A: I get asked this a lot. Here’s what I wrote not too long ago:
A: It depends on the breed and your goals.
How long do you plan to raise them? It depends on the size you want. Somewhere between 6 and 8 weeks should do it for Cornish. I like big birds, so I do 8 weeks, but they are practically the size of small turkeys by then. By 10 weeks they may have problems from getting too big.
This is very efficient feed with no losses to dust and it is nutrient dense so you can figure on a "feed conversion rate" of around 2.5 to 1 if the weather is decent, more like 3:1 if it is not. What that translates into is, say you grow a bird to a butcher weight of 7 lbs. (That's live weight.) It will eat about 7 x 2.5 lbs. of feed, or about 15-18 lbs. of feed in its lifetime. Worst case would be 21 lbs., best case, 15 lbs.
If they were housed in perfect (inside) conditions) the conversion would be more like 2:1, or 14 lbs. You don't count on the grass for much of their growth. It's more like salad. It makes them healthier, it makes them happier and they like to eat it, but it doesn't put much weight on them. If you like a smaller bird of course it will eat less and be ready younger.
If you had 100 birds, for the season you would use 100 x 18 lbs. = 1800 lbs. of feed (1500-2100). Fifty birds = 900 lbs., etc. How big you want them, the weather, etc. all figures in.
Q: How about Ranger-type birds? How much do alternative breeds eat in a pasture model?
A: Figures based on the various alternative breeds are less well studied and less reliable, because of the many differences in management inherent in pasture based systems in different areas, different climates and so on. The best numbers I have found so far (and I’d love to see more) are from a SARE grant I’ll cite here:
and the graphs are here:
My takeaway from their study was that the Rangers were going to have to live longer, and although they ate about the same amount of feed per day as the Cornish, they ate for more days and so came out eating about 30% more by the time they were harvested. They found that if a Cornish bird used between 15 and 18 lbs. of feed (see above), a Ranger might eat from 19.5 lbs. to 27 lbs. I did the math myself from their figures, so they are not written in stone, and they are based on extrapolation from their data. I think those numbers might be a little high. Chances are, the Rangers will be harvested smaller than the Cornish, so the lower number is probably more accurate. But between weather, management practices and quality of feed, your mileage is likely to vary quite a lot.
That’s my take on it from what I have seen to date.