Lakeland Dairies has moved to raise its milk price for last month, but all the other major processors have held returns at November levels.
Lakelands will pay 30.5c/l including VAT (29c/l excluding VAT) for December supplies, an increase of 0.5c/l.
However, Glanbia, Kerry and Dairygold all said that they are holding at 30c/l including VAT.
Centenary and Town of Monaghan are also staying on 30c/l including VAT, while Arrabawn is staying at 30.5c/l including VAT.
The Lakeland Dairies move has been welcomed by both the ICMSA and the IFA.
The IFA's Kevin Kiersey said Lakeland Dairies had taken a first step to pass back improved market returns to farmers.
He called on all co-ops, in the context of rapidly rising on-farm input costs, to examine the scope for an end-of-year top-up on last year's milk and to start passing benefits of the significant market improvements of recent weeks back to farmers.
The ICMSA's Pat McCormack called on co-ops which had yet to pay an end-of-year bonus to do so as soon as possible.
There was surprise across the industry that the Irish Dairy Board (IDB) failed to raise returns to co-ops for last month's supplies -- returns for butter were cut by €150/t and skim milk powder (SMP) by €100/t in November.
"We calculate that gross spot returns for Dutch food-grade SMP and butter this week, before processing costs, exceed 37c/l -- an improvement of over 3c/l since late November," Mr Kiersey said. "With fast rising input prices, we estimate that, to maintain margins, dairy farmers will need an average 2011 price at least 10pc (3c/l) higher than the 2010 average."