Forage is a great thing to give them! I give it instead of pellets
Giving a forage mix and selective feeding are two totally different things.
A forage is dried leaves and flowers. It is given as an enhancement to the hay based diet. Everything is healthy, they search through it but tend to eat it all if you are providing things they like.
It mimics the natural way they search and eat through grasses and leaves.
In human terms it would be like giving a plate of salad leaves, cucumber, fruit.
It is totally different to selective feeding. Selective feeding is when they are fed muesli mixes (which are not recommended) and they then pick out the bits they like. The problem with this is that the ingredients used cause them to pick out the sweetest and therefore unhealthiest bits of what is already pretty unhealthy.
In human terms it would be like giving a plate of chocolate and biscuits