Stepwise Reduction of Dietary Phosphorus in Diets for Piglets and Fattening Pigs of Different Genetic Origin Housed under Various Station Environments—A Ringtest

SIMPLE SUMMARY: To reduce the impact of livestock on the environment, in modern rations for farm animals the concentration of relevant nutrients is aimed to be lowered. The main nutrients in focus are nitrogen and phosphorus. Because phosphorus is essential for bone development, the degree to which dietary phosphorus can be decreased is limited. There are case reports that pigs indicate leg problems under specific practical conditions, and theories that this might be linked to reduced phosphorus level in feeds are circulating. To verify whether these observations are connected to the nutritionally recommended reduction of dietary mineral phosphorus and to increase the acceptance of these feeding regimes on farmer level, three diets with variable phosphorus levels were fed throughout grower and finisher stages. Performance, bone mineralization, blood parameters and selected mineral transporters involved in P homoeostasis were studied. The results did not show any indication of an insufficient outcome. Thus, it is concluded that the digestible phosphorus supply was sufficient within the range of this study. It is further highlighted that it is essential to decrease the calcium concentration in phosphorus-reduced diets to maintain high P digestibility and availability, which is often neglected under practical conditions. ABSTRACT: The reduction of emissions of nutrients from livestock is one of the main topics in areas with intensive animal husbandry. In order to minimize the loss of nutrients into the environment, it is common practice to feed animals as close as possible to metabolic demands. For phosphorus (P), there are various studies for swine and poultry, which showed that a reduction of dietary P levels is possible, if a sufficient level of phytase is added to the diet. The supplementation of a sufficient dosage of phytase to plant-based diets leads to an increase in digestible phosphorus (dP) upon the hydrolisation of phytate (InsP(6)) to P and lower inositol-phosphates. However, most of these studies were conducted under standardized experimental conditions. In terms of transfer to practical conditions with varying housing, management and genetics, there are concerns that have led to speculation by farmers and veterinarians whether the reduction of dietary P could negatively affect bone health and therefore animal welfare. In order to test whether a reduction of dietary P according to the recommendations for dP of the German Society of Nutrition Physiology (GfE) affects bone mineralization and growth performance, a ringtest was conducted where piglets and fattening pigs were fed at four experimental stations with three centrally produced diets from the same batches. The diets contained three different levels of P and were designed to reflect practical diets. The P level decreased from diet one to three, respectively. Diets one and two were calculated to contain P levels, which are typically fed under practical conditions in Germany. The third diet was optimized to fulfill the requirements of dP by the GfE. The animals were fed in two phases as post-weaning piglets (8–15 kg and 15–28 kg BW) followed by a three-phase fattening regime (28–60 kg, 60–90 kg and 90–120 kg BW). Individual body weight and feed consumption (pen basis or individually, depending on the experimental station) were recorded for every feeding phase. At the end of the experiment, animals were slaughtered. At one experimental station, additional blood serum, metatarsi of the left leg and kidney tissue were sampled to analyze serum P concentration, expression of P transporters in the kidney and bone traits. In two experimental stations, femur and vertebra were sampled, and bone ash was determined. Overall, animal performance and all other traits analyzed did not differ between the treatment with the highest and the treatment with the lowest dietary P concentration. The results demonstrate that it is possible to decrease dietary P according to the recommendations for dP of the GfE, without impairing the animals’ performance or mineral homeostasis and health. A reduction of total P by reducing mineral P to the levels of the present study require the supplementation of phytase to achieve sufficient concentrations of dP.