He told prosecutor, John Elvidge KC, cross-examining: “It was a tragic accident. I made a mistake.”
Mr Speakman agreed he had an “obligation” to ensure Albie was kept apart from moving vehicles.
“I checked every possible blind spot. I checked he was not there. I couldn’t have checked more,” he said.
Asked why he had not put Albie inside the farmhouse, he said: “It was 33 degrees.
“He wanted to play outside with his dogs. It was his family home.”
He said Albie “knew dangers” on the farm, but added: “I shouldn’t have left him in the garden. We all know that.
“Is it truly exceptional bad behaviour, neglectful behaviour? No.
“I messed up, I shouldn’t have left him there.”
He added: “It’s one bit of human error for a split second which has ruined my life.”
Mr Speakman said that “seconds before” the collision he had seen his son sat in the garden playing with the family dogs.
He said: “I have looked behind to the best of my ability. Not for a second would I risk his life. He is the best thing that’s ever happened.”
The prosecution alleges Mr Speakman disregarded a warning in 2020 from the Health and Safety Executive (HSE) about the use of another piece of farm machinery with a lifting bucket attached.
A letter was said to have been sent by the HSE to warn him of potentially fatal consequences after the emergence of a video posted on social media, which showed a teenager inside the bucket in the air as the defendant moves the vehicle and is heard to say: “I’m going to drop you”.
But Mr Speakman denied he had received any such letter and told the court the family had had problems with missing post.
The defendant has pleaded guilty to a breach of the Health and Safety at Work Act in failing to ensure, so far as reasonably practical, the health and safety of Albie.
The trial continues.