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Optimizing phosphorus requirement for nursery, growing-finishing pigs

Results from these four studies suggested that feeding excess dietary Ca negatively affected growth performance and bone ash concentration of nursery pigs when diets are deficient in digestible P.

Researchers: C.M. Vier, F. Wu, S.S. Dritz, M.D. Tokach, J.C. Woodworth, R.D. Goodband, M.B. Menegat, H. Cemin and J.M. DeRouchey, Kansas State University; M.A.D Gonçalves and U.A.D. Orlando, Genus PIC  
Phosphorus is essential for growth performance and development, and maintenance of the skeletal system. Besides bone mineralization, phosphorus is involved in different biological functions, such as energy metabolism, synthesis of nucleic acids and structure of cell membranes.

Yet, phosphorus is considered the third most expensive nutrient in swine diets after energy and protein. P supplementation is typically associated with lower safety margins in swine diets compared to calcium (Ca). This is likely driven by environmental and economic concerns.

Diets formulated with excess P can lead to increased P excretion, negatively impacting the environment. Coupled with the cost associated with P and environmental concerns, swine diets are typically formulated to avoid excess P, with low margins of safety.

Understanding P requirements of pigs and the impact of phytase and calcium level on P requirement helps producers achieve optimal growth performance and economic returns. It also helps reduce phosphorus output from pig production and alleviate environmental concerns.

K-State research
A research team at Kansas State University investigated the phosphorus requirements and the impact of calcium and phytase on the phosphorus requirement of nursery and growing-finishing pigs in four studies.

Two titration studies were done to determine the digestible P requirement of nursery and growing-finishing pigs for growth performance, bone mineralization and economics.

For nursery pigs from 25 to 50 pounds, the estimated digestible P requirement ranged from 0.34% to at least 0.53%, depending on the response criteria and statistical model used, which indicates that the National Research Council (2012) requirement estimate is lower than that needed to optimize performance and economic return.

For growing-finishing pigs from 53 to 287 pounds, the digestible P requirement ranged from 116% to 131% of the NRC recommendations for each phase, depending on the response criteria and statistical model considered.

Elements’ effect on growth
Two studies investigated growth performance and bone ash concentration of 13- to 25-pound nursery pigs in response to different combinations of dietary digestible P and Ca levels provided by inorganic P or phytase.

Appropriate dietary Ca and P concentrations are essential for nursery pig performance. Accurate formulation of Ca and P is even more important with the increased use of phytase and the desire to reduce P excretion in recent years. Research has shown that feeding excess dietary Ca impairs P digestibility and, therefore, reduces growth performance and bone ash concentration in nursery pigs.

Previous research has suggested that high concentrations of Ca reduce P digestion and absorption by forming a Ca-P complex, reporting a decreased growth performance observed for pigs from 24 to 55 pounds when more than 0.50% standardized total tract digestible (STTD) Ca (about 0.74% analyzed Ca) was added to diets containing adequate STTD P.

But observations from the K-State study, “Effects of dietary calcium and phosphorus concentrations and addition of phytase on growth performance of nursery pigs,” indicated that the detrimental effects of excess Ca were only observed when diets also contained a deficient P concentration.

Supplementing phytase with low P diets improved pigs’ growth over those fed sufficient STTD P without phytase. It is possible that the 0.12% P and Ca release value for 1,000 FTU of phytase underestimated the true digestible P and Ca release, resulting in more Ca, P or possibly other nutrients becoming available to the pig.

Also, this phytase response was more evident during Phase 1 of the experiment, when dietary P would have been more limiting than at Phase 2.

High Ca concentration has been demonstrated to decrease P digestibility and absorption by forming a Ca-P complex. In the previous study above, we observed that feeding excess Ca (1.05 vs. 0.58%) decreased pig growth performance when diets were deficient in P, but not inadequate P diets (0.45% STTD P during phases 1 and 2).

