I have a 5 month old weanling colt. He’s on pasture grass and I feed him some alfalfa every couple days. I also feed him some grain. Can I feed him coastal (yellow) hay free choice, and add alfalfa in when needed?
This is a uk forum and reading your terminology would mean I assume you aren’t uk - so most of us won’t know the nutritional profiles of what is available to you. What you feed will be dependent on breed, body condition and the nutritional profiles of your fibre sources.
Here the general advice would be grass/meadow hay(lage) plus a Youngstock vit/min supplement/balancer as a starting point.
But grass is usually a good starting point and I suspect a little Alfa hay will be fine (but why not daily). No clue what yellow costal hay is