I think that I would plow it under and start with something else. A little Alfalpha is good for young, growing horses but it can be too rich and can cause some problems for older ones. Too much protein. One of ours breaks out in hives after a few bites and becomes an itchy, scabby mess. Timothy is good if it will grow in your climate or just a good grass mixture. I've seen special horse pasture mixtures at the feed store.

I agree. Till it under and reseed. I would probably seed it with a good grass mix for your area.

First I would get a soil test. Take it to the local fertilizer plant so you will know what field needs to grow good grass. I think I would spray field to kill everything, work the dirt, fertilize and reseed. No-till drill would allow seeding with no plowing or disking of ground. You probably won't be able to use the pasture till spring. You need to allow new seed to grow, get more mature. Very tender roots will be easily pulled up or cut with hooves running around.

I would agree that alfalfa is way too rich for horses pasture, though I would say alfalfa is worse for young horses than older ones. Young horses get calcium--phosphorus balance off quickly when fed alfalfa as well as gaining too much weight. Causes tendon, bone problems in growing animals. Older horses have more trouble with alfalfa reacting in their systems. Mine get the runs. Some horses never have any problems with alfalfa.

There are several seed mixes available, you should check what is sold in your local area. Good grass in North Carolina probably won't grow well in Canada. If you EVER plan to have a bred mare you want no Fescue grass to prevent delivery problems. I found the Mare and Foal grass mixes to contain no Fescue seeds. Read ALL the contents labels. Buying something cheap that mostly or only contains annual Ryegrass is not a good seed investment for your money. If you can get grass seed drilled in with a seed drill or no-till drill, it gets a better start than broadcast seed.

Since my horse is now pastured on a former alfalfa field, I am going to disagree with the above recommendations to destroy the alfalfa before pasturing horses. You definately DO want to establish grasses before putting horses on the field, but SOME alfalfa is a good thing for your pasture. Indeed, most horse pasture seed mixes deliberately add a legume or two to increase the nutritional profile of the forage.

I will ask the farmer where I board my horse what he did, but I suspect he mowed the pasture as far down as possible in the fall and then overseeded grasses, possibly drilling. Alfalfa does not compete well with grasses and the grass seed should grow just fine. Frequent mowing will help the grass out-compete the alfalfa. By spring, you should have a good mixed stand.

However, your local county extention agent should be able to give you a more authoratative answer.