Table 9.

Egg quality of Chinese Yellow broiler breeders fed diets with different copper contents.

Variable	Dietary Cu content, mg/kg						SEM1	P-value3
	3.5	8.5	13.5	23.5	43.5	83.5		Cu level	Linear	Quadratic
Egg shape index	1.31	1.28	1.32	1.29	1.32	1.30	0.012	0.331
Shell strength, kgf2	3.82c	4.44a,b	4.58a	4.42a,b	4.18b	4.14b,c	0.107	0.001	0.509	0.293
Shell thickness, mm	0.345b	0.353a,b	0.358a	0.355a	0.363a	0.339c	0.004	0.001	0.066	0.001
Eggshell proportion, %	9.05	9.09	9.35	9.17	9.17	9.09	0.108	0.433
Yolk color score	4.02c	5.07b	6.70a	5.23b	4.93b	5.13b	0.296	0.001	0.972	0.673
Albumen height, mm	4.61b	4.70b	6.53a	4.42b	4.88b	4.96b	0.392	0.010	0.788	0.958
Haugh unit	66.1b	66.7b	79.5a	63.0b	67.6b	69.8b	2.75	0.006	0.984	0.966

^a-cMeans within row with no common superscript differ (P < 0.05).

¹SEM (n = 6).

²kg-force.

³Linear and quadratic contrasts examined only when Cu level was significant.