Abstract:
Ascertaining calcium (Ca) and phosphorus (P) digestibility values remain a challenge in the determination of nutrient requirements of laying hens. In recent years, limited research has been conducted in this area, and there is scope to update literature to reflect the requirements of the modern hen. The present study investigated the effects of time post-oviposition (POP), limestone particle size (PS) and phytase inclusion on Ca and P digestibility, blood ionized calcium (iCa), and eggshell percentage (PE) in commercial laying hens. The treatments were comprised of two levels of phytase (0, 600 phytase units (FTU)) and two particle sizes of limestone (0.25 mm and 1.5 mm) to yield a 2x2 factorial experimental design. Digestibility values were determined at 3h and 11h POP and blood iCa values were determined at (0, 3, 6, 9, 12, 15, 18, 21) hours POP. Ca digestibility was significantly affected by time POP (P<0.01), with a higher Ca digestibility being noted at 11 hours POP during eggshell calcification than at 3 hours POP, during the inactive phase of the shell gland. There was no significant effect of limestone PS and phytase on Ca digestibility (P >0.5). However, there was a two-way interaction between the limestone PS and phytase (P<0.05). Blood iCa was also significantly affected by the time POP. Blood iCa remained high during the shell gland inactivity period and decreased rapidly during the period of eggshell calcification (P<0.01). Blood iCa was also affected by a two-way interaction between limestone PS and phytase. A significant effect of limestone PS on PE was noted, with limestone grit providing an improved PE (P<0.01). P digestibility was significantly improved by the addition of 600 FTU phytase (P<0.001). The time POP affected P digestibility (P<0.01) with P digestibility being greater at 3h POP than 11h POP. The results from this trial illustrate the presence of diurnal circadian rhythm for Ca digestibility and P digestibility. The results from the blood iCa and PE demonstrate that limestone grit improves the Ca status of the hen and that the combination of fine limestone and the addition of phytase enzyme had a positive effect on Ca and P digestibility.
