"I think he felt Ewen was trying to take over the farm.''
He recalled a conversation after Mr Guy's death in which Macdonald said police had asked him if he knew who had killed his brother-in-law.
"He sort of laughed about it, saying they were hopeless and had no idea. He was referring to the police and the investigation.''
Callum Guy did not take much notice of the comment at the time, but after Macdonald's arrest thought it was the kind of thing someone would say if they had "gotten away with it''.
Under cross-examination by defence lawyer Greg King, Callum Guy said he thought the comment was about how the police did not know too much.
"I didn't see anything sinister at the time.''
Nikki Guy said she had noticed differences between Macdonald and her brother after Mr Guy returned to the farm from living in Hawkes Bay.
"When Scott came back they were both equals but the farm wasn't big enough for both of them to have the same, equal roles,'' Ms Guy said.
She said there were also personality differences.
Mr Guy was all about the "big picture'' while Macdonald was more task-orientated.
Ms Guy said she thought her brother had been looking forward to moving on and working with Macdonald in the future.
"I noticed that the last time I talked to him he didn't say anything about Ewen at all.''
The Guy siblings' mother, Jo Guy, told the court Macdonald felt like he was being shut out by "wild plans'' to restructure the farm. She and her husband Bryan had attended a farm management conference in the days before Mr Guy's murder and returned with plans to develop a lake and residential housing on the farm.
"Scott got really excited about that,'' Mrs Guy said.
But Macdonald felt like the plans would put less emphasis on the dairy farming side of the business, of which he was manager.
Mrs Guy said her daughter Anna, who is married to Macdonald with four children, raised the issue with her after the conference.
"Anna had said to us, 'Well Ewen is a bit worried that the dairy farm side of the business would fall off'.''
She said Anna told her Macdonald was concerned with the plans.
Under cross-examination, Mrs Guy agreed that a list of goals drawn up after the conference were for the benefit of all the family, including Macdonald.
Mrs Guy said that during earlier talks about growing the farming business, Macdonald would not raise issues directly.
"Often he would go through Anna so she would fire up and say something, rather than, he wouldn't say it himself,'' Mrs Guy said.
Scott Guy was more direct, once raising issues through a letter that was "quite harsh'', she said.
He was also upset when he found out the Macdonalds were going to move into the home he had grown up in.
Mrs Guy and children Nikki and Callum all recalled a tense run-in between Mr Guy and Macdonald on a night that was supposed to be one of celebration.
Jo and Nikki Guy had thrown a party to mark the fifth birthday of their clothing shop in Palmerston North, after which the family was to celebrate with dinner at a cafe.
Ewen and Anna Macdonald were late because Ewen had to stay on at the farm, while Mr Guy knocked off early at the farm so he could make it on time.
When Macdonald accused Mr Guy of "skiving off'', he and wife Kylee left the dinner.
Nikki Guy said Macdonald did not seem concerned by that.
"He was just sitting there with his arms crossed and he didn't look too worried or too concerned that ... he'd ruined our dinner party.''
Jo Guy said her son had been angry but the incident had not come to blows.
"That would be unusual for Scott to react like that.''
Under cross-examination, she agreed the incident was not common.
"Maybe it was more undercurrents perhaps, and lack of communication.''
The court is this week hearing evidence about the relationship between Macdonald and Mr Guy, which is crucial in establishing a motive.
Mr Guy's father Bryan, sister Anna and wife Kylee will give evidence before Justice Simon France and a jury of 11 tomorrow.
