Coarse limestone does not alleviate the negative effect of a low Ca/P ratio diet on characteristics of tibia strength...

The hypothesis was tested that an increased digestion of coarse compared with fine limestone can alleviate the negative effects of a low dietary Ca/P ratio on the growth performance and characteristics of tibia strength (CTS) in broilers. A total of 1,152 Ross 308 broiler chickens received a standard commercial starter feed from day 0 to 13. From day 14 onward, birds received 1 of 12 diets containing 1 of 6 Ca/P ratios (0.50, 0.75, 1.00, 1.25, 1.50, and 1.75) and 1 of 2 limestone particle sizes (<500 [fine] and 500 to 2,000 [coarse] μm) in a study with a 6 × 2 factorial arrangement of treatments. Total P content was fixed at 5.5 g/kg for all treatment diets. Each treatment was replicated 6 times with 16 birds per replicate pen. On day 20 and 21, twelve birds per pen were randomly selected from 4 of the 6 replicate pens for tibia analysis and digesta collection from different gut segments. The apparent Ca digestibility was higher for fine than coarse limestone in the jejunum (P = 0.043). However, this difference in Ca digestibility disappeared for the low, whereas it remained for the high Ca/P ratios in the proximal (P_interaction = 0.067) and distal (P_interaction = 0.052) ileum. In addition, coarse limestone improved apparent P digestibility in the proximal and distal ileum (P < 0.001) but not in the jejunum (P = 0.305). Regardless of limestone particle size, reducing dietary Ca/P ratio linearly improved apparent Ca and P digestibility in the proximal and distal ileum (P < 0.001). Moreover, decreasing dietary Ca/P ratio linearly (P < 0.001) and quadratically (P < 0.046) reduced the CTS. Reducing dietary Ca/P ratio linearly (P < 0.003) and quadratically (P ≤ 0.006) decreased body weight gain and increased feed conversion ratio. For both fine and coarse limestone, the optimal Ca/P ratio was 1.00 to 1.25 to optimize apparent Ca and P digestibility while maintaining growth performance and CTS. Reducing Ca/P ratio from 1.75 to 1.00 improved distal ileal Ca and P apparent digestibility from 36.6 to 53.7% and 48.0 to 58.3%, respectively. In conclusion, coarse limestone is equally digestible with fine limestone at a low Ca/P ratio but is less digestible at a high Ca/P ratio, and the optimal Ca/P ratio in the diet is 1.00 to 1.25 for both fine and coarse limestone.