Low-phosphorus diet: Maintenance of growth performance and mitigation of fecal phosphorus excretion in Xia-Nan cattle

This study was to evaluate how low phosphorus (P) diets affect growth performance and fecal P excretion in Xia-Nan cattle and to elucidate the underlying mechanisms. Thirty-two healthy Xia-Nan bulls with similar body weights (BW; 267.42 ± 45.77 kg) were assigned to four groups with eight individual replicate pens per group in a completely randomized block design, stratified by initial BW and average dry matter intake (DMI). Cattle were fed total mixed rations (concentrate-to-forage ratio 40:60) with varying dietary P levels (DM basis): 0.16% (XPA), 0.19% (XPB), 0.22% (XPC), and 0.25% (XPD; control) for 90 d. The first three groups were considered low-P treatments. Compared with the XPD group, XPA showed significantly lower average daily gain (ADG; P = 0.018) and DMI (P = 0.046), while XPB and XPC showed no differences (P > 0.05). XPB group showed a significant lower feed conversion ratio (FCR) than XPA (P = 0.049) and had the lowest FCR among all groups. Total-tract apparent digestibility of calcium (Ca; P = 0.006) and P (P = 0.006) decreased linearly with dietary P reduction. P intake (P < 0.001) and fecal P content (P < 0.001) were significantly lower in low-P groups than in the XPD group and decreased linearly with P reduction. Serum concentrations of parathormone hormone (PTH; P < 0.001), 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃; P = 0.005), Ca (P < 0.001), and P (P < 0.001) were positively correlated with decreasing dietary P levels, whereas calcitonin (CT; P = 0.016) and alkaline phosphatase (ALP; P = 0.006) were negatively correlated. XPB and XPD groups had higher molar proportions of propionate (P < 0.001) and butyrate (P = 0.006) but lower acetate molar proportion (P < 0.001) and acetate-to-propionate ratio (P < 0.001) than XPA. Fecal P excretion correlated positively with Ruminococcaceae_UCG-005, Saccharofermentans, and Butyrivibrio, and negatively with Rikenellaceae_RC9_gut_group and uncultured_bacteriumf_F082. These results suggest that reducing dietary P to 0.19% (XPB) maintained optimal growth performance while reducing environmental P pollution in Xia-Nan cattle.