By Jerry Shurson, University of Minnesota Department of Animal Science
Phosphorus is the third most expensive component of swine diets, and unlike all other grains and grain co-products, distiller’s dried grains with solubles contains a high concentration of total and digestible phosphorus. Therefore, when adding DDGS to swine diets formulated on a digestible phosphorus basis, significant reductions in inorganic supplementation and diet cost can be achieved.
In addition, many ethanol plants add phytase during the ethanol and DDGS production process which further improves the phosphorus digestibility of DDGS, but also contributes to variation in digestible phosphorus content among DDGS sources (Rodrigues-Reis et al., 2018).
Measurements of phosphorus digestibility and utilization
There are many ways of expressing the utilization of dietary phosphorus in pigs. Total phosphorus represents all of the phosphorus present in a feed ingredient, and includes the indigestible portion of phosphorus known as phytic acid (phytate) in grains and grain co-products. Therefore, if diets are formulated on a total phosphorus basis, overestimation of the digestible phosphorus content may occur because it does not account for the amount that is digestible and available for utilization by pigs.
Bioavailable phosphorus is the proportion of total phosphorus that is digested, absorbed and is available for use in biological functions or stored in the body. Phosphorus bioavailability is usually determined using a slope-ratio assay in digestibility experiments, and it theoretically estimates the digestible and post-absorptive utilization of phosphorus in body tissues. Bioavailability of phosphorus is also frequently described as available phosphorus.
The approach for determining available phosphorus with the slope-ratio assay involves fitting the slope of a linear titration of dietary phosphorus content provided by an inorganic phosphorous source with the associated response criteria (growth rate or bone ash), and comparing it with the linear titrated slope of the same response criteria using the phosphorus content of the test ingredient. However, this method has the disadvantages of assuming that the bioavailability of the inorganic source is 100%, which is not the case.
In addition, the bioavailability estimate derived from this approach varies and is based on different responses depending criteria chosen. Therefore, when using bioavailability estimates for phosphorus, it is important to remember that these estimates are relative to the bioavailability of the reference inorganic phosphorus source used in the comparison, and that it does not represent true bioavailability. If this is not considered, diets formulated on an available phosphorus basis will overestimate the actual amount of phosphorus being utilized.
To overcome these challenges, most recent studies use methodology to estimate apparent total tract digestibility of phosphorus or standardized total tract digestibility of phosphorus. The difference between ATTD and STTD phosphorus is that STTD corrects for basal endogenous losses of phosphorus, resulting in greater accuracy of estimating true digestibility. Therefore, using ATTD phosphorus values will likely underestimate the true digestibility of phosphorus because it does not account for basal endogenous losses. Shen et al. (2002) estimated that basal endogenous losses of phosphorus in corn accounts for about 26% of the daily phosphorus requirement for pigs. Therefore, after correcting for endogenous losses, STTD phosphorus values are additive for all feed ingredients and provides the most accurate estimate of the true digestibility of phosphorus in the diet (Gonҫalves et al., 2017).
Installments in the DDGS series
Part 9: Corn DDGS a good source of digestible phosphorus for swine
Summary of recent studies evaluating STTD of phosphorus in DDGS diets for swine
The National Research Council (2012) lists the ATTD and STTD of phosphorus in DDGS containing between 6% to 9% oil to be 60% and 65%, respectively. Results from recent studies have shown that these estimates are low and rather conservative. Almeida and Stein (2010) determined the STTD phosphorus content of DDGS to be 72.9%, and the addition of 500 phytase units per kilogram of diet of a microbial phytase did not improve STTD phosphorus (75.5%), unlike the addition of phytase to corn and soybean meal diets.
Furthermore, they showed that formulating diets on a STTD phosphorus basis does not reduce pig growth performance, and the use of phytase, DDGS, or the combination of both in corn-soybean meal diets reduces phosphorus excretion in growing pigs. In a subsequent study, Almeida and Stein (2012) showed that the addition of 130, 430, 770, or 1,100 FTU per kilogram of microbial phytase tended to improve STTD of phosphorus in DDGS from 76.9%, 82.9%, 82.5%, 83.0%, respectively. However, when these researchers developed regression equations to predict STTD of phosphorus in DDGS, the R2 value of prediction was only 0.20, and as a result, was not adequate for predicting STTD of phosphorus in DDGS.
Hanson et al. (2011) demonstrated the advantages of formulating DDGS diets on an available phosphorus basis compared with a total phosphorus basis in diets containing zero, 10% or 20% DDGS. Results from this study showed that increasing diet inclusion rates of DDGS reduced total dietary phosphorus content and fecal phosphorus concentration, but did not affect phosphorus excretion, retention, or digestibility. Baker et al. (2013) conducted two experiments to compare values of STTD and relative bioavailability of phosphorus (dicalcium phosphate was used as the reference phosphorus source) in DDGS fed to growing pigs. The STTD of phosphorus in dicalcium phosphate and DDGS was 86.1% and 58.8%, respectively, and the bioavailability of phosphorus in DDGS was 87% relative to dicalcium phosphate.
