*Dr. William A. Dudley-Cash is a poultry and fish nutritionist and has his own consulting firm in Modesto, Cal. To expedite answers to questions concerning this column, please direct inquiries to Feedstuffs, Bottom Line of Nutrition, 5810 W. 78th St., Suite 200, Bloomington, Minn. 55439, or email firstname.lastname@example.org.
ONCE upon a time, nutritionists felt comfortable that they had a handle on calcium and phosphorus requirements for broilers.
The total (analyzed) requirement for calcium was about 1% of the diet. The total (analyzed) requirement for phosphorus was about 0.5%. The ratio of calcium to phosphorus in the diet should be about 2:1. The phosphorus requirement was further refined to include 100% of the phosphorus from inorganic and animal sources but only 30% from plant sources.
For years — decades, really — the requirement values for calcium and phosphorus remained largely unchanged. They were considered settled science, basically a done deal. However, more recent research has dramatically changed the perception of the correct calcium and phosphorus requirements.
Much of this research has been associated with a better understanding of the activity of phytases and the factors that are involved in modifying the activity of phytases. It seems that the more we learn, the less we know.
Dr. R. Angel of the University of Maryland presented a paper on calcium-to-phosphorus ratios in broilers at the Australian Poultry Science Symposium. Angel traced the history of calcium and phosphorus requirements and ratios reported in the National Research Council (NRC) "Nutrient Requirements For Poultry" bulletins:
* The 1950 NRC bulletin listed requirements of 1.0% total calcium (tCa) and 0.6% total phosphorus (tP) with a tCa:tP ratio of 1.66:1.
* The 1954 NRC bulletin recognized the importance of the availability of phosphorus by specifying that 0.56% of the 0.60% tP requirement should be supplied from an inorganic source. The value of 30% availability for plant tP was also specified.
* The 1977 NRC bulletin still gave requirements in terms of tCa and tP, with a proviso that part of the 0.7% tP requirement from zero to eight weeks be supplied from inorganic sources of phosphorus.
* The 1984 NRC bulletin gave the requirements for phosphorus as available phosphorus (aP), and the aP values were given for some ingredients. No change was made to calcium. The recommended calcium:aP ratios were 2.22:1 to 2.28:1 throughout growth (zero to eight weeks).
* The 1994 NRC bulletin used the term non-phytin phosphorus (nPP) instead of aP, but there were minimal, if any, changes to the numeric values. The recommended calcium:nPP ratios were 2.22:1 to 2.67:1, depending on the stage of growth.
Recognition of the low digestibility of phosphorus in plant sources and the variable digestibility of phosphorus in animal and inorganic sources prompted the change to aP, nPP, digestible phosphorus (dP) or retainable phosphorus that better reflected the availability of phosphorus in various dietary sources. However, Angel pointed out, there has been no move toward a digestible calcium (dCa) system.
The extensive use of phytases in poultry diets worldwide has drawn attention to the negative effect of higher levels of calcium on phytase efficiency, which has prompted an interest in reducing the level of calcium in poultry diets.
Because there are few data on actual availability or digestibility of calcium in feed ingredients, a digestibility value of 100% for calcium is generally assumed. Angel states that this is not correct.
Researchers from the University of Maryland reported on the calcium digestibility in a corn/soybean meal diet with no added inorganic calcium or phosphorus sources and the same diet with added limestone. They calculated calcium availability in the corn/soybean meal portion of the diet of 20-33%. They calculated the digestibility of the calcium from the limestone at between 60% and 70%. The corn/soybean meal portion of the diet provided between 0.17% and 0.21% calcium to the diet.
Up until now, the low digestibility of this "organic" calcium was disregarded because it represents a small percentage (about 20%) of the calcium in a diet that contains 1% tCa. However, when the tCa content of the diet is reduced to 0.6% or even 0.5% in the withdrawal phase, then the calcium from the corn/soybean meal portion of the diet becomes a much greater proportion of tCa.
Angel emphasized that in order to formulate diets with concentrations of dCa and dP that meet the needs of the broiler, a dCa and dP system must be developed. Methodologies must be developed to determine dCa in ingredients and to review the methodologies used to determine dP. The expected contributions of dCa and dP also must be reviewed when phytases are used.
Commercially, tCa:dP ratios of 2:1 have been the standard for more than 30 years, according to Angel. However, does this standard optimize broiler performance? There are many reports in the literature where the tCa:dP ratios at "optimal" performance or bone mineralization are not 2:1.
