Cow horn is prone to delaminating, which is why it is not used in high stress areas such as the cores of horsebows (korean, chinese, turkish, etc).
Hide glue bonds extremely well to horn. That's real hide glue that you have to dissolve in warm water, NOT the liquid sold as hide glue. You might try soaking the horn in very dilute hide glue for several days. Some glue will diffuse into the horn which should make it a little stronger. If the horn is worked before it is fully dry, it should also be a little more flexible so perhaps a little less brittle.
If you can't find hide glue, Knox gelatin should work fine. So should fish bladder glue (Isinglass) but that stuff is pricy.
I understand it can go rancid during the extended soaking. I forget offhand what you do about that. I've not had to do this with any horn I've worked with (gemsbok, buffalo).
Info on the technique is probably available online somewhere. I haven't searched.
Gerald