that fir tree stuff grows dead quick, and horses don't like it so they won't pull it out for you... can't think what its called tho - loads of people have it round gardens..... otherwise hawthorn.
Hate to say it but conifers are quick! You could use gold ones instead of the usual green or alternate them. Normal hedging usually takes a few years to get tall and thick enough to be a screen. Laurel or cotton easter aren't bad although I wouldn't use tham as a field hedge.
Leylandii will wreck your soil and have very invasive roots. Think very hard about the long term implications of planting them.
Id go for a short term measure such as willow hurdles with a mixed species hedge planted behind it. You will have to wait a few years to see much more than a few sticks but longterm you are providing a superb barrier plus a great environment for wildlife.
Had not thought of sheep hurdles, I much prefer the natural Hedge I have a lovely hawthorn hedge down the length of the field. Its only really my Mums pet sheep that ever go up the top end of the field, so not to worried about conifers. basically we have just moved our access gate, so we can now park in a drive to open the gate ware as before we almost had to park on the lane then open the gate! But because of this people would have a much better view of the stables (The stables are arriving in the spring) Eventually it will look lovely we have banked the exes mud & the idea is to plant a hedge on top. I was thinking of getting some dog rose to fill in the gaps of the existing hedge.