Farmers have brought in emergency water tankers
Many say the grass they feed their cows effectively stopped growing weeks ago, prompting concerns about the food available to herds in the longer term.
The weeks of scorching weather follow heavy snow storms in late winter.
NFU Cymru has also warned farmers that they have "a duty of care" to animals, as forecasts predict two more weeks of soaring temperatures.
The warm weather reached a record high of 32.6C in Porthmadog in Gwynedd at the end of June and the heatwave has continued this week.
NFU Cymru President John Davies said he had heard farmers were already using their winter supplies of fodder because the lack of rain had stunted grass growth while the "intense" heat was "burning off crops".
Mr Davies said: "We've heard of farmers who are already using their winter supplies of fodder to feed livestock and with fodder already at a premium this could become a real problem later in the year."
The union has now relaunched its free fodder bank service to help members find animal feed or sell any surplus.
Comment: The present UK heatwave has already caused the earliest harvest in 40 years for farmers, with a significantly reduced yield. The heatwave seems to be extending towards the end of July, with the Met Office predicting that "the last third of July, dry, sunny and warm weather will probably dominate across the bulk of UK, especially in the south."