The Farmers Union of Wales (FUW) has called on more farmers to claim compensation for losses as a result of military low flying in the countryside.
The MOD says it tries to keep low flying to a minimum
The union claims hundreds of farmers are missing out because they are not aware of their right to claim for injuries to frightened animals.
The Ministry of Defence (MOD) was "surprised" at the union's comments.
It argues that low-flying training is kept to a minimum but was essential for an effective air force.
In 2005, there were 19 claims for compensation, worth £40,000, from Wales.
Peter Davies, the FUW's director of administration said: "Our research work suggests that many farmers are losing money as a result of animals aborting or injuring themselves as a result of the jets.
"It appears that some farmers are just not aware that they can claim for compensation." He said he based his claim on his work as regional secretary for the FUW in Carmarthenshire over a number of years.
"I know of one member who has had over £20,000 over the past 15 years. It's obvious that those planes went over other farms as well - and probably caused problems there, " he added.
Low flying - below 2,000 feet - is allowed in theory over the whole of Wales.
But mid Wales is one of only three areas in Britain when flying as low as 100ft is also permitted.
The Ministry of Defence argues that this type of training is an essential skill for an efficient and effective air force.
But John James, from Ty Llwyd farm in Felingwmuchaf, near Nantgaredig, Carmarthenshire, knows as well as anyone what effect the low flying can have on man and animal.
He has made 20 successful claims for compensation over a number of years.
"It's about 10% of the aircraft which have an effect on the animals - cows aborting, animals becoming very agitated and frightened - breaking their legs as they try to jump hedges.
"This has happened every year for the past 25 years," said Mr James who keeps a herd of pedigree Welsh Black cattle.
He says that the noise also had an adverse effect on the children when they were small.
The MoD deals with hundreds of complaints annually - more than 430 in Wales last year.
But they resulted in only 19 claims for compensation, worth £40,000.
Squadron Leader Peter Sinclair deals with the complaints from his office in Welshpool, Powys.
He said he was "surprised" by the FUW's comments about compensation.
"We do try to keep low flying to the minimum required to complete the exercise and any complaints are fully investigated - and claims are also fully investigated.
"It may be an awareness problem but I would urge anyone who has any complaint to get in touch with us."
Mr James says he cannot afford not to claim.
"Every farmer has to make his own decision but I certainly can't afford to lose thousands of pounds every year. And it's not just the animals but it's the effect on the family's health as well."
The FUW accepts the need for some low flying in principle but it is also adamant that farmers should be more aware of their right to claim compensation.