The sheer number of sick or injured racing dogs is putting a strain on animal welfare organisations, according to a greyhound protection group.
Greyhound Action said most are forced to retire by the age of three and about 15,000 dogs disappear each year.
Less than one tenth of greyhounds which retire from racing are found a home, according to the group.
The welfare of greyhounds has been placed in the spotlight by the case of David Smith who killed racing dogs with a bolt gun and buried them at his home.
The builders' merchant was fined £2,000 by a court on Friday after he admitted disposing of the greyhounds without a permit.
Spokeswoman Jan Lakey said the case was a "terrible way" to bring the problem to public attention.
"People who dispose of dogs in the way that David Smith did are not welcome in the sport"
She said: "Some trainers have up to 100 dogs and if lots of them fall ill or become injured that puts a lot of strain on organisations dedicated to looking after sick animals.
"For those that are ignoring the problem, ask yourself how many greyhounds you've met. How many old greyhounds do you see?"
The RSPCA is also calling on the government to reduce the number of greyhounds bred for racing.
The charity hopes that the new Animal Welfare Act, which comes into effect in England on 6 April, will lead to tighter regulation of the industry.
Spokeswoman Becky Hawkes said: "There is simply no justification for killing racing greyhounds just because they can't do their job anymore - it is shocking and unnecessary.
"An animal should be the owner's responsibility for life, not just until they are no longer useful."
The British Greyhound Racing Board (BGRB) disputed that 15,000 animals disappear each year and said it spent £4m annually on dog welfare.
Welfare officer Peter Laurie said: "We were disgusted by what happened at Seaham and there is no evidence that this is happening anywhere else in the country.
"10,000 racing greyhounds were registered with the National Greyhound Racing Club (NGRC) in 2006 and in the same year the Retired Greyhound Trust re-housed almost 4,000.
"Other organisations, like the Battersea Dogs Home, take them in and some are looked after by owners and trainers.
'Safer tracks'
"People who dispose of dogs in the way that David Smith did are not welcome in the sport."
He added that the board was working hard to make tracks safer and was training vets in specialist treatment methods to ensure that dogs can race for longer and live healthier lives.
An NGRC spokesman confirmed that approximately 4,000 greyhounds were retired and re-homed in 2006.
But the spokesman said that some dogs go on to race at tracks unregistered by the NGRC, which the club has no control over.
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