A major search is continuing for James Corfield, 19, who has not been seen since he left the White Horse pub in Builth Wells, Powys, in the early hours of Tuesday morning.
He had been due to meet family at the showground where he was camping with friends but never arrived.
His mother Louise Corfield said they were "desperate to know where he is".
She said at a press conference on Thursday: "This is totally out of character for James.
"He is so passionate about his farm animals and he hasn't come home to them.
"He would never leave them unattended, which makes us even more worried about where he is.
"James is a gifted cricket player and is due to play cricket this Saturday and his team Montgomery Cricket Club need him home for this.
"We're grateful for the many volunteers who are helping to search for him but we still can't find him and need your help. Where is he?
"We love you James."
Mountain rescue teams with kayaks and boats resumed their search of the River Wye at 08:00 BST on Thursday but were stood down at about 20:30.
Mark Jones, of Brecon Mountain Rescue Service, said gardens, sheds and grass verges were being searched.
Police dogs, officials from young farmers' clubs and the fire service's rescue boat are also involved in the operation.
Dyfed-Powys Police said about 200 volunteers were helping with the search.
Visitors to the Royal Welsh Show were being asked over tannoy announcements to contact the police if they had any information on his whereabouts.
On Wednesday, his aunt Gill Corfield said the family were "beside themselves".
Supt Huw Meredith, of Dyfed-Powys Police, said the force was growing "increasingly concerned".
Mr Corfield is 6ft 2in tall, of slim build, with short brown hair and was last seen wearing a blue Abercrombie and Fitch shirt and jeans.
The mother of a teenager who went missing from the Royal Welsh Show has issued an emotional plea to find him.