There are so many sites that warn against feeding chickens cut grass!
As children we always threw mower clippings in the hen yard. Nobody ever mentioned it could cause impacted crop. And as is often the way with knowledge, ignorance seemed to protect our chickens.
Now some people argue that if you mow your lawn, you shouldn't even leave the clippings lying around during free range periods in case your birds swallow some! Best to catch all those lawn clippings in a catcher and... what? Chuck them in the green bin for council to deal with, I expect. They certainly don't break down well unless they're mixed with other material to stop them forming a mat. But do they form a mat inside a hen?
I don't know. Perhaps it can. But the question I want to ask is: can people who claim this really be sure they're not seeing a combination of problems, rather than cut grass being the entire problem? It's not as if anybody has done studies... In fact the only evidence I've read for this (like the 'rest your eggs for 24 hours before incubating' idea) is anecdotal. Even superstitious! (Don't leave your eggs for 24 hours... Put them in the incubator as soon as you can. If it takes the core of an egg 24 hours to stop moving after being transported, then when you place them in the incubator you'll only be setting them in motion again... So set, set, set!)
Ahem... Back to grass. I've never used a catcher when mowing my lawn. Frankly, what a chore! Much easier to let the stuff lie while it dries out, then harvest the dried clippings for nestbox material or floor litter in the shed. The chook poop will mix through the stuff and it becomes extremely good compost. Meanwhile I've often let chooks out to range over the cut lawn, and never seen impaction. That's in 10 years of ranging my birds.
I've also thrown the stuff into the chook pen to cover up the range of sins chook pens are prone to... No problems with impaction there either. Then again, I always supply other greens such as spinach, so why in blazes would a hen prefer horrid scratchy mower-oil-infused grass?
But still... I'm sure they do eat some.
I suspect there are other factors at work in impaction cases. How many of those who warn against using it have no qualm with using antibiotics? Yet antibiotics are known to sometimes cause a breakdown in a chicken's digestive system, including the crop. I wonder whether a history of antibiotic use might predispose a hen later (because of upset or the wrong bacterial balance) to impaction when she eats grass?
But what if there's a purely mechanical reason? Feeding fine-particle foods arguably helps stop the birds' gizzards (the muscular stomach that grinds food) from developing the necessary strength to handle tough foods. Certainly flabby gizzards can cause digestive problems and may contribute to impaction. Once a bird's gizzard has failed to pass something, the blockage can easily back up to the crop, resulting in 'stasis' (or inactivity). (Pellets are of course only fine-particle feeds turned briefly solid by the addition of a hardener. Once in the crop, they tend to break down and partially dissolve.)
By contrast, feeding hard whole seeds like wheat and corn actively strengthens the muscles of the gizzard. This is one of the reasons why whole seeds are introduced slowly to chicks, so their gizzards have time to muscle-up. Now, I'm not saying gizzard and/or general digestive weakness is the main cause of impaction, or that feeding lawnmower clippings is harmless, but I do say that these questions don't appear to have been researched much by those who say you should never feed cut grass.
Have I ever had a case of impaction in my chickens? Why, yes. A single hen arrived here with a bad habit of feather-eating. She didn't eat what the other chickens ate, but pretty much survived on feathers. Eventually her digestive system stopped working, and on autopsy I found a blockage in the proventriculus caused by a bent, stuck feather shaft. So in essence it wasn't crop impaction that killed her, but a general stasis caused by a blockage in the tube that leads into the gizzard. However it did present as crop impaction, with the crop not seeming to empty properly (nor for that matter to fill properly, either). She hadn't been eating any grass.
At the same time, it's probably never a good idea (no matter what your chickens' gizzard health) to confine birds and give them mowed grass as their only green feed. Chickens naturally love grass and may try to consume too much in one go. Heck, maybe that might even cause impaction.
But I'd say it's still a very open case...