So what's the problem?
The last few decades have seen an unprecedented decline in the number of Britain’s farmland birds. The losses have been particularly prevalent in the western regions of Britain where pastoral farming (livestock grazed grass pastures) predominates. It is thought that the intensification of land management, such as heavier grazing and more frequent mowing, is reducing the availability of seeds and invertebrates and these are highly important in the diets of many farmland birds.
Studies have shown that pastures with a higher diversity of both plant structures and species support a greater variety of invertebrates which can, in turn, potentially support higher numbers of farmland birds. Conversely, the intensive management of grassland pastures often results in fewer plant species which leads to poor variety in the physical structure of vegetation. Ultimately this affects the number, and possibly the variety, of birds it can support. Compounding the problem, mowing machinery can cause the loss of nests and chicks of ground-nesting species.
What is this project doing?
In order to overcome these issues and to try and reverse the decline, Defra is funding a project aimed at modifying grassland management systems. This will promote a more varied vegetation structure in order to support greater numbers of invertebrates and birds. As opposed to a heavily grazed, bowling green-like pasture that lacks variety, the project aims to encourage short, medium and tall structures, such as grass leaves, stems and flowers, which offer a wider variety of niches for various invertebrates, all within the same pasture.
Our aims need to deliver substantial benefits for biodiversity (particularly invertebrates for foraging farmland birds) and the guidelines need to be simple, practical and low cost for farmers to implement.
CABI is working alongside the Royal Society for the Protection of Birds and ADAS on this project which is divided into three main studies. Throughout the project we will be testing lenient cattle grazing and early cessation of grazing, which should be widely applicable to livestock farmers across the UK.
Grazing grassland more leniently, leaves more vegetation in situ for longer and this allows a range of invertebrates to develop and reproduce. However, a tall dense pasture can inhibit some invertebrate groups (that need warmth from sunlight or bare ground for laying eggs) and can prevent birds accessing invertebrate prey. Reduced density of livestock encourages cattle to graze more selectively producing grassland with a mixture of tall and short patches that should maximise the benefits for invertebrates and foraging birds.
Results so far
It is known that the availability of invertebrates is key to the reproductive success of many farmland birds, and that grazing can influence invertebrate abundance. So far, after four years of research, we saw that lenient early season grazing increased invertebrate abundance by up to 71%. This abundance was linked to the increase in vegetation height created by lenient grazing compared to conventional intensive grazing. However, lenient grazing did not increase structural heterogeneity which may limit access to this resource by farmland birds.
Although lenient grazing had a large positive impact on invertebrate abundance, the vegetation was too tall and dense to promote bird foraging and costs to farmers were relatively high. The grazing treatments being tested in this study have been developed to meet the requirements of priority farmland birds (like yellowhammers and cirl buntings), while minimising costs and complexity for farmers.
However, whilst the invertebrate response to lenient grazing was encouraging, there were two serious drawbacks to it. First, costs were high because of the loss of the late season grazing and second, there was a marked deterioration in the quality of the vegetation.
We therefore want to test whether a vegetation height of 9-12cm enhances the abundance of invertebrates and birds. We will trial both continuous and intermittent grazing (where periods of lenient grazing are alternated with rest periods). Grazing intermittently will hopefully allow grasses to seed, and should fit readily into rotational grazing systems still common in the UK. We won’t apply inorganic fertilizer to either.
Our proposed treatments have the potential to provide a variety of wider environmental benefits. These include reduced emissions of greenhouse gasses (especially nitrous oxides through the cessation of inorganic fertilizer application), increased carbon sequestration (as a consequence of reduced stocking rates, improved water quality (as a consequence of reduced soil compaction and improved water infiltration through the soil). This way of managing pastures will also increase pollination facilities and the resilience of biodiversity to climate change impacts because of increased availability of invertebrate-rich habitat patches in the landscape.
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