A showing show had radio equipment and a generator stolen the night before it opened on Sunday (10 May).
The White Rose County Show, run by BSPS Area 3B, is held in Temple Newsam Park in Leeds.
Co-organiser Jane Cooper was puzzled to see tea, coffee and a carrot cake had been stolen from the marquee when she arrived first thing on Sunday morning.
The thefts became more serious when commentator Simon Richardson opened up the locked commentators’ box.
Several walkie-talkies and a generator had disappeared, although there were no signs of a break-in.
The theft was “inconvenient” said Mrs Cooper, but as Mr Richardson could see all the rings from the commentary box it did not hamper communications on the day.
Further thefts were reported by the catering vans despite the manager staying on site overnight.
The BSPS has been involved with the White Rose County Show for 12 years and “never before had any problems,” said Mrs Cooper.
Co-organiser Naomi Bradwell said the thefts have been reported to the police and did not make any difference to the success of the show.
“We had a fabulous day with takings well up in the first couple of hours,” she told H&H.
“We had people on site all night and did all we could to make sure everything was safe and still got burgled.”
This is not an isolated incident, with thefts occurring at shows across the country.
In March an event rider had her purse stolen from her locked lorry at Oasby Horse Trials.
Felicity Baker-Attew, 24, was competing in an intermediate section with The Wedding Horse.
When she returned to her lorry she found someone had broken in through the jockey door and stolen the contents of her purse.