Temporary suspension of mineral phosphorus reduces mobilizable bone zinc in adult laying hens irrespective of the dietary zinc supply

This study examined the effects of short-term dietary zinc (Zn) and phosphorus (P) variations on the mobilizable bone Zn pool and overall Zn status in adult laying hens. Forty-eight hens (50% Lohmann Brown Classic, 50% Lohmann LSL Classic) were housed in pairs (one hen per breed per pen) across 24 pens. The pens were randomly assigned to one of two dietary P levels (0.37% or 0.84% in DM) using a high-protein corn-soybean diet (11.4 MJ AME/kg, 21.5% CP) during a 14-day acclimatization period. Following acclimatization, pens from both P groups were further randomized into four dietary treatments in a 2 x 2 factorial design, varying in P levels (low vs. high) and Zn supplementation (28 vs. 131 mg/kg) over an 8-day experimental feeding phase. Performance metrics, egg production and quality, and tissue mineral concentrations (plasma, liver, bone, and eggs) were measured. Statistical analyses were performed using linear mixed models in SAS 9.4, incorporating random effects of pen nested within treatment group and fixed effects of dietary P, dietary Zn, breed, and their interactions. Tukey-corrected 95% confidence intervals were used to estimate effect differences, with significance set at P < 0.05. Performance metrics, including egg production and body weight, were unaffected by dietary treatments (P > 0.1), indicating no clinical symptoms of Zn deficiency. However, hens on low-Zn diets exhibited significant reductions in plasma Zn concentration (-0.83 mg/L; P = 0.0008) and liver Zn concentration (-6.78 mg/kg DM; P = 0.01), confirming subclinical Zn deficiency. Low-Zn diets also increased the femoral molar Ca:P ratio by 0.15 (P = 0.01), irrespective of dietary P supply. Interestingly, low-P diets led to a significant reduction in femur Zn content (-0.46 mg; P = 0.0009), regardless of Zn supplementation, following 21 days of reduced P feeding. These findings highlight the higher susceptibility of laying hens to phytate antagonism compared to broilers, as evidenced by measurable subclinical Zn deficiency under short-term Zn deprivation. Additionally, a temporary suspension of mineral P supply appeared to impair the mobilizable bone Zn pool. The underlying functional mechanisms driving these interactions remain unclear and warrant further investigation.