Driver et al. (2005a) reported that a 1:1 ratio of tCa to tP maximized bodyweight gain and feed:gain. Tibia ash was maximized at tCa:tP ratios of 1.07:1 to 1.35:1.
In a second paper (2005b), Driver et al. reported that broken line requirement values were calculated at tCa:tP ratios of 0.77:1, 0.99:1 and 1.14:1 for bodyweight gain, feed:gain and tibia ash, respectively. Based on calculated nPP values, the tCa:nPP ratios were 1.08:1, 1.39:1 and 1.60:1, respectively, at the requirement. These values are clearly different from the industry standard of 2:1.
Recent research at the University of Maryland found that maximum bone ash was associated with a diet containing 1.05% calcium and 0.65% nPP, which is a tCa:nPP ratio of 1.61:1. When retainable calcium and retainable phosphorus were measured, the ratio was 1.64:1 for maximum bone ash.
It has been known for some time that phosphorus digestibility is low in seed-based ingredients but variable and not 100% in animal and inorganic sources.
In one study, the digestibilities of calcium and phosphorus were determined in a corn/soybean meal starter diet devoid of inorganic calcium or phosphorus sources. The calcium digestibility was 33.6%, and the phosphorus digestibility was 67.9%.
The research also demonstrated the importance and large negative effect of calcium on phytase phosphorus digestibility. The analyzed value for calcium in the corn/soybean meal diet was 0.17%. After adding 0.5% calcium from calcium carbonate, the analyzed content of calcium was 0.65%. As the calcium in the diet went from 0.17% to 0.65%, the digestibility of phytate phosphorus went from 69.2% to 25.4%. The only dietary change was the addition of 0.5% calcium from calcium carbonate.
The digestibility of the calcium reported for the corn/soybean meal diet with no added inorganic sources of calcium or phosphorus was 33.6%. By calculation, the digestibility of calcium from the inorganic calcium carbonate was 62.7%.
In more recent research at the University of Maryland, true digestibilities of calcium and phosphorus in soybean meal, limestone and monocalcium phosphate were determined in trials lasting from eight to 96 hours. Purified diets were fed, and the specific ingredients tested were the only source of the phosphorus or calcium. The true digestibility of calcium in limestone and monocalcium phosphate was 34.1% and 67.9%, respectively, when determined at 40 hours post-feeding of the ingredients to 25-day-old broilers.
The most important information is the low digestibility of calcium in a feed-grade limestone compared to the digestibility of calcium in monocalcium phosphate when the absolute calcium concentrations were similar.
If the digestibility of calcium in monocalcium phosphate is two times higher than in limestone, then what is the effect on calcium:phosphorus ratios when phytase is used?
When phytase is added to broiler diets, the amount of inorganic phosphate added is usually decreased or removed entirely, and limestone is added to supply calcium. When phytase is added to typical corn/soybean meal diets, the amount of monocalcium phosphate in the diet would be reduced by 60% and the amount of limestone by 12%. Assuming a dCa:dP ratio of 1.6:1 in a diet without phytase, the dCa:dP ratio in a diet with added phytase would be close to 1:1. This change in the dCa:dP ratio is a result of the large decrease in the amount of monocalcium phosphate, which has a much higher calcium digestibility than limestone.
In diets containing calcium and phosphorus from animal sources, nutritionists sometimes find themselves adding inorganic phosphorus to maintain calcium:phosphorus ratios. Ingredient values for dCa and dP are essential for correctly formulating dCa:dP ratios.
Angel concluded that it is important for nutritionists to move to a dCa/dP system that allows them to formulate diets to more accurately meet the requirements of the bird.
We must accept that the digestibility of calcium in inorganic sources can vary widely. When phytase is used, the source of calcium must be changed from a calcium phosphate, which usually has a higher calcium digestibility, to limestone, which usually has a lower calcium digestibility that varies with the limestone source.
The current system of using the calcium:aP ratio that partially reflects digestibility no longer helps the nutritionist when formulating diets where the concentrations of phosphorus are minimized and alternative ingredients and feed additives are used to reduce costs. All of these changes alter the levels of dCA and phosphorus in the diet.
The Bottom Line
Angel's paper is a landmark review of the importance of digestibility values for calcium and phosphorus requirements and calcium and phosphorus values in ingredients. This is the most detailed information I have read about the digestibility of calcium, and I feel that it should be required reading.
The proceedings of the Australian Poultry Science Symposium are available at http://sydney.edu.au/vetscience/apss/proceed.shtml.