Effects of dietary-reduced nitrogen (N) and phosphorus (P) on N and P balance, retention and nutrient digestibility of contemporary fattening pigs fed .

The reduction of nitrogen (N) and phosphorus (P) in fattening pigs' diets is one possible approach to lower N and P excretion in livestock farming relative to N and P intake. Due to the implementation of the European Nitrates Directive and the consecutive amendments to the German fertiliser legislation since 2017, - and -reduced diets for fattening pigs are becoming more and more important and are increasingly used in practice. To investigate the effects of such diets on N and P balance and retention as well as on nutrient digestibility of contemporary fattening pigs, a balance experiment was performed with eight barrows (average live weight = 61.5 ± 2.1 kg) which were surgically fitted with a simple T-cannula at the terminal ileum. The pigs received a control diet meeting nutrient requirements (CON) and an - and -reduced diet (NPred) ( = 4/diet) in a 3-phased feeding regimen (3 weeks/phase). In the last week of each phase, faeces and urine were collected quantitatively for 5 days followed by a 2 × 12 hours collection of ileal digesta. Daily feed intake, live weight gain and feed-to-gain ratio did not differ between CON and NPred. NPred-fed pigs consumed 10.5% ( = 0.006) and excreted 28.3% ( = 0.028) less N than CON-fed pigs. Phosphorus excretion was lowered by 15.1% in NPred-fed pigs ( = 0.012). N and P retention did not differ between CON and NPred, but were elevated in comparison to other studies. N and P efficiency, expressed as nutrient retention divided by nutrient intake, was higher in NPred - than CON-fed pigs (N: 68 vs 60%, P: 54.2 vs 49.3%). Apparent post-ileal digestibility coefficient (DC) and apparent total tract digestibility coefficient (DC) of crude protein were higher in NPred - than CON-fed pigs ( < 0.013), but apparent precaecal digestibility coefficient (DC) of crude protein was unaffected by diet. DC, DC and DC of P were similar for CON- and NPred-fed pigs. NPred-fed pigs showed an elevated DC and DC of organic matter, N-free-extractives and starch compared to CON-fed pigs. DC of calcium was also higher in NPred-fed pigs. In conclusion, the results suggest that - and -reduced feeding of fattening pigs remains an effective strategy to lower the N and P release into the environment. Furthermore, results indicate that - and -reduced feeding leads to a higher N and P efficiency in contemporary fattening pigs.