In regards to feeding alfalfa and grass and wondering which is best. You can feed a horse straight alfalfa. You can feed straight grass. You can feed a mix. All 3 strategies are good and all 3 strategies are not good. Below are reasons both bad and good.
The idea of a horse getting too "hot" with alfalfa compared to grass is not really true. It depends on when the grass was cut and what cutting of alfalfa it is. Grass typically has more sugars (carbohydrates, fructan, starch) as compared to alfalfa yet alfalfa will have higher protein and calcium and magnesium. When people are thinking their horses are "hot" or in other words have to much energy and are hard to handle they blame it on feeding too much protein. In reality it is too much sugars that cause problems like spikes of energy, and laminitis.
Think of it this way. When you eat a bunch of candy, cake, cookies, pop, etc you have a lot of excess calories to burn off. Your metabolism slingshots up and you feel like you can go all day. Then when the sugar rush disappears you get even more lethargic, tired, lazy, etc and you don't feel like doing as much other than hanging out.
Now lets say all you are eating is hamburgers, steak, chicken, drinking milk. You'll have a lot of protein and calcium to build up muscle. You'll be stronger and have more endurance to go longer. You just won't have spikes of energy for your energy level gradually grows and then it gradually wains depending on our biorhythms as compared to artificially boosting it with caffeine and or sugar.
So now that you can relate to foods for you and how it makes you feel lets think of the horse. People misconceive the idea that higher protein causes the horse to be hot when in reality it just gives them energy to last longer. When we are feeding a lot of sugar and starch the horse gets the quick energy boost shortly after eating but it doesn't last long and so they then become more easy going after the rush.
People who feed twice a day high quality grass hay usually go riding shortly after work but before they feed the evening meal. The horse has come down form their sugar high hours before and so the horse is more lethargic. If they ride shortly after a feeding and they think their horse is really energetic they probably lunge him for a little bit to burn off that "excess" energy and don't relate it to the idea of recently feeding him.
When a horse is only getting alfalfa or is just accustom to having higher protein levels the horse has more endurance and strength as a whole. So if the horse is feeling frisky it takes a much longer time to "tire" out because they are physically able to go longer. People who typically complain about their horse being too hot are people who don't know how to channel the energy into something productive and so they blame it on the excess protein.
Don't get me wrong. There can be problems associated with feeding excess protein to horses too. The kidneys work harder and the horse metabolism keeps trying to absorb the protein beyond what it needs and you can have other complications too. The horse will need more water. The horse will have a more lathery sweat which is not as efficient at cooling compared to just sweating. You will also notice the urine is thicker, cloudy, and have a stronger ammonia smell.
Alfalfa is higher in calcium and magnesium compared to grass and lower in other minerals. So one should supplement with a horse mineral block or have some grass in the diet too to even things out. But don't let people tell you that you can't feed straight alfalfa. It is perfectly acceptable and at times even desired to feed straight alfalfa. In the SW United States that is pretty much the only thing the horses get because the climate is too dry for grasses. Alfalfa has lower levels of fructan in general compared to cool season grasses. Lower levels of fructan are generally better for horses since the microflora in the hindgut responsible for fermentation can get out of whack when there are excessive levels of fructan.
You may think that grass / alfalfa mixtures are best. They are if you feed separate alfalfa and separate grass flakes. If you use hay already mixed there are problems with that too. Mainly because of when the farmer cuts the hay. Dairy farmers are the biggest users of hay. They want very high protein, calcium, and energy hay for the cows so that the cows will produce more milk. In order to get this the alfalfa needs to be at around 10% of bloom. The grass at this stage is just at head emergence which is when it is highest in sugars/fructan This is not ideal for the horse because it is more energy, carbs, and protein than it needs but because people see the greener and higher levels of nutrition in the feed we think more is better.
So you may think I'll just feed less to my horse so he won't get excess energy and I'll avoid the problems. I'll also save money that way because I won't be feeding 20+lbs of hay a day but only 15lbs. This is good in theory and alright for a day or two but because the horse is designed to graze 16 hours a day they have problems when their gut doesn't have the fiber passing through it and so they start having gastral problems which can lead to colic. Also when the horse doesn't have things to chew or keep him busy he will find other things to chew like wood and he'll more likely become a cribber and wear his teeth wrong so in his latter years his teeth will be bad where he can't break down the hay before it goes into his stomach and then the nutrients won't get absorbed into his system and then he'll loose weight and ... etc.
Okay you say, I'll feed high quality hay and supplement it with straw for the fiber so he has something to keep his mouth and tummy busy. That doesn't work either for the long run because straw is extremely high in starch and carbs so you'll run into lamitis problems and equine diabetes.
The more I get into what types of feed you should feed your horse the more questions you'll have and there are no definitive answers. You just need to look at what works best for your horse.
What we do is buy hay that hopefully has no mold and isn't that dusty. The early and mid summer is best. The farther you get into winter the more likely you'll run into poor kept hay. Hay kept in a barn is best but most don't have that so it sits outside and continues to get rained on so mold can form. The good news is the longer hay is washed the more the sugars get washed away. However, so do all the other nutrients which is why hay should be fed within a year from when it was cut otherwise the nutrition content of the hay disappears.