Source: Zuidhof et al 2014. Editor's choice: Growth, efficiency, and yield of commercial broilers from 1957, 1978, and 2005 - PMC (nih.gov)
The effect of commercial selection on the growth, efficiency, and yield of broilers was studied using 2 University of Alberta Meat Control strains selected since 1957 and 1978, and a commercial Ross 308 strain (2005). Mixed-sex chicks (n = 180 per strain) were placed into 4 replicate pens per strain, and grown on a current nutritional program to 56 days of age. Weekly front and side photographs of 8 birds per strain were collected and the results are shown in this figure:
From 1957 to 2005, broiler growth increased by over 400%, with a concurrent 50% reduction in feed conversion ratio, corresponding to a annual rate of increase in 42 d live BW of 3.30%. Forty-two-day FCR decreased by 2.55% each year over the same 48-yr period. Pectoralis major (chest muscle) growth potential increased, whereas abdominal fat decreased due to genetic selection pressure over the same time period. From 1957 to 2005, pectoralis major yield at 42 d of age was 79% higher in males and 85% higher in females. Over almost 50 years of commercial quantitative genetic selection pressure, the intended beneficial changes have been indeed achieved.