Digestibility of phosphorus in laying hens fed a wheat-maize-soyabean diet and the excreta phosphorus fractions

Availability of phytate phosphorus (P) was determined in ISA Brown hens 21 and 47 weeks old, fed a wheat-maize-soyabean diet, without a phytase supplement. The diet contained total and phytate P at 4.99 and 1.58 g/kg, respectively. The average body weight and hen-day egg production were 1.56 kg and 0.52, and 1.92 kg and 1.00 for younger and older hens, respectively. Phytate in the feed and faeces was determined by a capillary isotachophoretic method. Availability of phytate P and proportion of dietary Ca retained was non-significantly higher in older hens than in younger ones (47.3 and 56.7% vs 38.7 and 44.4%, respectively). Amount of P retained was similar in both groups (199 and 194 mg/day in younger and older birds, respectively). Amount of Ca retained was significantly higher in older than in younger hens (2.87 and 1.69 g/day, respectively; P<0.05). Dry matter of excreta of 47-week-old hens contained significantly (P<0.05) more total and phosphate P than excreta of 21-week-old hens (14.8 and 8.9 mg/g vs 12.3 and 5.9 mg/g dry matter, respectively). Phytate P represented 30.9% of excreta P in younger and 23.0% in older hens. Corresponding proportions of phosphate P were 48.0 and 60.1%. Differences between younger and older hens in proportions of P fractions were statistically significant (P<0.05). Excreta of 21-week-old hens and those of 47-week-old hens contained 78.1 and 63.0 mg of Ca/g dry matter, respectively (P<0.05). Thus, the age and different egg production significantly affect retention of Ca and excreta composition in laying hens. The effect of age on phytate availability was less pronounced.