Howard Simmerson, 43, is claimed to have arrived at the remote farm with the body of Julie Turner hidden in an oil drum on the back of his Ford Ranger pick-up truck.
His intention had been to bury the drum but during the three-hour visit he had sex with 20-year-old Charlotte Melland in her father's shed, Sheffield Crown Court heard.
Two days earlier he allegedly called Miss Melland and asked her to lie about his movements 48 hours hence - the night that 40-year-old Miss Turner was shot in the head with a revolver in Simmerson's Mercedes.
Miss Melland told the hearing that she was in love with Simmerson, a man she had known since she was a child through his deliveries to the farm from his gas bottle distribution company.
She explained the relationship between them had developed a few weeks before Miss Turner's death and they discussed having sex through text messages. Miss Melland, who has a one-year-old son, admitted they made love for the first time in a shed at her father's farm at Flagg, near Bakewell, Derbys.
She said they had not had sex before because Simmerson told her he would not do it "whilst he was with Julie".
He told her Miss Turner, a mother-of-two, from Sothal, Sheffield, would not leave her common-law husband Darren Akers for him and they were not getting on well.
Simmerson then went to see her father to ask if he could bury the oil drum on their land, a request which was refused.
Miss Melland told the court: "Howard said there were some guns in it and the guns had been involved in a shooting, that was all."
She admitted that she had lied to the police at first to cover up for Simmerson because she was "scared". He had told her to telephone him on the night of the alleged murder, making it appear as if he was at home watching television.
His request came in an hour-long telephone conversation she had with Simmerson two days before the alleged murder in which he said he had fallen out with Miss Turner. She said he told her he would be going to Birmingham to pick something up and if he got caught he would "get sent down".
He also asked her to telephone him at about 10pm on the evening of the alleged murder and again at midnight, with both calls lasting an hour, Miss Melland said.
"If the police became involved he said I should tell them he was at home all night watching television," she added.
Miss Melland's father, Christopher, said Simmerson had turned up at his farm with the truck and asked if he could dispose of the oil drum there.
He told the court: "He said he had some rubbish to bury."
Simmerson later told him there were two guns in the drum, which had been welded shut, but he would not open it for inspection.
Miss Turner's body was discovered in the drum six weeks later after it had been left at the premises of a car valeting company.
Simmerson had lavished gifts worth £180,000 on her during their four-year affair, including buying three designer handbags worth £2,800 on the night she died.
The jury has heard that he initially told police he had dropped her off near her home at her request on the way home from a promotional night at the Harvey Nichols store in Leeds.
He later claimed she was shot in the right side of the head in the passenger seat of his Mercedes with the gun in her own hand after she pulled it on him and "jabbed" it in his face. An Italian Derringer double-barrel pistol was later found at his business premises.
Simmerson, of Creswell, Derbys, denies murdering Miss Turner in June this year.