Your horse will more than likely be weaned many times in its life.
The separation from its mother is simply the first time.
The three weaning(s)
At our farm there are in fact three "weaning". The first one is the weaning from the milk, the second one is the separation from the dam and the third one is the separation from the herd as the foal leaves for its new home.
Because of today's modern equine management techniques the first two usually happen simultaneously (away from mom, cut off from the milk she produces) followed a bit later by the third one (leaving the herd). They are however distinct and it is important to distinguish between the two first ones because they lead to different recommendations as to when and how one should wean.
Let's get the first one out of the way because it is the easiest and most straight forward: weaning from the dam's milk. What is traditionally thought of as weaning.
Weaning from Milk
We know from the analysis of the mare's milk from birth to 6 mont post birth and from the maturation of the equine digestive track that between the age 
of 3 to 4 months of age the foal no longer needs milk. In the comfort of domestic life, most of its dietary needs can be filled by consuming various types of forage. When necessary, younger foals then this CAN be weaned if they are supplemented with easily digested proteins, fats and sugars. Foals above the age of 4 months can fare nutritionally on their own. In most of today's well managed stables young horses are given access to good forage and supplements that are probably superior to what their dam can provide for them after the age of 4 months.
Without intervention most mares will wean their foal from their milk between the 8th month and the arrival of the next foal. There is no drama involve the mare simply refuses the access to the teat and the foal, while a bit sullen, usually complies without a fuss and that is that.
If you wait long enough, chances are the mare will wean her foal herself-but not all of them will. So if the foal is 9 months old and the mare shows no sign of wanting to wean her foal it will likely nurse indefinitely.
Separation from the mare
The second and far more impactful weaning is the removal from the dam's overall protection and social influence in the herd.
Because of the pressure of our management technique and expectations, the weaning most often involves both weaning at the same time. For various reason , at some point the foal needs to become and independent equine from its mother.
This separation from its dam is actually the first of many "weaning" it will experience in its life. Think of how often a horse changes living arrangement. Every time this constitute of a "weaning" of sort. We sometime conveniently forget how stressful it is for a very social animal to be moved away from its pasture mate, herd buddies ect... It is indeed required of them and we quite admire the ones that can do it stoically and with minimal drama because they are generally the exception and not the rule.
In a non-managed (think wild or feral) setting, equines remain with the herd they were born into until sexual maturity at which point the males are driven out. However most foals today need to be weaned from their dam because they will be moving to new homes or they will move to a different pasture where they can be managed more appropriately.
What ever the reason, weaning a young horses is a completely human made issue and as such it needs to be addressed like any other cases were we impose an un-natural situation on the horse: with respect for its nature and a concern for its safety.
Every time you take a horse away from its herd you are in effect weaning him and it is stressful. However a mature horse is better able to handle the stress, and can rely on its past experience of being pulled from its herd but in finding safety in another. Learning from past stressful experiences in which the outcome was good is one way horses can (and must!) learn to navigate life with us humans.
But the first weaning of all is particularly stressful because there is NO previous experience on which to rely on.
Ok- So when should we wean?
So let's be considerate and prepare for weaning in the foal at what ever age in the following ways.
Ensuring that it is the right time for this particular foal to be on its own.
When is a foal ready to handle the world without its dam?
Foals will gain independence and confidence at various rate. Some are timid and will need more time to learn to rely on their own instinct and reactions. Until they do, they will be glued to the dam, not letting her out of their sight and acting quite worried when they can't see her. But, in time, they will all gain confidence and independence and stray further and further away and sometime spend more time with other mares than with their own dams.
When they are able to handle minor scare without rushing to their dam we can make a note that this particular foal is starting to gain some independence. One can observe how they react when their mother rounds a corner or wanders down the hill and out of sight. For most foal at around the age of 4 to 5 months they start to spend time away from their dam although some will go through a period of insecurity where they regress and seem once more dependant on their dam. Sometime because of a scare sometimes for other reason I cant quite figure out but I've observed it many times.
Eating separately from their dam these foals are starting to gain some independence.Some foals remain quite skittish and nervous without the reassuring presence of their dam and those foals should be given more time to gain some confidence. To separate those foals too early would just lead to a lot of unnecessary stress and increase the chance of injuries.
Will it make them neurotic maladjusted horses later on? I seriously doubt it but that is not a reason to not allow them a bit more time to make the whole process less stressful then it has to be.
Other foals are born supremely independent and could probably handle weaning much earlier.
I keep a close eye on each foal behaviour and relation with its dam.
Do I see it spending more time with the other foals than with its dam? How does it act as I take it away from its mother as it learns to lead and explore the world outside the familiar paddock with me? When feeding it apart from its dam, does it fret and pace or does it eat, far more interested in the food than in the proximity of its dam? When something odd occurs do they run with the others, investigate on their own or run to find their dam? Those are all clues that the foal is gaining its much needed independence and confidence.
When a foal is able to handle minor stress, eats its meals and forage out on its own, without its mother by their sides; it shows sign of being ready to face weaning.
This could be anywhere between 5 and 7 or 8 month in my experience.
Doing it as safely as possible
No matter what, there is always the possibility that there will be some moment of stress when the foal will try to find its way to the reassuring presence of its mother. So it is very important that the place to wean be chosen carefully. The fences need to be solid, imposing and without any gaps that could tempt the foal to squeeze through. Particular attention to gates must be paid if the foal challenges the fence or attempt to get though it either it is not ready to be weaned or the fence is not suitable as an imposing barrier that should be respected.
There is no shame in reconsidering the timing or re-evaluating the physical situation you have. Without re-doing the whole fencing system consider confining the foal to a safer area- at least at first.
