The motive for the murder, the Crown says, was Macdonald's unhappiness at the small amount of work he felt Mr Guy was doing on the farm, and his fear that his role on the property could be jeopardised by Mr Guy's assertion that he expected to inherit it from his father.
Macdonald has pleaded not guilty to the charge.
Farm consultant Simon Redmond, who had the Guys as clients, said Macdonald and Mr Guy were friends but the friendship soured, probably when Mr Guy took on more work, and they began to drift apart.
Mr Redmond considered Macdonald to be the superior farmer, saying he was "streets ahead" of Mr Guy and could have worked anywhere. He was not sure Mr Guy would find work as easily.
Mr Redmond read to the court a copy of his statement to police, in which he said Macdonald told him Mr Guy was "braver than most" - although in cross-examination he said he did not link this to Mr Guy's death and it could have been about his rodeo riding.
He also said Macdonald had told him that shotguns weren't traceable, although the wadding might be.
Farm worker Simon Asplin said Macdonald would plan his days but occasionally Mr Guy would arrive and tell them he needed a staff member to help with a task, such as moving stock.
"We would have to down tools to help Scott."
Mr Asplin said workers wondered "what he did some days" but said that when Mr Guy did work, he worked very hard.
He tried not to get involved in differences between Mr Guy and Macdonald, "but sometimes I did; I felt like the man in the middle".
He believed there was a lack of communication between the two men, and issues weren't resolved.
Andrew Short told the jury he began working on the farm part-time during high school, and noticed a competitive nature to the relationship between the two men.
"If one would get a tattoo, the other would get a tattoo; if one got a new car, the other got a new car," he said.
"Scott and Ewen had competition between each other ... competition to be better."
Mr Short described the relationship between the two as "hot and cold".
"Scott sometimes had his own opinion about Ewen, but they still worked together."
One occasion he could recall was when he asked Macdonald where Mr Guy was and he replied, "Probably skiving off" - but he had heard similar comments from both of them.
Jackson Gilbert, who worked on the farm part-time and later full-time for six months, said there was rivalry between Macdonald and Mr Guy.
"I could definitely tell there was a bit of rivalry between them ... just comments that were said from Ewen sometimes, not very often."
He remembered one incident when Mr Guy and his brother Callum came off the farm because it was too wet, causing Macdonald to call Mr Guy "cowboy" and joke about him being farmer of the year.
Despite that, he told defence lawyer Greg King, he considered Macdonald the best boss he had had and the work environment was a fun, happy one.
