Growth hormone and prolactin levels in nonselected and selected broiler lines of chickens from hatch to eight weeks of age.

Abstract

Plasma growth hormone and prolactin levels were measured in two lines of commercial broiler chickens (lines 2 and 4) and two nonselected randombred broiler lines (lines 1 and 3). The body weights of the 4 lines differed by only a few grams at day of age but by 1 week of age the selected lines were 60-80 g heavier than the unselected lines. At 8 weeks body weights of the selected lines exceeded 2000 g while the unselected lines were approximately half this. At all bleeding times between 1 and 8 weeks of age GH levels of both nonselected lines were significantly higher than those of both selected lines. GH levels of the two selected lines did not differ from each other, nor in general did GH levels of the unselected lines differ from each other. In both experiments GH levels of the unselected lines increased between 1 and 3 weeks then dropped week by week. In contrast, GH levels of the selected lines were rather stable from 1 to 3 weeks and gradually declined thereafter. Prolactin levels of the unselected lines tended to exceed those of the selected lines, but the differences only achieved significance at 2-3 weeks of age. GH levels at various ages were found to be significantly correlated to relative growth at different time periods. These significant correlations were generally positive and occurred most often in the selected broiler lines. Overall analysis of variance showed a significant relationship between GH levels and growth in Expt. 2, but not in Expt. 1.