BREEDING AND GENETICS Growth Hormone, Thyroxine, and Triiodothyronine Levels of Japanese Quail Selected for Four-Week Body Weight1

ABSTRACT Plasma growth hormone (GH), triiodothyronine (T3), and thyroxine (T„) levels were compared in four lines of Japanese quail (C,P,T, and S) from 1 to 4 weeks of age in two experiments. Lines P and T were derived from line C by 59 generations of selection for high 4-week body weight. Line P was selected under the same nutritional regimen as line C (28 % protein) while line T was selected under a restricted protein (20%) ration containing.2 % thiouracil. Line S was developed as a subline from T by continuing the thiouracil stress but with the birds on 28 % protein ration. Marked differences in body weight have resulted with C line birds being lightest followed by T, S, and P, line birds, respectively. There were significant line, age, and experiment differences in GH. Levels decreased with age in all lines. The GH levels of the largest and smallest (P and C) lines were higher than those of the intermediate-sized birds at 1 and 2 weeks of age but not thereafter. Levels of T3 decreased with age in Experiment 1, but in Experiment 2 they increased from 1 to 2 weeks and declined thereafter. In general, T3 levels of all three selected lines were lower than the control line with S and T being lowest and P intermediate. At 1 week of age, however, T 3 levels of the T line tended to be higher than those of the other two selected lines. Levels of T „ increased with age. The largest line (P) had the lowest T4 levels and the S line had the highest. Thyroxine levels in the C line were intermediate between the P and S line, while those of the T line vacillated from age to age in no apparent pattern. (Key words.- growth hormone ^ triiodothyronine, thyroxine