Wheat, soybeans, buckwheat, and rice all have to be harvested using your sickle. It will not tell you when it is ripe, so...you have to know what the ripe crop looks like. If it is not ripe, then it will just disappear when you cut it with the sickle. If it is ripe, you will get the raw grain.
For soybeans, you can use the raw grain in recipes. For buckwheat you can't. I don't know about wheat or rice yet (haven't harvested any yet).
Appearance-wise - For soybeans, wait until the beans turn BROWN. No, cutting the soybeans early while they are still green will not get you edamame - in this game you have to buy edamame from the store.
For buckwheat, it will get little puffy things on the top that will change colors several times. Wait again until the puffy part turns brown before harvesting.
For wheat...I don't know. I'm gonna check the crops section on Cher's site to see if she's updated a picture of the full-grown crop...otherwise I'll probably just have to keep cutting one every time the appearance changes to what I think might be full-grown, until I actually get the crop, like I did with soybeans and buckwheat.
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Ah, thanks for that. Hmm...looks like the "full-grown" color changes for wheat and rice are less drastic than the ones for soybeans and buckwheat. I'll be sure to watch closely...(I've got some wheat that should be near harvesting soon...)