UNDERSTANDING FEED-GRADE CALCIUM PHOSPHATE SOURCES “HIGH DIETARY LEVELS OF MONOBASIC OR DIBASIC PHOSPHATE AFFECT LAYING HENS DIFFERENTLY”
This is the last in a series of four issues of Milwhite’s Journal dedicated to understanding more about monobasic calcium phosphate (MCP), dibasic calcium phosphate (DCP) and tribasic calcium phosphate (TCP) which are high quality feed-grade phosphate sources used by the world-wide animal industry. In 1994 Dr. K. Keshavarz, a faculty member in the Department of Animal Sciences at Cornell University (USA), reported results of several experiments with commercial egg-type laying hens that indicated they responded quite differently to high levels of P when the phosphate was in either the monobasic (H2PO4-) or dibasic (HPO4=) form. He attributed this difference in response to the strong acidic properties of the monobasic form of phosphate compared to the less acidogenic dibasic form. In his experiments Dr. Keshavarz formulated the laying hen diets using purified reagent-grade sources of these forms of phosphate [i.e., monobasic calcium phosphate monohydrate, Ca(H2PO4)2•H2O) and anhydrous or di-hydrated dibasic calcium phosphate, CaHPO4 and CaHPO4•2H2O, respectively]. , however, he did not use any commercially available feed-grade phosphate sources. Therefore, being familiar with this research, in 1997 Miles and Oelfke at the University of Florida, Gainesville, FL (USA) fed high dietary concentrations of P to laying hens using a feed-grade source of monobasic calcium phosphate and reported similar findings as those of Keshavarz (1994). Their data indicated that feeding a diet containing a high concentration of P supplied from a feed-grade source of monobasic calcium phosphate resulted in a decrease in laying hen performance. No decrease in performance resulted when the same dietary P concentration was supplied by a reagent-grade source of dibasic calcium phosphate. The following is a discussion of the laying hen experiment reported by Miles and Oelfke (1997).
EXPERIMENTAL APPROACH: Prior to the experiment conducted by Miles and Oelfke, these researchers surveyed the feed industry and purchased several commercial sources of feed-grade phosphate and had them subjected to X-ray diffraction analysis in order to document the exact form of phosphate present. The analysis was conducted by the Florida Industrial and Phosphate Research Institute located in Bartow, Florida (USA). From all of the feed-grade sources tested, a feed-grade source was selected that contained approximately 93% of its phosphate in the monobasic form. Also, one feed-grade source of tribasic calcium phosphate (TCP) was selected. A corn soybean meal basal diet was formulated to contain a concentration of 0.55% total P with 0.2% of the total P being supplied to the diet by a feed-grade source of dibasic calcium phosphate with the corn and soybean meal supplying 0.35%. Along with the basal (control) diet, four additional diets were formulated to contain 1% supplemental P supplied from reagent-grade dibasic calcium phosphate and monobasic calcium phosphate as used by Keshavarz (1994), feed-grade monobasic calcium phosphate and feed-grade tricalcium phosphate. This provided the five experimental diets, four of which contained 1.55% total phosphorus (Table 1). All diets contained the recommended concentrations of calcium, other nutrients and energy which met the bird’s requirement for their age. Each diet was fed to 5 replications of 5 individually caged laying hens in an open-type house. The hens had ad-libitum access to their experimental diets and water during the entire 28 day-experimental period.
EXPERIMENTAL RESULTS: Within 24 hours after having access to their experimental diets there was a significant (P < 0.05) 60% decrease in feed consumption by the hens eating the high-P diet containing the phosphate supplied from either reagent-grade or feed-grade monobasic calcium phosphate. A low average feed intake by hens in these two experimental groups continued throughout the entire experimental period (Table 1). The immediate decrease in feed intake observed in this experiment was also observed by Keshavarz (1994) in hens fed high P from reagent-grade monobasic calcium phosphate. In his experiment, after 7 days the average daily feed intake was 107 grams whereas, hens fed the monobasic form averaged only 29 grams of feed intake each day. It is not known whether the decrease in feed intake which occurred as a result of consuming the diet containing the high concentration of P in the monobasic form was a result of poor palatability because the hens did not prefer the diet due to its acidogenic nature or whether the decrease in feed intake was a result of metabolic acidosis, as proposed by Keshavarz (1994). There was no decrease in feed-intake, egg production or overall laying hen performance resulting from hens consuming the diet containing high P in the dibasic form of calcium phosphate or from tribasic calcium phosphate (Table 1). Since X-ray diffraction analysis indicated that none of the phosphate in tribasic calcium phosphate was in either the monobasic or dibasic form, the normal feed intake by these hens was expected. Keshavarz also reported that feeding the dibasic form of phosphate did not affect hen performance, even when this form provided a supplemental P level in the diet of as high as 2.41%. In contrast, hen performance was seriously impaired when a supplemental P level of 1.02% was supplied in the monobasic form.
As pointed out by Keshavarz (1994), depending on processing conditions such as the ratio of limestone to phosphoric acid, concentration of phosphoric acid, purity of raw materials, temperature of processing, etc., the percentage of monobasic and dibasic forms of phosphate can vary considerably in commercial feed-grade calcium phosphate sources used in the animal feed industry. Normally, companies that manufacture these feed-grade phosphates have strict quality control standards so that the ratio of each form of phosphate is usually known and kept somewhat constant from batch to batch of their product. Even though the monobasic form of phosphate is known to promote metabolic acidosis, no problems in performance of laying hens should be anticipated when this form is present in a feed-grade phosphate that is being used at normal dietary levels to meet the P requirement of the birds. No nutritionist would ever deliberately formulate and feed laying hens a diet containing such a high concentration of P which was used in the experiments mentioned above. However, when a diet unknowingly contains an unwanted high P concentration, such as when a feed mill mixing error occurs, major problems with feed intake and performance should be anticipated when the majority of the P in the feed-grade phosphate source is in the monobasic form, but not the dibasic form. The magnitude of the problem, as a result of the monobasic form, will be directly related to the actual amount of the monobasic form used in the diet and ultimately being consumed by the laying hens.
The information presented in this issue of Milwhite’s Journal was compiled by Dr. Orlando Osuna, Director of Health Science at Milwhite, Inc. and Dr. Richard Miles, Professor Emeritus, University of Florida, Gainesville, FL USA
Miles, R.D. and E. Oelfke. 1997. Laying hens respond differently to high dietary phosphorus from commercial sources. Poultry Sci. 76 (suppl. 1:143).
Miles, R.D. and P.R. Henry. 2004. Effect of time and storage conditions on interior quality of eggs from hens fed vanadium. J. App