Farmers in 11 counties in the south, midlands and east will be taking to the fields today to spread slurry, the first day this is legal under new EU environmental law.
Millions of gallons of slurry have been building up since last October which was the last time farmers could spread manure from their cattle on fields.
The rules have been put in place under the EU Nitrates Directive to protect the nation's water quality and to ensure slurry and manure do not enter watercourses.
The country is divided into three zones - Carlow, Cork, Dublin, Kildare, Kilkenny, Laois, Offaly, Tipperary, Waterford, Wexford and Wicklow farmers may spread slurry from today.
Farmers in Clare, Galway, Kerry, Limerick, Longford, Louth, Mayo, Meath, Roscommon, Sligo and Westmeath will have to wait until Tuesday.
It will be January 31st before slurry can be spread in Donegal, Leitrim, Cavan and Monaghan where soil types, rainfall and other conditions make it more dangerous to spread slurry.
The new rules were criticised before Christmas by Irish Cattle and Sheepfarmers Association president Malcolm Thompson.
He said it was "EU bureaucratic nonsense" that farmers were not allowed to spread slurry at that time as conditions were ideal.
It had not rained for weeks and farmers could get into their fields and spread without causing any environmental damage, he said.
A Department of Agriculture spokesman said farmers must take into account weather conditions even during the open season and its inspectors would be monitoring the new season activities carefully.