I introduce my chicks to greens starting at about 3 weeks. They don't really eat them until 4 or 5 weeks, until then they are just playing around with them for the most part. Chickens are very wary of new foods. They are not at all good at throwing up, and they even have a hard time spitting something out once it's in their beak. So they are quite slow to try something totally new. It often takes days or even weeks to get them interested. The earlier in life you can introduce new categories of food, the better. Start with pieces that are definitely smaller than they are, even pieces that they can easily fit in their beaks and swallow. First they will look at it with one eye then the other. Eventually someone will get up the courage to peck it. They will probably take one taste and wait a while. If it makes them feel ill they won't eat it again. If it makes them feel good they will come back for a little more. After a while it will be something they remember eating and liking. Once they like one type of fruit, they'll be more likely to try another kind of fruit. Once they like one type of greens, they're more likely to try another kind of greens. Whatever type of food they are used to, they will be more likely to try something that looks only a little different. Also you might try bringing in new foods in the afternoon. They've been eating their usual food all morning, now they are a bit bored and maybe more likely to try something new.