I don't know what you hope to achieve by going to a dairy farm and then arriving so late that you miss most if not all of the early milking shift and bugger off so early that you miss all of the late shift. That is what you're there to do, to be 'trialled' on, not feeding calves and watching cows have their feet trimmed. Milking is all about the early starts, the physicality of the work, the muck, being tired all the time, working with animals, etc.
You have to prove yourself to them, prove that you can be trusted, prove that you can do the work, prove that you can work with animals, etc. If you can't convince a farmer how are you going to convince an admissions tutor? From my experience if you put the hard work in, show an interest, be eager for more work to do, etc you quickly gain their respect. Bear in mind that you will receive short shrift on any of your other placements if you're forever disappearing before any hard work can be done.
Yes it was a 'trial' but for mine I phoned up and asked what time they wanted me around and when I could be of most use. I didn't dictate what hours I would be working, I was after all there for work experience, to get an accurate representation of the job. It so turned out that I was there at 6 and ended up working a full day. If you absolutely can't get to any early shift make sure you get along to the evening shift.
It's up to you. Personally I'd go along the next day, apologise even if I wasn't totally at fault, ask when they wanted me there, make an effort to be there on time and slog my guts out to prove myself to them. Veterinary work experience is on the whole hard to get hold of and having farmers bad mouthing you behind your back isn't going to help matters. I have made a large amount of contacts, even had job offers, while on placement. Help them and on the whole they'll help you, whether it's putting a good word in for you, writing a good reference, etc.