How do pigs deal with dietary phosphorus deficiency?

Feeding strategies for growing monogastric livestock (particularly pigs) must focus on maximising animal performance, while attempting to reduce environmental phosphorus (P) load. Achieving these goals requires a comprehensive understanding of how different P feeding strategies affect animal responses and an ability to predict P retention. Although along with Calcium, P is the most researched macromineral in pig nutrition, knowledge gaps still exist in relation to: 1) the effects of P feed content on feed intake (FI); 2) the impact of P intake on body composition; 3) the distribution of absorbed P to pools within the body. Here we address these knowledge gaps by gathering empirical evidence on the effects of P deficient feeds and by developing a predictive, mechanistic model of P utilisation and retention incorporating this evidence. Based on our statistical analyses of published literature data, we found: 1) no change in FI response in pigs given lower P feed contents; 2) the body ash-protein relationship to be dependent upon feed composition, with the isometric relationship only holding for pigs given balanced feeds; 3) the priority to be given towards P retention in soft tissue over P retention in bones. Subsequent results of the mechanistic model of P retention indicated that a potential reduction in P feeding recommendations could be possible without compromising average daily gain; however, such a reduction would impact P deposition in bones. Our study enhances our current knowledge of P utilisation and by extension excretion, and could contribute towards developing more accurate P feeding guidelines.