How to decide When/what horses go into stud, and what ones do not?

How long is a piece of string? hahahahahaha
Still working that out myself
For me - if you love seeing them win...keep racing them until you think their form starts to tapper off.
If you're in a hurry to get a really strong stable then probably retire them at 5yo.
By 5 - if they are the bees knees then they would have won a lot of big races by then.

For me, it depends on where I am in the game.

When I'm starting out, I retire all of my starting horses who are over age 5 and buy all cheap fillies and mares at auction to use immediately for breeding. If I need a stallion, I try to pick up a reasonably priced one unless I skipped a season. Then I just breed with the game stallions. I don't bother looking much at anything early on since you can sometimes get a super horse out of two nags. I also don't look at age much. My best mare (won over 60 G1s) in SO5 came out of a mare that was nearly 20 which is well beyond the age some here breed their horses. Both her dam and sire were multiple group 1 winners, though.

After I start to pick up some group 1 winners (whether by buying them or breeding them), I only retire horses that have won at least 3 G1 races. After a bit, I might up it to 5, and then 10. Similarly, I only breed with game horses with the same requirements.

My horses stay in the barn until they die unless I need the room. Then I only retire those who have poorly performing foals. Early on, I might have 5-10 stallions but later on, I might have 2-5. Similarly, I might have 50 mares early but shrink that to 10-20 once I get some real winners. I also breed with the game horses so I typically get 25-30 foals. Some foals I just end up breeding to sell while others I keep to run.

I vary how long I run my horses. Some games I might retire them at 4. Some I might run them until 5. Others I might go until they stop winning and their rating drops. It also just depends if my barn is getting too full. I try to keep under 50 horses racing. I tend to base who I retire to stud off group 1 wins, but from time to time I retire horses to stud with high stat bars like battling qualities or extra speeding rating when I'm lacking those in my bloodlines. If a horse ran consistently despite never or rarely winning, sometimes I might retire them to stud, too, especially if they kept being runner up to another of my horses. Only horses I keep running after my retirement age cutoff for that game are geldings, and I only keep them around as long as they win. Once they stop winning, I permanently retire them. Some I've kept going until 8 and 9 years old. I avoid running in claimers and sellers just to not have to buy my horse back when they win. Sometimes I forget and I'm out a horse.

Some here use spreadsheets to see how their horses perform. I have one myself that I use to keep track of my racing horses as well as to see who's my best breeding stallion and mare. I'm contemplating tweaking it further to throw in a dam sire tracker as well. Just have to figure out what works for you. Those who do the league have some incredible horses and might have tips on who to breed and who to skip, but I don't do the league personally. I like to go at my own pace and figure out what works for me. Ultimately, it's often just down to spending a LOT of time breeding. Each generation improves on the last so even if you start with two nags, after 10 generations, you'll potentially have a super horse who wins 50+ G1s.

Great post by Titan, reflects my own thoughts on it quite well. I do have a smaller racing roster however (8-10 usually), and a similarly sized breeding roster so I retire the 8-10 year olds from the barn to make room for the next generation that will refresh the bloodline.

Experimentation is key, and when playing I always know what horses I need to keep racing and which are probably finished for me. It's not really about group wins, or ratings etc, instead I only race high potential horses and (after the first season or two) only breed from my ex racing stock. Once they are in the barn I mix and match by distance and group wins but that is about as far as I go with it.

+2 for Titan. Great post!

I am just reaching where Titan is, I think. This is my first season with 50 horses in my stable - 51 to be exact. 50 is a LOT of WORK! I am going to try make 50 the limit if possible.
I've been racing for 60 odd seasons and only started breeding around the 45th season. It took me at least 40 odd seasons to start getting consistent group wins with game bred/bought horses. Within that 40 seasons I only had a handful of group winners.

So what Titan says is true....it does start off slow!
Personally I don't geld colts anymore - I ignore that prompt!

I race all my good horses til at least 8yo - as long as they are winning Group 1's - that should give me a good 10 years breeding - as you may have noticed, the game does kill of horses randomly at the end of each season and the ages are random too!

As Titan mentioned - the further you go into this game the higher you keep setting your bench marks along the way ie at first you may just breed with good listed horses as these will be your best horses at first - then Group 3 and 2 winners then Group 1 winners. I am at a stage now - to make room in my breeding barn - mares and stallion's must have at least 5 Group 1 wins and no doubt that requirement will change in the future to 10 Group 1 wins!?
Recently I have been trying to breed with horses that have good performance bars - like 'finish application' 'speed rating' etc. That stuff is a mystery in itself! It appears that a long red ' deteriorate with age' bar can be over ridden by a long 'speed' bar etc.

I am not surprised at all about the use of spread sheets! Especially for breeding - I actually think that this will probably be my next step!
I also avoid claiming/selling races like the plague! I also now race my 2yo's sparingly keeping them fresh for the Group 2yo races which are big in Australia.
Don't sell 2yo's off unless they are really bad - some horses improve big time as 3 and 4yo's. So if they are around the mark - wait until the end of their 3yo season before selling etc.

Could you share some spreadsheets with me, please? It would be nice to know what should I be writing down.

Another question i have is, how do you increase the breeding barn limit?

From what I can see....you can't.

I asked similar a while back. The limit is the same as the number of horses your stable can hold, so at the beginning it's 6, and fully upgraded it becomes 150. Ignore the capacity on the Breeding Barn screen - just keep adding horses and it'll fill up nicely.

There is no way to increase beyond 150 in the barn and 150 in the field/racing, seems 300 is the top limit for how many horses you can own.

Thanks for your info. That explains a couple of things for me.