There are a few assumptions to make here before answering, most of which fall under the lines of 'define cow'.
If you are going for buffalo:
- A cavalry charge (in knight terms) was at its most destructive from sheer momentum. Ya, the mounted rider can help with the invent of the lance, but the ability to break a defensive line has more to do with the weight of a horse being thrown at the weight of a man (hence pikes being an effective defence). In terms of sheer weight, a stampeding buffalo is significantly heavier and would have considerable more breaking power when hitting a defensive line.
- Powerhouse head. A horse's head is somewhat small and leaves horse's chest open while it gallops forward. A charging bison looks like a giant armored skull designed for headbutting coming at you and leaves little for weak spots.
- Buffalo are big-ass creatures and can take surprisingly large amount of damage before being brought down.
- They are significantly more brutal creatures than horses and would easily gore and toss around a human soldier like he's nothing. While a horse doesn't do as much post charge, a buffalo can gore and toss around people like they are paper dolls.
- I would think a charging buffalo wouldn't think twice about going head-on with a horse while a horse would either need some pretty specific training or risk spooking instead. This was an effect of the elephant as well, elephants were particularly nasty when ran at horses.
Downsides are more in maintainability:
- Buffalo aren't an easy ride and require some pretty specific animal handling to control. Long treks on buffalo back don't seem feasible.
- Larger and lazier than horses. Bigger creatures tire quicker and don't have the same stamina horses ultimately possess.
- Food consumption. A herd of buffalo requires a significant land mass to feed.
- Larger target. Arrows (and bullets) are more effective against these giants as they are easier to hit from sheer size.
- Less sure on this, but in comparing them to Elephants, I would think the possibility of bison/buffalo 'spooking' and rampaging over their own troops is a possibility. Romans developed the tactic of covering pigs in oils, lighting them on fire, and sending the squealing herd into elephants. This would panic the elephants and cause them to rampage. As dangerous as these large 'beasts of war' are to the enemy on the battlefield, they also posed some degree of risk to friendly troops as well.
- Bison/buffalo are ultimately wild creatures. Horses have been domesticated for thousands of years and, like dogs, a relationship with humans is at the core of their spirit. This simply isn't true with Bison. You may be able to get around this point with your setup had this nomadic culture began forming bonds between Bison/rider in the same way a horse/rider does, but odds are, the Bison are going to retain some 'wild' qualities making handling of them far more difficult.
Ultimately, my answer is "yes, more than feasible". They come with specific advantages and disadvantages, but so did elephants and they saw their use. If you have access to them and can handle them, why not?
If this question is only about cows... I hate to say it, but human involvement has selectively bred cows to be eating machines. The majority of their internal make-up is focused on digestion and we have focused evolutionary pressures on these creatures to be the best eaters they can be, not much else. Unless cud can somehow be weaponized (or perhaps using the tremendous amount of methane that these creatures emit). They are also lacking in the endurance domain (once again, they are bred to eat not run).
I really hope you're not referring to dairy cows - that would seem udder nonsense.
On more add...
Unlike the previous 'unicorn charge' question, it's debatable if a horse's head and neck are really capable of withstanding the impact of their own charge. Buffalo, on the other hand, can fully use their horns for charging and goring as their physical make-up is designed to support themselves during these impacts. If you want to see nasty youtube footage, deaths from yellowstone bison annually is a measurable number and I remember at least one bison video where it charges a car and the car pretty obviously loses.
I guess if you lose the battle, you've unintentionally provided the opposing army with a great deal of food reserves ^^ Opposing generals can rally their troops with 'Tonight, we eat steak!'
Buffalo vs car