The union said growers in the South West were facing a disaster because supermarkets were paying around 18p per head of cauliflower, but it costs 28.6p to produce.
NFU horticulture board chairman Richard Hirst said around half the growers in Cornwall met with the NFU last week and expressed "great concern about where this is all going".
"This threatens the long-term viability and sustainability of the vegetable industry in this country," he said. "Growers can no longer afford to grow crops at a loss."
He added that the current glut in vegetables had been caused by last summer's rains, which delayed planting and harvesting. He would not name specific supermarkets but said all the main chains were beating down pay rates on crops.
The NFU told the meeting of 49 growers it would amass more details on costs and production before deciding on a plan of action.
Hirst added: "But we will talk to the retailers and highlight the problems and long-term implications. If people stop growing brassicas, where will the supply come from? If retailers are happy importing, that's fine. But in reliability terms I suspect they don't want that, especially when you look at the volatility in Kenya and elsewhere."
Brassica Growers' Association (BGA) chairman Phillip Effingham warned that the draw of more solid returns from cereal production may prove too much, as growers could switch from the "hassle and uncertainty" of brassicas.
He added: "I think there will be a natural drift from brassicas, especially from small and medium-sized growers, which is a tragedy because they are the very heart of the industry in terms of quality.
"In fairness, what can the supermarkets do? If they raise their prices it will depress the market and lower the amount sold. It's the survival of the fittest and economics must prevail."
BGA vice chairman Alistair Ewan forecast a "mammoth" reduction in brassica production in 2009.
He said cereals like wheat and barley had almost trebled in price, while brassicas were high risk but low-paying.
"There's no point going on and on about it - growers have to do something and take action. Maybe it's like the dairy industry, where so many moved into other areas."