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The role of weeds in a balanced pasture
By David Mason-Jones
It may be time for a bit of counter intuitive thinking about the role of weeds in a healthy pasture. Obviously poisonous weeds may have no place because they defeat the whole reason for the pasture existing in the first place - but what about the inedible weed plants that do not harm any stock?
Our intuition says that weeds are bad; that they are taking up space which could be filled by grass; that they are competing with grass for water; and also nutrients. Intuition says that weeds diminish the grazing resource and should be eradicated whenever and wherever they pop up.
Caution, however, is needed in taking the intuitive approach. That is the way that many people once thought about trees as well - "If there's a tree growing there, there can't be any grass growing there!" Have you ever heard that one? It sounds sensible in theory but is quite wrong in reality. Research has now proven that land productivity actually goes up when a proportion of the ground is replanted with trees.
Can our intuition with weeds be wrong? What is the case for allowing the presence of weeds in a balanced pasture?
By thinking against your intuition it is possible to realise that the very facts that you may have once despised about weeds may be the very features which are most valuable in your pasture.
Roots of weeds and grasses compared.
Generally speaking the roots of most edible grasses are shallow in comparison to the overall soil profile. As a stand of grazing pasture gets taller, its roots go deeper. When a stand of pasture is grazed, the below ground portion of the plant, the roots, withers to a shallower penetration of the soil to support the smaller area of leaf above the ground. Strongly grazed grasslands - particularly in set stocking arrangements - are therefore characterised by a shallow penetration of the grassland roots into the soil. This means that shallow rooted grasses can only access shallow nutrient sources and shallow water reserves.
Weeds, however, generally have a much deeper reach into the soil profile with their root systems. This means that deeper-rooted weeds can access deeper nutrient sources and deeper sources of water.
The conclusion from both these facts is that the root profiles of weeds and grasses are not necessarily in competition because they are drawing resources from different sections of the soil profile.
Solubility of nutrients.
Many of the nutrients in the soil profile are soluble. This means that, after heavy rainfall where water soaks into the ground and down through the soil profile, there is a danger that the water can dissolve nutrients and remove them from the reach of the shallow rooted plants. After many successive years of rain and nutrient leaching, the total nutrient availability to the shallow rooted grasses gets less and less. It is for this reason that there is a view that a shallow rooted mono-culture grassland is unsustainable because it will always tend to degrade into a desert.
The function of a deep rooted weed in this situation is to continually reach down deep into the soil profile and recover the nutrients that are leaching away from the surface. The weed brings these nutrients to the surface and re-deposits them on the ground in the form of mulch when the leaves fall off. If there is a grazing animal which will eat the weed, then the process of conversion of the nutrients brought to the surface by the weeds will be faster than through the process of mulching.
Fact of being inedible.
Most so-called weeds are edible to some form of animal somewhere. However, let us assume for the purposes of this article, that weeds are the plants which grazing stock such as cattle will not eat. With the old thinking about weeds this led to the conclusion that weeds were just a waste of space and a drag on the grazing resource.
With the new thinking about weeds it is possible to see that their inedible nature is actually one of the factors that guarantees the performance of their nutrient recovery role. Inedibility guarantees the weed's survival. If weeds were edible, then the grazing animals would be constantly eating away their foliage and the root depth of the weeds would die back in the same way as the shallow rooted grasses die back when grazed. Being inedible means that their roots can continue reaching below the depth of the grass roots at the very time that the grass roots are under pressure from the grazing herd.
Soil Binding.
The deep roots of the weeds mean that soil is bound together more effectively than if there were no weeds. This assists in holding soil profiles together against wind and water erosion and becomes particularly important when paddocks are bare - as often happens at the end of drought. Droughts often break with heavy rains and the presence of weeds can help hold the land together.
Covering bare ground.
With poor grazing manage-ment and at the end of events like droughts, large areas of bare ground open up, totally devoid of any grass or vegetation cover. These are potential disaster areas and start-points for major erosion events. Effectively, they are areas which have been lost to your farm management and are totally - in their present form - incapable of producing grass.
Weeds are hardy 'pioneer' species and will be the plants which quickly establish on these bare ground areas. This is the best way of getting some form of vegetation cover reestablished. Once the cover is reestablished - albeit with 'weeds' - the owner of the farm can then manage the stand of weeds to replace it slowly with a mixed stand of weeds and grass and, finally, a mainly grassed area. It really makes no sense to spray a stand of weeds which has established on previously bare ground - all you will be doing is to restore the bare ground.
Biodiversity.
Like it or not, the presence of weeds represents an increase in biodiversity and a reduction of weeds represents a decrease in biodiversity. Highly biodiverse environments are generally regarded as better for the health of the soil than environments of poor biodiversity.
All the reasons why weeds contribute to the health of a balanced pasture are not to say that they should be allowed to predominate - that is not healthy either. It is also not to say that poisonous weeds should be allowed to survive.
A re-think of the role of weeds in a balanced pasture can lead to an improvement of the productivity of the soil and the health of the grasses. In order to carry out their role of recovering leached nutrients from deep in the soil profile, weeds need to be present in pasture and spread throughout grasslands. Healthy pasture does not mean that the weeds are the main plant variety - in a grazing operation edible grasses have to be. But anyone who wants to see the very last weed eradicated may not be building the best pasture possible.
