Comparison of embryo physiological parameters during incubation, chick quality, and growth performance of three lines of broiler breeders differing in genetic composition and growth rate.

In broiler breeder management, stringent feed restriction is practiced to reduce body size in order to improve egg production and meet broiler production demand, but this practice has raised welfare issues. The potential for the dwarfing (dw) gene to reduce feed intake and body size of breeders under ad libitum feeding or less stringent restriction while maintaining improved egg production has been reported. In this study, we compared embryo physiology, quality of chicks, and performance of broilers from eggs of dwarf breeders with those from a standard broiler breeder. Hatching eggs from 3 commercial lines of broiler breeders were compared for incubation parameters, 1-d-old chick weight, chick quality, and broiler growth to 41 d of age. The lines included a standard heavy (S) line, an experimental (E) line, and a label-type (L) line. The E and L line breeders carry the sex-linked dw gene and are being used to assess the potential for dw to reduce feed intake or lower feed restriction and improve reproductive performance in heavy female broiler parent stock. Two separate experiments were conducted. All female parent stocks were mated to Cornish males, and fertile eggs were collected. In the first experiment, eggs were incubated for 21 d under standard conditions to determine, during final stages of incubation, corticosterone and thyroid hormone levels (triiodothyronine, T3; thyroxine, T4) in embryos and hatchlings, CO2 partial pressure (pCO2), and O2 partial pressure (pO2) in air cells, heat production by eggs and 1-d-old weights. In the second experiment, eggs were incubated for 21 d to compare chick quality, chick weights at 1 d of age, and broiler growth to 7 and 41 d. Average egg weights were higher for the S and L lines than the E line, but weight loss during incubation was lowest for the E line. Plasma T3 and T3/T4 ratio was similar between lines at IP, but corticosterone was higher in the S line. At hatch, T3/T4 ratio was higher in the S line compared with the E and L lines, but corticosterone was higher in the S and E lines than in the L line. Heat production by embryos was different among lines (S > E > L). The pCO2 was also higher in the S line than the E and L lines. These incubation parameters suggest different metabolic rates among lines (S > E > L). Incubation duration was shortest for the S line. Chick weights at 1 d old were not different between lines. Chick quality scores were also not different when expressed as a percentage of high-quality chicks or as an overall average score of each line. However, broiler BW at 7 and 41 d were different among lines (S > E > L). Chicks of higher quality (score of 100) in all lines had higher BW than those of lower quality (score of < 100). For corresponding quality groups between lines, the S line had higher BW, and those of the L had the lowest. These data suggest a link between the levels of embryo metabolism and growth potentials of the lines. We concluded that the dw gene has potential for reducing feed intake in heavy broiler parent stock (as in the E line), improving reproductive performance (as in the L and E lines), and maintaining progeny broiler chick weights at 1 d of age and quality but with lower weight at slaughter.