There are a whole lot of different products out there. I’ll group them into groups but there can be differences in each group. It just depends on what is available in your area. Other than Layer, the only real difference is in the percent protein.
Starter – Around 20% to 24% protein
Grower – 16% to 18% protein
Layer – Usually around 16% but with extra calcium
Combined Starter/Grower – Usually around 20% protein. Some is 18%
Grower/Developer = Usually 15% protein. Intended for weeks 13 until you switch to Layer
Flock Raiser – Usually around 20% protein. Supposedly for a flock that has both future layers and future meat birds (not necessarily Cornish X but dual purpose birds), but it can be fed to a pure laying flock.
What the different brand call their product can vary too. Instead of Flock Raiser they may call it All Flock or something like that, for example.
The normal progression is Starter for between 4 to 8 weeks, then switch to Grower, then when they start to lay, switch to Layer. The extra calcium in Layer can be harmful to growing chicks so it is best to not feed Layer to growing chicks. One bite of Layer won’t kill them, but if they eat the extra calcium in Layer for a time period, it can damage their organs, like liver and kidneys. There are studies where they cut the chicks open to look at their organs that show this. But a few days on Layer won’t hurt them. It takes a while for the effect to build up. If they forage for a lot of their food, it will take even longer if at all. The percent protein is not what hurts them. It's their total daily intake of calcium that is important.
Another possible difference is that some Starter is medicated. Not all, but some. Every major brand I’m aware of that provides medicated feed for chicks that will be a laying flock medicates with Amprolium. I haven’t seen it myself but I’ve seen posts by people on here that I consider reliable that some feed for broilers can contain other medications.
I’m not going to get into broilers, but for chicks that will part of a laying flock, it really does not matter that much which feed you feed at any stage, other than the problems with the calcium in Layer. The general idea I use is to feed them a higher protein feed their first month or two, then switch to a lower protein feed once they have gotten a good start. They really don’t need the extra protein to develop their bodies since their skeletons and internal organs will mature at a slower pace. But many people on this forum feed differently and do fine.
There is not really much difference in Starter/Grower and Flock Raiser, except you might be able to get Flock Raiser in pellet form. The different forms of the feed, mash, crumbles, or pellets have nothing to do with nutrition. Those different forms are mainly available because different automatic feeding systems work best with different forms of feed. You do need either mash or crumbles for baby chicks. The really young ones cannot handle the pellets.
I don’t worry too much about which feed I feed the first few weeks except that it be at least 20% until they feather out. That’s about 4 to 5 weeks. Then, when the bad of Starter runs out, I switch to a lower percent protein feed. I’m happy with anything between 15% to 20% protein. They’ll grow faster on a higher protein feed, but faster is not my goal. I want them to mature as they grow.
A lot of people like to make this complicated. It’s really not. Avoid the Layer until they start to lay or reach 20 weeks because of the calcium. That’s basically the rule. Anything else is up to you.
When I have a mixed age flock, which is most of the time, I feed either Grower or Starter/Grower or Developer to all of them. The chickens will not self-regulate. I offer oyster shell on the side. The ones that need the calcium for their eggs will normally eat it. The ones that don’t need it may taste it, but they don’t eat enough to harm themselves.
