For what it´s worth - and I have very little experience of keeping a horse around a normal working day - I think horses can cope with a lack of routine very well. My example :
Our four travelling horses have a routine of not having a routine. They get hard feed when we find it. It could be corn, oats, barley, or a mix. It could be between 2 and 7 pounds of feed, depending on what they´ve worked that day, what they´re going to work the next day, and how muh feed we´ve got left. Salt, vitamins etc are occasionally added. They get fed anywhere between 4pm and 11pm. They have been in situations where they haven´t seen grain for ten days.
Fibre-wise, we look for grazing and / or hay every night. Only once in five months have we not been able to find anything, and on that occasion they were tethered under a tree whose seed pods and seeds they eat. They get alfalfa or grass hay or grass, of all qualities, or dried/green maize stalks, or willow brush, or poplar brush. They aso graze during the day on whatever we find, but not at regular fixed times. If they´ve got hay, they get about a half bale a day - if we´re in one spot for 24 hours, then they get their hay in two batches : if we´re only there overnight, they get it all in one whack.
Not once, in months, have we had the slightest indication of colic. All the horses are healthy and shiny. Once they´re turned out or picketed, they know to eat what´s in front of them. They are extremely unstressed horses...
Four horses, two people. 4000km in to the South American leg of our trip,and now in Brazil. Drop us a line on The Great Horse Trip