27/11/2018

Understanding catchment management issues

Soil moisture is gradually increasing following the summer drought and we approach that time of year when arable farmers have to consider when to apply propyzamide to control black-grass in oilseed rape. This is the herbicide that is considered to be the most effective at controlling this major arable weed, but timing is everything. The temperature needs to be falling and the ground needs to be wet, but not too wet or there is an enhanced risk of runoff to water, mainly associated with eroded soil particles. Once in water, the herbicide readily exceeds the 0.1µg/L limit for drinking water and is difficult for water companies to remove. While the 0.1µg/L limit is contested on the basis of lacking scientific evidence for negative impacts on the environment or human health, the fact remains that this is the legal limit for drinking water supply.

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There are linkages here between farmer objectives for productive and profitable crop production, water company objectives for reducing pesticides in water, and environmental objectives for the conservation of aquatic wildlife affected by sediment.  Compacted, poorly functioning soils have a negative impact on each of these.  Because sediment clogs drainage channels downstream, there are also implications for flood risk management.  The herbicide is at the centre of a web of interacting issues.

By using four years of herbicide concentration data from the study catchments in our Water Friendly Farming project, together with rainfall, stream flow, and crop area data, our partners at York University could estimate that, to keep the herbicide concentration below the 0. 1µg/L limit, we would need to restrict the oilseed rape area to less than 5% of the catchment area.  Additional or complementary mitigation options include extending the rotation with vigorous hybrid barley which can compete against black-grass, reducing tillage intensity, and monitoring soil moisture and compaction to guide herbicide applications and soil management to reduce runoff.

We put these options to the farmers in one of our study catchments.  In some years the oilseed rape area is around 5% of the catchment, but in others it is up to 30%.  It depends which part of each farm is within the catchment boundary in any one year.  How could land use be coordinated so that the 5% limit was not exceeded? The consensus was that it could not!  Such a restriction would impose too heavily on the economics of the farm business and create tensions between neighbouring farms who do not normally work together.  Multiple tenure arrangements between neighbouring farms complicate the issue, and as small farms are increasingly managed by contractors, this restricts long-term planning and makes timely operations more difficult.  There was also concern about the economic risks associated with adopting a no-tillage approach to reduce erosion on clay soils, and a lack of long-term government support for more sustainable soil management.

Introducing hybrid barley into the rotation was well received as a means of extending the rotation and controlling black-grass, a principle that has additional potential benefits for pest and disease control, and for farmland wildlife.  There was also interest amongst the farmers in improving understanding of compaction with a view to carefully targeted management that would reduce erosion and improve crop performance.  And while farmers were reluctant to collaborate, they agreed that there was an important coordination role for local trusted advisors.

Because of its adoption of rigorous science in a landscape scale practical setting, the Water Friendly Farming project provides an excellent platform for understanding constraints and opportunities for developing future land use policy and practice.  As a result of this recent exercise, we all have a better understanding of how a wide range of objectives interact, where the constraints are, and what potential opportunities we could explore to meet the multiple objectives we all have for agricultural landscapes.

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