When I first started training dogs, I started with a few old ewes which where fairly wild.
That was a big mistake. Old ewes are not suitable to train pups or inexperienced dogs on. If they are worked for a while, they tend to get sticky and may refuse to work or charge the dog, not a desired situation. This can frighten the dog and cause him to lose confidence, try to escape the pen, or panic and start biting. This is what happened to my Koolie in the beginning, before I learned the hard way. Her biting was very bad for quite a while.
The sheep will need to be dog broken, this is done by working them down with an older experienced dog, until they become quieter and flow well. If an older dog is not available to break them in, it might be possible to purchase broken in sheep from someone who has experienced dogs. Expecting a young dog to gather sheep that are racing around in all directions is too much to ask. They need to come easily to the handler when the dog goes around them.
The best sheep to start young dogs/pups on are about 8 months to two years old. Ewes or wethers. As we only have merinos I cannot comment on how different breeds work, but some breeds can be very flighty while others are more sticky and harder to work. The terminology for this is light sheep and heavy sheep.
I have three trainer mobs at present, one of about 30 sheep, one mob of three sheep that I keep in a smaller pen and five weaners that I usually use for the pups as they are unlikely to stand up to a pup and they flow nicely. My three are older girls now and were born in 2004.
I had five originally and we called them the Dave Clark Five, (A singing group from the sixties for those too young to know this) just as a way of referring to them. One of them ceased to work nicely and one started to confront the dogs so I removed them. They are still called the Dave Clarke Five though, which confuses people. Even though they are older, they work well, so I will keep them permanently. They are quiet but will run if pushed and are ideal for the older dogs.
Another mistake I made when I started out was to constantly change my sheep around. I thought when they got too quiet they were of no use and would let them out and get in some wild ones. I wondered why my poor pups got uncontrollable, as they tried to gather the sheep which the day before were quiet. I talked to a number of experienced trainers who said it is better to keep the same sheep. To train a dog well you need quiet sheep, so you can direct and control the dog without the sheep disappearing over the horizon. One person said he still had a couple of sheep that were over ten years old, which is old for a sheep. They were nice workers he said, why change them. I come to see the point of this with my Davos, they are quiet and will follow me calmly. When my husband shore them recently he remarked on how easy they were to handle and they did not fight or struggle.
As the dogs get more experienced, using some wilder sheep will keep them challenged. It is a good idea to keep a couple of quiet ones with the new wilder ones as this keeps them together better and still provides a challenge without making it too difficult for the dog. Also adding extra sheep will encourage the dog to cover better. When you do suddenly put your young dog onto wilder sheep don’t expect him to obey all the commands as he may have the previous day. Allow him to simply work the sheep and balance up until he develops confidence with them. It is too much to ask him to stop if the sheep are tearing around. Once he is working them nicely you can start putting commands on him again.
Sheep will work differently depending on the time of day, the weather or how much wool they have on them. Freshly shorn sheep can be very flighty, so best to handle them carefully for a week or two until they settle. A windy day or the approach of rain or a storm may also cause the sheep to be jumpy, so keep this in mind when working them.
Interestingly a strange dog working them can upset them, they seem to get used to the same dogs. If they are regularly worked by a number of different dogs they seem to cope better, perhaps you have visitors who come to work your sheep often and they get used to them. If they are only worked by your dogs a different coloured dog can upset them.
I visited a friend once with a red pup of mine who was working well. Border Collies had mainly worked her sheep and they were quiet and well trained but when Caleb entered the round yard they panicked and raced around, trying to confront him in their fear. The owner was surprised as they are well worked sheep, so we found him another mob of sheep that seemed to cope with him better. So be aware of a different looking dog which may upset your sheep.
Try not to harass your sheep too much on a hot day, especially if they are woolly. An overheated sheep can collapse and die, although it does not happen too often. Once their mouth is open and they are panting, they have had enough.
If you shear your sheep in summer be aware they can get sunburned, so provide shade for them,
Keeping trainer mobs fresh can be a problem as they do get 'dogged down' if worked a lot by inexperienced dogs. It is not the length of time they are worked that does this but the way they are worked. Sheep constantly harassed, pushed and bitten will quickly 'switch off', and become difficult to work. So try not to allow your dogs to do this. Pups can be a bit pushy but as soon as they are old enough they can be trained to steady up and keep off the sheep. An experienced dog can calmly work the sheep without 'dogging' them by keeping a good distance and not harassing them. If you can have about 15 sheep you can swap them around, as a break can freshen them up. You only need to work three at a time, so the others can be spelling, and swap around every two weeks or so
If it is not possible to have any more than three sheep, it is a little more difficult. You can replace them but then the new lot will have to be re-broken in. Replacing just one or two is another option but a lot of messing around.
It is probably better to spell them for a few weeks if they show signs of getting dogged down.
As I mentioned earlier though, if worked correctly they will not get too dogged down, and some sheep don't tend to get dogged down too much. Perhaps ask people with different breeds, what breed they prefer. I find merinos work fine and when spelled, freshen up well, in fact can become a bit wild if not worked for too long.
So, even if you can only keep a few sheep, if managed properly, they will serve you for a long time.