The payments are set in Euros and then converted into sterling.
This year they will be worth 73p to the euro - that is 4p down on last year's rate.
In Northern Ireland it means about £236m pounds will be paid out this year - about £15m less than this time last year.
The Department of Agriculture has said it will do what it can to ensure the bulk of payments are made on time in December.
The Ulster Farmers Union (UFU) said the weaker euro meant farmers faced an "income plunge".
It said there had been a "gradual slide" since the 2013 rate of 83p.
And it said it would come as an added blow to farmers already facing cash flow problems due to weak prices for milk, lamb beef and grain.
UFU president Ian Marshall said it showed the extent to which farmers were subject to circumstances outside their own control.
"This makes it all the more important that the farm minister, Michelle O'Neill, and DARD officials ensure every effort is made to improve on the 93% of applicants that received their payments in December last year," he said.
"This is especially true this year, considering the number of sectors experiencing seriously reduced or even non-existent margins."
However, the news could have been much worse for farmers.
Had the rate been calculated at the start of August when the euro was worth about 70p, it would have knocked another £9m off the amount to be paid out.
Farmers have been told the exchange rate at which their EU subsidy payments will be paid this year.