We harvested and have begun eating fennel seed this year, and I think it can play a small but useful part in providing a low-maintenance calorie-dense crop in the perennial food system. We didn't do a great job of measuring how much space our fennel seed sources took up, but we have some estimates. We think we harvested from about 15 plants, and figure they take up a combined total of about 40 square feet. (This gets especially fuzzy when you try to distinguish between space at the base vs space at the top after the stalks have branched out a lot. We interplant a lot of shorter plants with the fennel, so we're estimating more of the base/root exclusion area.) We harvested a total of 35.5 ounces (3600 calories), but we estimate we only harvested about 2/3 of the available seed. So we could have gotten about 5400 calories from 40 square feet, which means 135 calories per square foot...right on par with good yields from greens and roots.
We also harvested an unknown but sizeable bunch of greens from the fennel plants through the season. I also harvested many flowers (perhaps 5% of the total?) for salads, believing mistakenly from my early experiments with eating the fennel seed that the strong flavor meant we wouldn't actually be able to use it in much bulk, so I might as well eat up the flowers.
I can't eat much fennel seed raw; I use it in small amounts as a strong spice or nibble. But I can eat at least a tablespoon (quarter ounce, about 25 calories) in my omelette each day, as long as I put the fennel seed in early so it cooks well. That knocks the strong flavor back a lot. I can sprinkle fennel seed on beef patties before cooking them, or add it in bulk to soups and stews. (Haven't measured exactly how much I've used in these situations, but I'd guess at least half a tablespoon with 1/3 pound of beef.) I've also soaked fennel seed overnight with nuts (hazelnuts, walnuts, sunflower seeds) and eaten them all together with dried fruit the next day. (Again, maybe half a tablespoon at a time with a quarter cup of other nuts.) All in all, I think I can easily average 25 calories a day year round, and if I wanted to push it maybe 50 per day. Not a staple food by any means, but an easy concentrated source of calories. I'm learning the value of that as I find my limits of how many greens or bulky roots I can eat per day. A tablespoon of fennel seed providing the same calories as three cups of greens has a lot of value.
Growing fennel takes no work at all once it gets established. It comes back from a taproot year after year, probably mining some nutrients from deep below, and definitely feeding beneficial insects with its carrot family flowers. You can eat the roots of excess seedlings, though I've had trouble finding a good time to harvest them--young plants have nice enough roots (though very strong tasting, benefiting from cooking) but weigh almost nothing, while older sizable roots have become too tough for me to enjoy. So I'll probably just adopt an approach of over-harvesting to death the greens from unwanted seedlings, leaving more greens on our main plants to maximize seed production.
Our chickens love eating the seed! We had never tried feeding them fennel seeds until this winter, and I wish I'd figured it out sooner. Definitely worth growing a bunch of fennel plants in the chicken yard, then cutting down the heads throughout the fall and winter for the chickens to access.
It took me a while to process the fennel seed. Tulsi had already harvested the seed heads, by cutting them from the stalk and filling a four gallon bucket packed fairly tight. (It maybe took her 15-30 minutes to harvest it all??) I spent about 4 hours total watching movies and stripping the seeds from the seed heads. So I processed about 900 calories per hour. In general, I take much longer to do manual hand-processing work than Tulsi and many other people, so take that figure as an upper end of expected time required. Also, it took me a little time to figure out the most efficient strategy, and even once I did, I had a hard time making myself follow it.
You'll get the quickest results by accepting some losses--strip as much seed as comes off the seed head easily in one or two passes, then move on to the next seed head. The fennel seed you miss will feed the chickens. However, I tend to demand too much perfection in these matters--spending a long time picking the last bit of walnut out from the deep groove, carefully cutting off bad spots from jerusalem artichokes so as to preserve as much good flesh as possible (despite still having 100 pounds out in the yard to work through, more than we'll be able to eat!), and spending 4 times as much time to get the last 4 fennel seeds as I spent getting the first 16 seeds from the seed head. I need to cultivate an attitude of strategic waste; accept that I don't need to extract every last morsel of food from the project at hand, when there's plenty more where that came from, and the chickens have much more combined time on their hands (er, beaks and claws) than I.
I don't know whether fennel seeds contain anti-nutrients, which of course eliminate many seeds (especially grains) from the paleodiet. I would guess fennel seeds rely more on their intense flavor to keep mammals from eating very many of them, so maybe cooking them to mellow the flavor deactivates their main line of defense? And if they do contain anti-nutrients, I would guess that a quarter ounce per day won't constitute enough consumption to really cause problems. But I'd love to see more definitive information on this question.