Also consider the footing and the temperature. As the foal gets worried it will start to run, and pace and if the ground is hard and slippery or thick with mud the chances of injury greatly increases. If it is very cold or very hot this will only add to the stress, try to pick a streak of reasonable weather if at all possible.
Foals should learn to respect electric fences it will prevent them from challenging them later on.For those that will mention that if the weaning is done gradually it will not cause this stress I have to disagree I have yet to see a weaning where there was not at least one moment of worry and anxiety upon the (sometime delayed) realization that the mare is no longer either within physical or visual contact.
However if the foal is ready for weaning it will be short lived. Better be safe and expect some running and sizing up of the fences or the barrier between the foal and the mare than to expect things to go smoothly just because you have separated more progressively and end up with a disaster on your hands.
I would look at the forecast and pick a stretch of days when it will not be bad weather, not horribly cold, not terribly hot, no storm ect...
Looking at ways to minimize the intensity and the duration of the stress on the foal.
There is a few different method each with their advantages and draw backs. Always ensure that the foal is with safe and known companionship before weaning. It will rely on the safety of its herd (no matter how little) to cope.
Method 1: Cold turkey, out of sight and out of mind.
By far my favourite method if the foal has showed all the signs of being ready to be weaned is to move the mare to another property until the foal is ready to leave to its new home. The mare is better able to handle the stress of going off site and with no signs (calling, sighting or even smells) of the mare being around the foal quickly settles. They remain on the lookout for a few days but they are within the herd they know well and in a locations they are familiar with. They will call a few time every day as if to locate their dam but with no answers they give up and accept their situation.
I generally give the foals some medication agains ulcers for a few days to a week. Half a dose, mostly as a preventative measure.
If the foal is to leave the farm I would give that foal either a week before it heads out or over a month. The reason is that the trip of the farm will be stressful and the settling in a new situation will take a bit of time of course. So removing their mother and then transporting them to another location a week later or so will mean about 3 weeks of a somewhat stressful time. Removing the dam waiting two weeks just long enough for the stress to come down and then having another stress full time just seems to me like a lot of time to be stressed. If the time between weaning and transport is over a month then it allows the foal to quiet down, find a new normal before another disruption to its life. Again before the foal is to leave I will give it some medication to prevent ulcers and send some along with it.
If the foal is to remain on the property how long should the mare remain away: a few months to be safe. The foal needs to learn how to be independent and find its place in the herd before the mare is to be re-introduce or the work will need to be done all over again.
The mare will also need to dry up completely or the foal will try to nurse and stimulate the production of more milk. Some mare will refuse the foal after a few weeks but others will let them nurse so you will have to wait a bit longer.
A foal will always recognize its mother no matter how long they are apart so you can expect the foal and mare to meet again on very friendly term down the road.
Method 2: Cold turkey but within sight and sound but no physical contact.
That is my least favourite method: the calling and the pacing, the fretting. The moment the mare or the foals quiets down, the other calls and the stress starts up again. Everyone is miserable, including me. It has the highest chance of injury. Since the foal can see the mare there is always the temptation to somehow get over (or under, or through) that fence to re-join their dam. I almost never use this method unless there is a damn good reason. ( And I cant think of one right now....)
Method 3: Cold turkey but within sight and sound but with physical contact, just not nursing allowed.
If you have a set up for this method it seems to be the best. You need a very solid fence, shelter and water on both side. There will however be some time when the dam will wander further away than the foal will like and there will be calling and running so it might not be as ideal as it might sound, unless they are both in smaller fields. Generally mares are ready sooner to leave their foal behinds than the other way around. Careful as the foals will stick their head and neck into very doggy position in order to catch a drink and the mare might cooperate! There is still some calling and disruption to the regular routine with this method for sure. Best, as with the other method, is to move the mare and leave the foal with his current herd. But that can also cause issue if the mare is quite attached to the other mares ect....
Method 4: Gradual, separating the mare and and foal for longer and longer periods of time.
Its very time and handling extensive and require the right set up with safe enclosures that will not be challenged. The day the separation is complete and permanent can still be surprising stressful, but usually not. Foals have had plenty of opportunity to learn to cope on their own. But I see that as spreading the stress over many days and weeks. The few times I attempted this, there were numerous setback. Some days the foal was fine for hours on his own with his friends and some food and other days after 15 minutes he was calling and restless. I felt it was borderline cruel to basically constantly separate them and sometime it felt like Groundhog Day.
The third weaning
When the foal is loaded up unto the trailer and heading for its new home is in fact a third weaning. Leaving its herd and the comfort of everything it has ever known is hard and comparable in terms of stress as the weaning from its dam. Some would suggest that it is even more so because at least when the dam is separated the foal remains within its herd for a while and there is some continuity. However foals usually settles well into the transport and if they were well handle they put their trust into their handlers. This is perhaps where the importance of handling foals and making most experience with humans a good and positive one takes on its most critical importance.
Teaching foals to load will help minimize the stress of that first trip off the farm to their new home.They often trailer with other horses that also settle into the rhythm of the transport and quite often foals will lay down and sleep once they realize that they are not being chased or attacked and in fact quite safe in their traveling box stalls. Away from familiar smell and horses they stop calling and focus on making friends, finding food and drink and thus they travel well.
One last thing I would like to dispel is the notion a less than ideal or even rough weaning, or very stressful one can have a long lasting impact on the horse down the road. I have seen no evidence of this. I know many adult horses that had a rather stressful weaning and if they were well handled after that they became well adjusted equine citizens. Certainly a foal that is insecure to start with might have a difficult weaning no matter how dedicated and careful the owner might be and will remain an insecure horse down the road. The best weaning would likely not have prevented this either.
One should not anthropomorphized beyond reason foals are not children with very fragile and complex emotional make up. That is not an excuse to not do our best to reduce the stress of weaning however.
