As to separating fence - a cattle panel pen will work fine. Can be as big or as small as you like. The panels are 50" tall, 16' long. Then can be attached to steel fence posts or to wooden ones. you can make your pen any size - square, round or rectangular. For short term paddocks, you may not need posts every 8', but I would recommend that. Better for the panels and much stronger. The posts can still be pulled out of the ground. You can also use the panels to build a shelter as well (detailed in another post). I use these panels for smaller pens to separate mares and foals for a while, my 50' round pen and also in much larger pastures.
I don't have any pictures lately of fencing on wooden posts - but here is one of our mares in a 16' x 16' pen with her newborn filly. We knew this mare was "good" in a fence and that corner behind her has no fence post. If those panels weren't strapped tight to each other the one in front of her would fall down... She and her filly stayed in that for a week, before going back out in the pasture with the group. I put Bell and her filly out in the pasture first, then started turning the others out in the pasture, too. It was the same group that Bell had been with before giving birth. We'd already set up the pen, but had left Bell out that night since she had no impending signs of foaling... she foaled the next morning out in the herd, of course! The others in the pasture were all experienced brood mares that I knew wouldn't "steal" the new baby, but ... Bell was quite relieved and happily let us pick up "Classy" and take her to the pen on grass.
Your herd is
very different in dynamics! Normally, 2 stallions housed together with a mare would literally KILL each other or at least harangue each other enough to cause loss of weight and injuries. You may find as yours get into better condition and they aren't just worried about "stuffing their bellies" that they will start fighting. Even a gelding and a stallion housed with a mare can create issues. Sometimes 2 geldings and a mare don't work. Sometimes the geldings will fight over the mare just as much as they would as stallions. Sometimes the problem is the mare.
Is your mare a maiden? If so, she could completely change into a slavering, human and horse killing witch when her new baby arrives. Even if she's not a maiden - having other horses stick their noses in her baby's direction could be a problem. If she is an alpha horse already - you may have already seen her drive the boys around. If not, then what she "might" become will come as a shock... Just because she is a tiny mini doesn't mean she won't be upset by the presence of strangers - she may do anything to protect that baby. OR she could be just fine! She's a mare.
Separating her from the boys is safer for everyone. If you don't know what she is like as a mom, never getting between her and her foal is a good start. We often handle our mares a lot before the foal comes. Then, with a maiden or a nervous/upset mare, we handle the foal with either two people (one always handling the mare with a halter and leadrope and if needed a chain either under or over the nose) or the mare is haltered and tied up before the foal is handled. You'd be surprised how fast an angry mare can turn on you if her baby squeals wrong. That said - most of our mares have been a dream to handle them and the babies - but again - we handle the mares A LOT. Two mares that I have - I don't handle the foals with the mares loose in the first couple of weeks. I'd be placing myself into a position to be seriously injured and I don't currently have anyone who can care for all of our ponies besides myself... Once the newness has worn off their foals - they don't care anymore.
This could have been bad news... Bell has been great with us and her foals since I purchased her in 2009. She ended up nursing two other foals besides her own that fall when I shipped them from IA to NC and the two other foals "needed" her.
In the first photo, there could have easily been a problem. This is Satin's 5th foal for us. Her first one came with her when he was 2 weeks old. Satin had to be sedated every time that colt was handled until he was 6 weeks old. Both my vet and I had to have stitches from injuries she caused with her teeth when she got mad. Her foals would squall like "stuck pigs" when they got upset and if she was loose and anywhere close, she'd come running and "all on the prod"... not any different than a wild boar, IMO. Our daughter should not be between the mare and the foal. In the 2nd pic, she's shifted and the filly's bum is bumping against the mare's chest - however if satin got mad, she could still reach Madira. Satin is tied very short...
Also, some stallions and/or geldings will kill/injure/"steal" foals. Some will raise foals. I have had one that would put up with new foals actually trying to suckle on him (OMG, that's too funny!!) and would actually "clean and scrub" them with his tongue just like the dam would. For some of our mares, that was a relief. For others, it was a problem. in our herd situation, when we did turn our stallion out, we learned which mares were ok with him with the "new" foals. A couple of times, he corrected the mares when they tried to turn on him (he actually knocked one mare down - then he "bunted" the foal over to it's dam. We caught the mare and foal and put them separate for a bit longer - in our round pen for more than a month before that maiden mare went back out to the pasture. The mare had foaled in a stall and had been turned out when her filly was a week old. Later, that same mare would not stand for breeding - even though our boy said she was definitely ready. Her whole attitude showed in her constantly touching her foal who was tied in front of her, flattened ears, knashing teeth while tied, stomping hind legs - like she was saying ..."you created that - you ain't comin' near me!!" LOL. We waited until her foal was older and she'd relaxed her "mommy 'tude", then she stood for rebreeding. She was never like that again - for a total of 6 more foals.). Our stallion (1st one) could be used as a weaning buddy when the mare was removed between 4 - 6 months after the foal was born. That worked great until the foals - either sex - were yearlings (in the spring of their yearling year - not all of them would have been true year old yet). Then he'd attack both sexes and drive them away from his herd (we had 4 mares for quite a number of years). He couldn't be turned out with geldings - he'd chase, harass and constantly be on the attack and could weave a gelding around into a corner and keep him there for hours on end. He'd go w/o water/food/hay to "teach that one" a lesson... And he wasn't happy to be caught while in that mode of "operandi"...
Our 2nd stallion was good with the mares and foals together - didn't have him out with the mares/foals all the time like we did with our first stallion. He was often kept in a smaller paddock with mares that were already pregnant or with larger horse mares & we had one gelding that he could be put with before we used him for breeding a lot. That gelding went home before we'd had this stallion for long - so don't know if that arrangement would have continued to work or not. The gelding was a "lower totem pole" 3 yr old arabian - over 15 hh, our stallion was 10 yrs old and 40" tall.
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From the pics you posted I can't really see anything with the horses' hooves. Can you do an angle more from the side and on hard ground (cement or a board or rubber mat) would be easiest to tell what's going on. If they haven't had proper hoof care in a while, it can take several trimmings to get them back where they should be while also keeping them sound.
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Hay - I've often found that our larger Shetlands did terrible on Timothy hay. I've found that timothy hay here in NC is shipped in usually from NY. I've had 4 different suppliers (plus a couple of feed stores) and it's always been rough and stemmy. I've never had it tested for it's nutrient levels. TSC usually has local hay or shipped in hay - and nope - it's not always the best. Also, your humidity will take a toll on any hay when it's stored. I know it does here. With how small yours are - it may work best to have the small squares instead of the larger rounds.