Table 7.

Thickness and strength of the eggshell and the yolk color score of the eggs produced by laying hens fed rations containing different concentrations of caffeine for 12 weeks.

Caffeine in diet	Experimental period
	4 weeks	8 weeks	12 weeks
Thickness of eggshell (in mm2)
0 (control)	39.4 ± 0.63a	39.6 ± 0.48a	39.6 ± 0.54a
150 ppm	38.0 ± 0.36b	37.5 ± 0.37b	35.6 ± 0.66b
300 ppm	36.58 ± 0.44b	36.5 ± 0.39b	35.0 ± 0.68b
450 ppm	36.5 ± 0.59b	36.1 ± 0.65b	35.6 ± 0.73b
p	<0.0001	<0.0001	<0.0001
Strength of eggshell (in kg/cm2)
0 (control)	5.19 ± 0.17a	4.41 ± 0.16	4.88 ± 0.24
150 ppm	3.33 ± 0.16b	4.21 ± 0.21	4.64 ± 0.20
300 ppm	3.27 ± 0.11b	4.20 ± 0.17	4.21 ± 0.19
450 ppm	3.53 ± 0.15b	4.49 ± 0.19	4.35 ± 0.21
p	<0.0001	n.s.	n.s.
Yolk color score
0 (control)	7.25 ± 0.14	6.37 ± 0.12b	6.00 ± 0.20c
150 ppm	7.08 ± 0.13	7.25 ± 0.12a	6.58 ± 0.16b, c
300 ppm	6.96 ± 0.09	6.96 ± 0.09a	7.04 ± 0.23b
450 ppm	7.04 ± 0.13	7.04 ± 0.13a	7.25 ± 0.23a
p	n.s.	<0.0001	<0.001

Data are shown as mean ± SEM.

^{a, b, c}Different letters in the same column show significant difference (p < 0.05, Kruskal-Wallis test followed by Student-Newman-Keuls test).

n.s., non-significant.

n = 24.