The issue was brought to stark attention this week when local UFU member Raymond Douglas lost two cows which had become stuck in sewage that has built up along the river bank.
The UFU has raised the issue with Northern Ireland Water and the Environment and Heritage Service and now intends inviting Environment Minister Arlene Foster and Regional Development Minister Conor Murphy to see the problem for themselves.
UFU Deputy President Harry Sinclair, who visited the River this week, said: "What has happened to Raymond Douglas is totally unnacceptable. He has lost two cows in horrific circumstances. We will be holding Northern Ireland water to account for this. We are very annoyed that NI Water have issued a statement trying to absolve thenmselves from responsibility for what has happened.
"I was at the river and saw the sewage problem with my own eyes. The state of the river is an absolute disgrace and we will be raising this at the highest level. If a farmer was responsible for pollution like this he would be prosecuted immediately.
"The UFU won't tolerate double standards and who ever is responsible for this sewage problem needs to be held to account."
Harry Sinclair added: "The resolution we want is the river cleaned, the source of the pollution fixed, and the farmer compensated for his losses. The UFU members who farm along the Closet River deserve to be treated fairly and we will be fighting their case strongly. This is also an environmentally important area; for example the field is a lapwing breeding site and to see pollution like this is a disgrace. This issue will be a real test for the new Northern Ireland Environment agency which is soon to be launched."