I find it ridiculous that he only got a community sentence
He didn't.
"Brown was also given a 30-day rehabilitation requirement and ordered to complete 150 hours of unpaid work. He was also banned for 10 years from owning or working with livestock and ordered to pay £600 compensation."
several calves died because of him
The article says "there was no direct evidence that Brown, who had no previous convictions, had caused the death of any of the animals."
Even though I agree with you that it's likely they did die because of him, you can't convict someone of a crime if there's no evidence.
I presume there is something very wrong with him which means rehabilitation is probably more constructive than gaol.