Growth-associated traits in parental and F1 populations of chickens under different feeding programs. 1. Ad libitum feeding.

Two experiments were conducted to compare body composition, plasma concentrations of glucose, lipid and protein, gastrointestinal tract (GIT) morphology, and digestive enzyme activities among populations of chickens exhibiting wide differences in growth. In the first experiment, males from a high weight (HW) line, a low weight (LW) line, and crosses between the lines (HL) were examined. In the second experiment, the same high weight (HW') line, a White Leghorn (LEG) line and their crosses (HLEG) were compared. In both experiments, comparisons were made at both 25 and 61 days of age. The HW chicks were heavier and had larger breasts and longer shanks than the LW or LEG chicks. Although the crosses (HL, HLEG) exhibited values for these traits intermediate to those of the parental lines, HLEG chicks tended to resemble one of the parental lines more closely than did the HL chicks. Differences among all mating combinations were significant for body fat, but the crosses (HLEG, HL) tended to more closely resemble their HW parental line. The HW chicks had significantly higher concentrations of glucose, lipid, and protein in plasma at 25 but not 61 days of age. No differences in glucose, lipid, or protein concentrations in plasma were observed in the second experiment. When expressed as a percentage of body weight, LW and LEG chicks tended to have larger GIT than HW chicks, whereas crosses more closely resembled the HW line. Although significant differences in digestive enzyme activities were obtained, these differences were generally associated with differences in body weight. It was concluded that genetic variation in the traits examined was dependent on both population and age.