Dairy co-operative First Milk, which has a large cheese-processing plant in Haverfordwest, Pembrokeshire, is trying to address the situation with all 700 of its farmers across the UK - including 200 in west Wales.

The group has recently worked out the nutrient footprint of each farm - closely scrutinising use of bought-in feed and fertiliser.

This data is used alongside new maps, produced with the help of satellite imagery, showing the risk of causing river pollution on a field by field basis.

Farmers will be expected to adjust where they spread fertiliser and slurry accordingly, and introduce other mitigation measures to improve soil health and protect rivers.

They will have to report annually on their progress.

"It's obviously looking to protect watercourses and reduce run off from the fields, but it's also trying to prevent excess nutrients being brought on farm," said area manager Sarah Bennett.

"It's a missing metric for responsible farming," added Lee Truelove, First Milk's director of regenerative farming.