The shotgun pellets entered Mr Guy's neck horizontally from the left, in an upwards direction.
They fractured part of his jaw, sending bone fragments into the surrounding tissue, and severed the carotid artery that supplies blood to the brain.
An x-ray showed pellets passed through the roof of his mouth, through the back of his eye socket and into his brain.
Some 258 to 260 shotgun pellets were found in Mr Guy's body. Plastic shotgun wadding was also found in his throat.
Dr Rutherford said pellet injuries were also found on Mr Guy's left hand and forearm, which could be explained by Mr Guy raising his arm to his face.
There were no other external injuries.
Under cross-examination from defence lawyer Greg King, Dr Rutherford said it was his opinion that all the injuries could be explained by one shotgun discharge.
However, he could not exclude two or even three discharges.
Dr Rutherford said there were no specific pellet patterns for him to distinguish how many shots were fired.
A post-mortem examination alone could not determine how many shots were fired, and other evidence was needed.
Dr Rutherford was also unable to say for certain how quickly Mr Guy died, but due to the damage to the carotid artery it would be "within a matter of seconds''.
There were no powder burns or contact wounds, which meant the killer was not extremely close.
"We're looking from many centimetres to a few metres, I would guess.''
ESR forensic scientist David Neale examined the murder scene on the day Mr Guy died and found pooled blood around the top of his body.
The blood had begun to flow into a footwear impression near Mr Guy's head and under part of the driveway gate.
A cap found in blood near Mr Guy's head had numerous areas of damage to its peak, possibly from a shotgun.
The court earlier heard evidence on the shoes worn by Mr Guy's killer, which have never been found.
The Crown alleges Macdonald wore size nine Proline dive boots with a distinctive wavy pattern when he ambushed Mr Guy at the bottom of his driveway as he left to do the morning milking.
Mr Neale said all the wavy-patterned footwear impressions at the murder scene had come from the same boot, which he concluded was a Proline W375 neoprene dive boot or similar.
He said the shoes worn at the scene were size nine or 10, as measured by the forefoot of the impressions.
But under cross-examination by Mr King, he agreed the size nine boot he had seen as an example could not have been the same as those worn at the scene.
Mr Neale said he counted about 32 to 33 rows of the wavy pattern on the forefoot of three partial footwear impressions at the scene.
But under instruction from Mr King, he counted about 29 rows on the forefoot of a size nine Proline boot produced as an exhibit in court.
Referring to the impressions at the crime scene, Mr King said: "If your calculation is accurate, there is no way in the world that it can be a pair of size nine Proline boots.''
Mr Neale agreed the size nine example boot he had seen did not have 32 to 33 rows of waves.
He said discounting the number of waves, the impressions at the scene matched a Proline boot, or a boot that closely matched it.
Raising his voice, Mr King asked: "Why would you discount the rows of waves?''
Mr Neale said the impressions at the scene were from a boot he had not seen, and may or may not have been from a Proline boot.
He said he had seen only one example of each size of boot.
Mr Neale agreed the boot could be from a "rogue batch'' or may not be from a Proline boot at all.
The trial before Justice Simon France and a jury of 11 continues tomorrow with more forensic evidence.
'Uneasy' about moving off farm
The court also heard today that Macdonald was uneasy with a suggestion that he or Mr Guy should move off the family farm to grow the business.
His wife Anna Macdonald, Mr Guy's younger sister, gave evidence in the High Court at Wellington today about a conference her mother and father, Bryan and Jo Guy, attended shortly before Mr Guy's death.
She said her parents attended the conference to find out more about running the farm and expanding the business.
Mrs Macdonald said both her parents returned from the conference "very excited'' and suggested a family get-together to discuss their ideas.
She said her husband was "a little concerned'' about the direction the farm was going "because the farm wasn't big enough for the three families to live quite comfortably''.
Mrs Macdonald said the farm could not get any bigger and the family needed to look at other avenues to earn money.
One of the ideas Bryan Guy came back with was for either Macdonald or Mr Guy to move onto a beef stock farm to grow revenue for the company.
Mrs Macdonald said she and her husband discussed the idea but they did not want to move. Macdonald said he loved dairy farming and wanted to stay.
"We decided that if we were asked we would say we wanted to stay on the Byreburn farm and run the dairy unit.''
Mrs Macdonald said she was worried herself because she did not like change and did not want to move.
"Ewen may have been a bit worried with what he heard.''
Mrs Macdonald asked her mother about the proposal but was told not to worry because there would be no changes until the family got together to discuss it.
"Mum and dad were not going to make a decision until we were all together and had talked about it.''
She told Macdonald not to panic and there would be a meeting soon.
"He seemed happy with that.''
Another idea up for discussion was to develop a lake on the farm with some type of adventure activity.
Macdonald brought it up and "rolled his eyes''.
"It just seemed a little far-fetched, maybe.''
Under cross-examination by defence lawyer Greg King, Mrs Macdonald agreed it was made clear that no decisions would be made without consensus.
Bryan Guy told the court there were six options up for discussion with the family.
Among them was for either he, his son Scott or Macdonald to manage another farm but "nothing really specific came out of it''.
Another option was to subdivide the farm because it was so close to Feilding township.
Mr Guy said "a really long-term pipe dream'' was to build a recreational lake - an idea Scott Guy was enthusiastic about.
He said the business was management-heavy and the farm needed to grow its business.
The idea was to use the farm's skills to manage another farm or to start a sharemilking venture.
The plan would not necessarily mean the families would have to relocate.
Mr Guy said the lake was not something they would embark on for five to 10 years.
He said he tried to alleviate Macdonald's fears over the future of milking on the property, and said in the short term nothing would change immediately on the farm.
The whole family was to get together in September to discuss goals and aspirations so they all had a shared and clear vision.