However, these researchers concluded that the relative phosphorus bioavailability in DDGS overestimates the true utilization of phosphorus from DDGS, and estimates for STTD of phosphorus cannot be accurately calculated from relative bioavailability phosphorus values in DDGS. Therefore, it is necessary to determine and use STTD values for phosphorus in feed ingredients fed to pigs to achieve optimal phosphorus nutrition.
Rojas et al. (2013) determined the STTD of phosphorus in DDGS was 82.8% with microbial phytase added at 870 FTU per kilogram of diet, and 76.5% without added phytase, but these estimates were not significantly different. Presumably, the lack of minimal, if any, improvement in STTD phosphorus in these studies from the addition of phytase was due to the relatively low phytate content in the DDGS sources evaluated.
She et al. (2015) determined the STTD of phosphorus in DDGS sources containing 10.3% (high-oil), 9.1% (medium-oil) and 3.5% (low-oil) when 600 FTU per kilogram of phytase was added to the diets. The STTD of phosphorus estimates in this experiment were 71.2%, 70.8% and 71.8% for high-, medium- and low-oil DDGS sources respectively. These data suggest that oil content of DDGS had no effect on STTD of phosphorus in DDGS.
Diets containing DDGS should be formulated on a STTD phosphorus basis to achieve the greatest accuracy in providing optimal phosphorus nutrition to swine. Results from recent studies have shown that STTD of phosphorus in DDGS may vary from 59% to 77%, but most estimates are greater than the value of 65% reported by NRC (2012). Unfortunately, adequate prediction equations for estimating STTD of phosphorus in DDGS have not been developed for swine. Therefore, the conservative estimate of STTD of phosphorus from NRC (2012) appears appropriate for use when formulating DDGS diets to avoid overestimation of digestible phosphorus content of when using various DDGS sources.
The addition of phytase to DDGS diets had minimal effects on improving phosphorus digestibility, which is likely due to the relatively low phytate content in most DDGS sources. Although the relative bioavailability of phosphorus in DDGS has been estimated to be 87% relative to dicalcium phosphate, this value appears to overestimate the true utilization of phosphorus from DDGS for pigs.
Almeida, F.N., and H.H. Stein. 2012. Effects of graded levels of microbial phytase on the standardized total tract digestibility of phosphorus in corn and corn coproducts fed to pigs. J. Anim. Sci. 90:1262-1269.
Baker, S.R., B.G. Kim, and H.H. Stein. 2013. Comparison of values for standardized total tract digestibility and relative bioavailability of phosphorus in dicalcium phosphate and distillers dried grains with solubles fed to growing pigs. J. Anim. Sci. 91:203-210.
Gonҫalves, M.A.D., S.S. Dritz, M.D. Tokach, J.M. DeRouchey, J.C. Woodworth, and R.D. Goodband. 2017. Fact sheet — Ingredient database management for swine: phosphorus. J. Swine Health Prod. 25:76-78.
Hanson, A.R., G. Xu, M. Li, M.H. Whitney, and G.C. Shurson. 2011. Impact of dried distillers grains with solubles and diet formulation method on dry matter, calcium, and phosphorus retention and excretion in nursery pigs. Anim. Feed Sci. Technol. 172:187-193.
NRC. 2012. Nutrient requirements of swine. 11th rev. ed. Natl. Acad. Press, Washington, DC.
Rodrigues Reis, C.E., Q. He, P.E. Urriola, G.C. Shurson, and B. Hu. 2018. Effects of modified processes in dry-grind ethanol production on phosphorus distribution in coproducts. Ind. Eng. Chem. Res. doi: 10.1021/acs.iecr.8b02700.
Rojas, O.J., Y. Liu, and H.H. Stein. 2013. Phosphorus digestibility and concentration of digestible and metabolizable energy in corn, corn coproducts, and bakery meal fed to growing pigs. J. Anim. Sci. 91:5326-5335.
She, U., Y. Su, L. Liu, C. Huang, J. Li, P. Li, D. Li, and X. Piao. 2015. Effects of microbial phytase on coefficient of standardized total tract digestibility of phosphorus in growing pigs fed corn and corn co-products, wheat and wheat co-products and oilseed meals. Anim. Feed Sci. Technol. 208:132-144.
Shen, Y., M.Z. Fan, A. Ajakiye, and T. Archibold. 2002. Use of the regression analysis technique to determine the true digestible phosphorus digestibility and the endogenous phosphorus output associated with corn in growing pigs. J. Nutr. 132:1199-1206.