The broiler industry has come a long way in the past four decades. Today we can grow a bird to 4.7 lbs in 42 days, whereas, 34 years ago it would have taken 100 days just to get to a weight of 3.5 lbs. Part of the reason for this dramatic increase in growth rates is improvements in poultry nutrition. For instance, if we took a flock of birds from the 1950's and fed them a 1990's diet their growth rate would be 20 percent higher than birds fed a 1950's diet. The main reason, however, for the change in rate of weight gain is improved bird genetics. If you were to feed a modern bird a 1950's diet it would reach a weight of 3.9 lbs in 42 days, an increase in growth rate of 350% (Havenstein et.al, 1992).