1.  When we take a photograph of a moving subject we have to have a quick shutter speed in order to 'freeze' the motion.  However, because of this fast process, the amount of light getting into the camera sensor is limited.

2.  The faster the shutter speed, the lower the aperture in order to let more light in, however with a low aperture number,you don't get a very big 'depth of field', therefore you have to focus very precisely on the subject in order to get it in focus... not easy when they're moving and at a distance!

3.  ISO setting, the lower the number the less sensitive the sensor is, the more you increase this though in dark situations the 'grainier' the shots are!

...

So if I decrease my shutter speed in order to let more light in and get a better depth of field so that the picture is more in focus... this results in 'blurred' motion ...

But if I increase my shutter speed in order to freeze the motion then there is less light and less depth of field making the photo's dark and out of focus...

I actually end up shooting at as slow a shutter speed as I can get away with (which is still fast enough to freeze the motion), but to compensate the therefore low aperture, I increase my ISO settings on the camera...  the low aperture leaving some of the pictures not as in focus as I'd like and the high ISO leaving some of my pictures grainy.

There is no solution... we can't use a flash like you would normally in dark conditions, I don't think the horses would appreciate it...

... so that is why professionals refuse to do it, because it is actually impossible to get a 'professional' shot under those conditions...

Debs