Do you know the fat content of what you feed your chickens?
This sort of question only makes sense if one is giving a homogenized feed.
If what's offered is diverse, and the chickens can select what they want to prioritize from it (given that they're competing with each other, and there will be more choice at the start and less at the end of the feeding session), then each chicken may be getting quite different 'content', each day. On the fat front a particular issue is the mealworms, which supply is very variable, because (a) I make a point of offering them to anyone who hangs round the back door alone (generally a sign of needing a bit of TLC and/or food boost), chicks, moulters, etc., (b) because some of them hog the bowl when mealworms are the top dressing, so they get more than others, and (c) some of them dash off with their prize to eat it quietly somewhere, so they get less. I also give pork fat sometimes, and the same variability applies. The pork fat is what melted during grilling or roasting of bacon or pork btw, scooped up from the pan and usually spread on some bread as a carrier. They love it.
And then there's the intrinsic variability of the feed. The figures cited are always averages, as if any variety of any crop X grown on any soil in any weather at any time anywhere in the world was the same as any other, which is rubbish of course. It's fairly well recognized that some soils are deficient in a, b or c (e.g. selenium) so anything grown in them will be too, but that variability actually applies to every nutrient in every crop. Less well known is that different varieties of 'the same' crop can vary hugely in their nutrient uptake and content, particularly of the micronutrients that matter. When I realised that, I stopped calculating.
I don't know where you got your figures for the amount of fat in earthworms, but the studies I've seen on the amount of fat in mealworms, black soldier fly larvae, crickets etc. are not consistent, and again, it depends some on what they ate, in the same way as a plant's nutrients depend on what is in the soil it grew in. To get consistency you have to grow them in/on a blank slate and control all the inputs: that's industrial farming, which is not where I want to go, and certainly not just to 'know' i.e. get some precise figures.
So I accept that there will be variability, in what I offer, in what each chicken needs at any given time, and in what each chicken actually gets, and I trust that if I offer a diverse range of real foods, they will each select what they instinctively know they need there and then. The proof of the pudding is the health and fecundity of the birds.
On fats specifically, and the see-sawing of scientific opinion about which fats are good, bad, or other, you might find this interesting
https://zoe.com/learn/carbohydrate-fat-protein-macronutrients
https://zoe.com/post/fat-quality
https://zoe.com/learn/how-many-grams-of-fat-per-day
I essentially do this at every feed; I drain off the liquor (which might or might not get used for something else) and refill the jar with fresh clean cold water. There's usually enough old liquor left in there and on the remaining grains to kick-start the next batch, and if not, I mix in a teaspoon of plain natural live yogurt (i.e. naturally containing lactobacilli plus or minus other good bacteria) or sourdough starter.
went off, there was no doubt. It smelled so foul
I had it happen once when I was starting out fermenting, and as you say it's obvious it's gone bad. Our senses are reliable guides - and even if ours aren't, the chickens' are, so a fail-safe policy is, if they turn their noses up at something, take it away and give them something else that they do want to eat, and no-one will come to any harm.
It really makes me cross when I see some people on BYC telling others to make their chickens eat something they obviously don't want to by stopping 'treats' and taking away any alternative until their poor chickens do eat it. Starving chickens, like people, will eat anything; bad food/feed, leather, and each other.
