Thyroxine Secretion Rate and Growth In the White Pekin Duck

THE phenomenally rapid rate of growth of the White Pekin duck is well known. Reported average weights at nine weeks of age are 4.85 and 5.77 pounds (Horton, 1928; Elford, 1925, 1927) as compared to 2.00 and 2.60 pounds (Hoffmann, 1946; Klein, 1949) for relatively rapid growing chickens. The marked difference in growth rate between the species is also reflected in the maximum size attained. A White Pekin drake attained the remarkable weight of 10.5 pounds at 68 days of age (Hunter and Scholes, 1943), as compared to a New Hampshire cockerel that weighed 5.12 pounds at 77 days (Jull, 1949). Recent work with chickens has established a relationship between rate of growth and thyroxine secretion. Fast growing strains have an inherently higher thyroxine secretion rate than slower growing strains (Glazener and Shaffner, 1949). The question naturally arises whether differences in growth rate between species can also be partially accounted . . .