators, recovering from injury and resisting diseases.
Plant secondary compounds may also be therapeutic for herbivores and, eaten in small doses, can improve animal health, just as humans take medicines. Benefits that have been identified in animals include disease and parasite control, and even reduced emissions of the greenhouse gas methane.
The grazing preferences of an animal can thus change with:
- i) the types of plants on offer in a particular paddock
- ii) the time of year and growth stages of the plants
iii) past experiences of eating different types of plant
- iv) times of drought and times of plenty.
While grazers can detect what tastes good and can pick nutritious food accordingly, they can also learn from bad eating experiences in the same way we do. You may have heard accounts of animals poisoning themselves when put into a new paddock, in contrast to the herd that had been surviving just fine in the same paddock for years. The experienced animals had learned to avoid the poisonous species, or eat it in small quantities, or only eat it in a particular season.
It has also been suggested that young animals can learn what foods to seek and what to avoid by growing up in a herd already experienced in these matters. In other words, a culture of wise food choice can develop in stable groups.
Of course the fundamental requirement for livestock to regulate their diet in a beneficial way is the provision of a pasture that supports a diversity of plants. This includes species that might not seem desirable at first glance – that is, the fibrous ones and the nasty tasting ones as well as the delicious juicy ones. What does a diverse pasture contain? In my experience, it could have 100-300 species of grasses, herbs and shrubs for livestock to choose from.
For those that have read the August 2015 article on grazing (see the article at the bottom of this page), there are some papers available that describe the relationship between grazing, pasture patchiness, pasture condition and biodiversity. Please send me a message (see comment box below) if you would like copies send via email.
McIntyre, S. & Tongway, D. (2005) Grassland structure in native pastures – links to soil surface condition. Ecological Management and Restoration, 6, 43-50.
McIntyre, S. (2005) Biodiversity attributes of different sward structures in grazed grassland. Ecological Management and Restoration, 6, 71-73.
McIntyre, S, McIvor, J. G. & MacLeod, N. D. (2000) Principles for sustainable grazing in eucalypt woodlands: landscape-scale indicators and the search for thresholds. In: Management for sustainable ecosystems (eds P. Hale, A. Petrie, D. Moloney & P. Sattler) pp. 92-100. Centre for Conservation Biology: University of Queensland, Brisbane.
This book also discusses the topic in detail in non-technical language (along with many other topics) but I cannot send you a copy. It is available through CSIRO Publishing.
McIntyre, S., McIvor, J. G. & Heard, K. M. (2002) (eds) Managing and conserving grassy woodlands. CSIRO Publishing, Melbourne.