Results from the second study, “Effects of dietary Ca and P concentrations on growth performance of 13- to 25-pound pigs,” suggest feeding high Ca diets worsens the feed-to-gain ratio (F/G) independent of STTD P concentrations in Phase 1. But in Phase 2, the detrimental effects of high Ca on average daily gain and F/G were only observed in pigs fed 0.40% STTD P (NRC) but not for pigs that received 0.52% STTD P (> NRC).

It is possible that 0.40% STTD P just met, or was marginally below, the requirement of pigs during Phase 2, which resulted in a P deficiency when high Ca was added to the diets. This marginal deficiency in STTD P was also supported by the observation that feeding high levels of STTD P (>NRC) tended to improve ADG from day 0 to 24.

From day 24 to 45, all pigs received a common Phase 3 diet. No interactive or main effects of Ca and STTD P concentrations were observed for ADG, ADFI or final BW. However, pigs previously fed increasing dietary Ca had improved (linear, P = 0.003) F/G regardless of the STTD P content previously fed in Phase 1 and 2 diets.

As a result of this compensatory gain, overall (day 0 to 45) growth responses were not affected by the Ca and P concentrations fed during phases 1 and 2.

Takeaway for the pig farmer
Results from these four studies suggested that feeding excess dietary Ca negatively affected growth performance and bone ash concentration of nursery pigs when diets are deficient in digestible P. The digestible P concentrations estimated by NRC (2012) met the requirement of nursery pigs when diets contain low Ca concentrations, but resulted in decreased growth performance when diets contained more than 0.90% Ca. Moreover, adding phytase to P-deficient diets improved pig performance and alleviated the negative impacts of high dietary Ca on growth. 

Here are some key takeaways from the studies:

• NRC (2012) may underestimate the digestible P needs, resulting in suboptimal growth performance of pigs in all production phases.
• Overfeeding Ca negatively affects pig growth performance and bone development when diets are deficient in digestible P.
• Feeding phytase improves nursery pig performance and alleviates the negative impacts of high dietary Ca on pig growth performance.

In summary, phosphorus is considered the third most expensive nutrient in swine diets. Thus, its supplementation is typically associated with low safety margins. However, data generated from the K-State studies have shown that supplementing phosphorus above levels currently recommended in the NRC publication results in improved growth rate and efficiency of growth throughout all production phases.

Although feed cost is increased with the inclusion of higher P levels, improved growth leads to more revenue per pig, resulting in increased income over feed costs. Under the conditions of our studies, the profit margin ranged from about 45 cents per pig in the nursery phase to $1.20 per pig in the finisher phase.

Researchers: “Standardized total tract digestible phosphorus requirement of 25- to 50-pound pigs”: C.M. Vier, F. Wu, S.S. Dritz and M.D. Tokach, Kansas State Univeristy; M.A.D Gonçalves and U.A.D. Orlando, Genus PIC; and J.C. Woodworth, R.D. Goodband and J.M. DeRouchey, K-State. “Effects of standardized total tract digestible phosphorus on growth performance, carcass characteristics, bone mineralization and economics of 53- to 287-pound pigs”: C.M. Vier, F. Wu, M.B. Menegat, H. Cemin, S.S. Dritz and M.D. Tokach, Kansas State Univeristy; M.A.D Gonçalves and U.A.D. Orlando, Genus PIC; and J.C. Woodworth, R.D. Goodband and J.M. DeRouchey, Kansas State Univeristy. “Effects of dietary calcium and phosphorus concentrations and addition of phytase on growth performance of nursery pigs”: F. Wu, M.D. Tokach, J.M. DeRouchey, S.S. Dritz, J.C. Woodworth and R.D. Goodband, Kansas State Univeristy; “Effects of dietary Ca and P concentrations on growth performance of 13- to 25-pound pigs”: F. Wu, M.D. Tokach, J.M. DeRouchey, S.S. Dritz, J.C. Woodworth and R.D. Goodband, Kansas State Univeristy.

For more information, contact Fangzhou Wu.

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