Not that I need to put my nose in other's business, but you have a great post going here, and nobody said it yet. Have an exit interview with your departing employees.
I can't speak to how many of the other replying posters have worked in your operation, let alone stepped foot on it, but I would imagine your previous employees know better than anyone. If they are willing to come back and visit as  you stated, they should be willing to open up and talk about their decision to leave. Your intention with the interview shouldn't be an attempt to keep them, they made up their mind and you should respect them for doing that. Your goal is to understand what they value and how can you correlate that to helping secure your current employees. As well as helping you to improve the same discussion with your current employees who you may not yet know/understand as well. 

And hopefully you can make it a priority to ask your future/current employees what they value prior to their departure. Make it a part of an annual or more frequent process during the less busy time of the year to talk about your vision and how they play into it. Let them know you value them. Be willing to show them you value them and respect their values. What you may find is not the lack of time off, it's the lack of value on working everyday for the man. You, yourself are working for the man but it's yourself. What if they worked for themselves as well? Would they be able to explain that to their significant others and mother(s)? 

While too many people gravitate towards it's a generational problem (as others have stated); without ownership, it's just a job. The cost of entering into any business these days is astronomical and the youth doesn't have as many chances are before. Utilize your help and their opportunities for securing additional working capital - for example, you own the land, build a livestock facility. Your labor force can pay all/partial for the livestock to fill it and you work out agreeable terms on facility rent/labor trade. (just one idea, I'm sure you and your employees could come up with more in a worthwhile, planned discussion)

Believe it or not, you will need a succession plan. No better time than now to start talking about it with qualified successors. If you aren't willing to let your help become an owner, I agree with everyone of them that left. 

-$0.02
Adam Ryan
Age 33