A mixture between a fortified senior grain like purina or triple crown senior and then high percentage of alfalfa hay mix and/or alfalfa hay pellets . And if they don’t have great teeth water it all down so it turns to mash. You can explore supplements, I’ve had great luck with DAC oil and bloom which also are for weight gain. For packability into the mountains Sr grain and pellet hay probably will be the best combination. I would expect them to loose a bit of weight anyways.
Like someone else suggested I’d definitely look into having the vet inject any combination of hocks/stifles/front feet/ knee joints or at the bare minimum a round of Adaquan. The old guys do so well with a bit of this.
If you’re on Facebook you can also look up a ground called Jim the Feed Guy. He’s full of a ton of great information and is one to not promote supplements but science based fortied feeds and it’s worth checking out the conversations over there
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