My mare is an extremely easy keeper, swear she could stay fat off air. I only give her half a scoop, sometimes even less of grain and she just gets grass hay. How do you guys ensure they get all the vitamins and nutrients they need for ones that you can't feed a lot? I would like her to have a bit more muscle tone but I'm afraid to feed her more than I already do because she will get fat. She is a little fatter than what I would like her to be right now regardless. It has been very cold where I live so I'm not riding her as much as I was in the warmer weather, but even then she still was fatter than what I would like her to be. Any recommendations? My other one is the total opposite and I have to feed him quite a bit, especially during running season. Are there any supplements that will help her build some muscle, but not a belly lol.

Triple Crown Light might be good or a ration balancer. Pretty much all the bigger feed companies have one available.  Triple Crown 30, purina enrich, nutrena empower, are just a few that come to mind. High levels of nutrition with a 1 to 2lb feed rate to meet all the vitamin mineral requirements.  I think TC light is 5lb max if I remember correctly.  Bluebonnet makes one too I just can't remember what it's called! 

I agree about the ration balancers! I have had really good luck with triple crown gold balancer and mad barn omneity mix. they are super cost effective as well.

I have two of those easy keepers! One of them has to be really rationed as she foundered a couple of years ago. I use the Bluebonnet Ex-Factor, low starch feed, mostly for the mare. I also feed Stride 101 diet balancer to provide the other nutrients they miss out on in feed since they are getting less than 3 lbs per day. During the spring/summer I highly recommend using a grazing muzzle. That helps ALOT during the spring when the grass is super green.

I have a lot of easy keepers these days...might be a direct correlation to my age. :)  But I have started using the Hay Chix slow feed nets and that has really helped.  It takes my easy keepers longer to get their breakfast, dinner down.  I am not having issues with pacing, chewing on the fence, talking to me everytime I go past the pen. Its not the whole answer to this problem but I have been happy to have it as a tool to help keep extra weight off and save some hay. 

I generally don't feed grain unless I'm working one, the exception being my 29 year old who needs it for weight. I do hay chix nets for their hay, just keep it full 24/7 if I can. If you have good hay and a mineral available to them , they should nutritionally have what they need. For muscling, you could try just adding rice bran. I can get one pretty muscled up and ripped looking just with that, its the base of most muscle supplements. I have one that does no grain now even when working and one that does 2lbs of grain, so minimal grain diet. I've never had to feed more than a cup a day to get results. I will try to attach a photo to what rice bran does to mine as far as muscling. She is worked,5-6 days a week but mostly lunging

I honestly probably keep my horses "fatter" than most. But I think it is extremely healthy for their bodies (and their minds) to be turned out 24/7. Their stomachs were meant to graze all day, and their feet were meant to slowly move all day (circulation).

During the winter when they aren't getting ridden much, yes, mine are going to put on some weight. Who cares. It's the off season and I don't have an indoor (some day I will!).

During the season, they get adequate exercise and that keeps the weight off and keeps them in good physical condition. Once you start doing long weekends and going hard, I prefer they have a little extra fluff on their bodies because they don't by the end of the weekend, even though I keep hay in front of them at all times.

What is your exercise program during the season? Are you actually working your horse hard enough? (I feel like a lot of people don't.)