Hi
I've been enjoying the game for a few weeks now, and have finally decided to tackle the breeding side of things. I'm at the stage where I have a couple of good stallions and a dozen good brood mares, so it's all working fairly well so far.
The problem I've encountered is that I can't seem to find a way to sell off, or otherwise dispose of the breeding mares which are no longer up to standard. The "enter into next auction" button is missing and the "retire from game" option is greyed out. I'm probably missing something obvious, but I can't seem to find a way to cull my unwanted breeding stock.

Thanks in advance for any help.

Try press Send to field and you should be able to sell them then.

Either that or click the return to stables button and you can then auction them off. You can't sell a horse when it's in foal however, so just make sure they haven't been bred the same season you're attempting to sell them.

This

Thanks for the advice guys, ironically all my mares are in foal as of this moment, so I'll have to wait to do the deal.

I wouldn't sell your brood mares, breed them with the cheaper stallions and sell their offspring to earn some cash.

Also this. But as an unraced 2yo, not as yearlings. A little extra time brings a lot more money.