We
keep a close eye on our flock, and I’ve started noticing that chickens
don’t hatch from the egg and grow steadily until they reach full
size. Instead, just like humans, they go through a series of
growth spurts.
My obsessive
note-keeping on feed
conversion rates as we
slaughtered our broilers confirmed my observation. Just like last
year, I got the best feed to meat ratio from birds just shy of or right
at 12 weeks. As our pullets and cockerels reached the end of
their third month of life, they were growing so fast that I felt like I
could see a difference in size every day. Just a week later,
though, the chickens hadn’t grown at all, which set my feed to meat
ratio plummeting. No wonder heritage breed broilers were
traditionally slaughtered at or before 12 weeks — that’s simply the
economical way to raise them.
After extensive internet
searching, I compiled the data above, showing
the growth rate of various breeds of chickens over time. Cornish
Cross, of course, is the primary commercial broiler breed, Paraiso
Padres is a Brazilian broiler, and ISA is a commercial brown egg layer.
What I found interesting about
the chart is that you see the growth
curve peak at 6 to 10 weeks for each type of chicken. This is why
the big chicken farmers kill their broilers at 6 weeks — their
chickens have reached the peak of their growing curve and will probably
start eating more feed for each pound they put on in the future.
Of course, with heritage breeds (and especially if growing layers for
meat), you have to weigh the con of lower feed to meat conversion rate
with the pro of a heavier bird that’s more worth your while to
slaughter and dress.
What I don’t know is
when (or if) further growth spurts occur, and
whether if we waited until the traditional fryer age (14 to 20 weeks),
grain conversion efficiency would rebound. I suspect that people
grew chickens to the fryer stage not for efficiency, but to allow the
characteristics of individual birds to become clearer so that they
could cull those they were less interested in from the flock, but
perhaps chickens go through another growth spurt?
I am curios how a bird like a jersey giant would “weigh” in in the chart?
Interesting question. I suspect they’d line up with other heritage breeds — even though they get big, my understanding is that Jersey Giants don’t grow any faster than other non-Cornish Cross. However, I don’t have any personal experience with them and would love to hear from someone who does.