feeding alfalfa to older mare

With a heart murmur involved, your vet is the best person to advise on what she should be eating

I agree with the vet idea.  But why wouldn't you be feeding Senior feed?
I have a VERY hard keeper and Senior feed has worked well.  The vet recommended switching to a complete feed because she feels he can't chew well.  I consulted my feed web site and all I had to do to make it a complete feed was to add alfalfa pellets to what he's already eating. (it's an ADM product - SeniorGlo)  It's rounding them out nicely.  I have one senior besides this horse.  I also have my riding horse who needed to gain a little, acording to one trainer.  He eats HALF of what the other two eat and he's FAT. I also have a 3yr old and she eats the same as my fat horse.  The grazing is waning - I need to start setting out round bales. 
btw, I soak my feed because it's pelleted and I had a choke episode with one horse (not the same feed but still... similar size pellets).  They all get soaked. 

Hi,

Yes, alfalfa is *generally* suitable for *most* horses as a part of their forage ration, & good for weightgain. But I agree I'd be wanting vet's or at least nutritionists advice with her problems.

It is entirely possible that feeding her grain is her problem. It is generally not a healthy feed for horses and high starch/sugar feeds can cause weightloss, among other problems. If you are going to feed starchy/sugary feeds, it is vital to be aware of the problems and to feed little & often, as big &/or infrequent meals per day will be more problematic. More roughage - in the form of chaff perhaps, if her teeth are no longer capable, and some oil in her feed would also help.

It's also possible that if she is not getting a good complete supplement that it's nutrition rather than calories she's lacking.