Effects of Vitamin D, Phytase, Calcium and Phosphorus in Broiler Performance

The metabolism of vitamin D, calcium and phosphorus within the chicken is uniquely integrated. The absorption of intestinal calcium depends on many factors but one of the most important is vitamin D (Ameenuddin, Sunde et al. 1985). There is obscurity regarding the exact mechanisms of calcium absorption across the intestine, however it is known that vitamin D is essential for the synthesis of calcium binding protein (CaBP) in the cells of the intestinal wall and CaBP actively transports calcium across the epithelial wall into the plasma (Wasserman and Taylor 1966). Recent research on the modern broiler genotype indicates there is an increased physiological demand for calcium (Whitehead, McCormack et al. 2004). Unfortunately, high levels of calcium in the diet interfere with the availability of other minerals such as P, magnesium (Mg), manganese (Mn) and zinc (Zn) (Vandepopuliere, Ammerman et al. 1961). One way to minimize the negative effects of Ca on P availability in the chicken may be to increase vitamin D concentration in the diet to escalate calcium absorption and retention rather than increase calcium levels per se. The addition of higher levels of vitamin D may also increase the rate of absorption of phosphorus thereby also reducing its availability to form insoluble calcium salts.

Vitamin D has also been shown to increase phytate digestibility and reduce the rachitogenic nature of low calcium and high phytate diets in chickens (Mellanby 1950; Steenbock and Herting 1955). It has been shown that low levels of dietary calcium fed with elevated levels of vitamin D permit greater utilization of calcium and phytate-P and reduce the requirement for inorganic P (Mohammed, Gibney et al. 1991). Subsequently, it was demonstrated that vitamin D and exogenous phytase have a synergistic effect when fed together to broiler chickens (Edwards 1993). This synergistic effect of vitamin D when fed in combination with exogenous phytase was shown to improve leg health by significantly increasing tibia strength and tibia ash (Philipps, Aureli et al. 2007).

Finally, vitamin D and strontium (Sr) have a close physiological relationship following similar metabolic pathways to calcium absorption, bone deposition, and renal re-sorption. In poultry feeding, the major sources of Sr in the diet are the bulk mineral ingredients such as limestone powder or chips, dicalcium or mono-calcium phosphate. Excessive Sr has been shown to produce rickets because strontium inhibits vitamin D metabolism in the kidney (Corradino and Wasserman 1970). Conversely, small amounts of Sr (in combination with 800 IU vitamin D + 1000mg Ca per day) significantly reduced the number of fractures in older women patients with osteoporosis (Meunier, Roux et al. 2004). The exact role of Sr in poultry nutrition requires further investigative research. It was therefore the purpose of the experiment reported herein to explore the interactive effects of vitamin D, Ca, P and phytase in broiler chickens and to elucidate the effects of these on the retention of Sr and other minerals. 

A total of 288 day old male Cobb broiler chicks were fed for 28 days in an experiment with 8 treatments and 6 replicates in a complete 2x2x2 factorial design (36 birds per treatment) (Table 1). All diets were fed ad libitum in a mash form with the starter diet being fed to 14 days of age followed by the finisher diet to 28 days of age. The principal ingredients in both diets were wheat, soybean and canola meal. On the 20th day of the trial, a 48 hour total collection procedure was undertaken to measure mineral retention. On day 28, all birds were humanely sacrificed and measurements of bodyweight (BW), feed intake (FI), toe ash, tibial length, tibial dyschondroplasia (TD), and the retention of the minerals calcium, phosphorus, magnesium, strontium and sodium were undertaken. The Ca:AvP ratio of 2:1 was used based on the recommendation of the NRC (1994) and current commercial practice.

Table 1. Summary of treatments

 

The addition of higher levels of vitamin D did not produce a significant difference in BW, feed conversion ratio (FCR) or mineral retention (Table 2). However, exogenous phytase improved FCR when added to the low Ca/P diets but not in diets with high or adequate levels of Ca/avP, resulting in a diet*phytase interaction (P<0.05). The addition of exogenous phytase increased P retention in the low Ca/avP diet but not in the high Ca/avP diet resulting in a significant diet*phytase interaction. The diet with reduced Ca/avP returned poorer (P<0.01) FCR but superior (P<0.01) Ca, P and Sr retention compared with the high density diet. Furthermore, there was a direct linear correlation between Ca and Sr retention (R² = 0.98). Bone mineralization was not changed in any treatment group as measured by toe ash, nor was there any difference in TD prevalence (data not shown), growth rate and FCR. 

The lack of response to the addition of vitamin D was not expected and is contrary to the results of Whitehead et al (2004) who found that 10,000 IU D3/kg in broiler diets maximized growth and tibia ash. BW and FCR were close to Cobb500 breed standard of 1324g and 1.45 respectively at 28 days of age. Similarly, the fact that higher levels of vitamin D did not produce an additive or synergistic response with exogenous phytase is in contrast to the findings of Edwards (1993) who found the highest retention of phytate-P (79.4%) was obtained when both phytase and additional vitamin D were present in the diet. An explanation for the response in this experiment to extra vitamin D may be that the differential between 5,000 IU D3/kg and 10,000 IU D3/kg was insufficient to detect differences in Ca and phytate-P utilization. In future research higher levels of vitamin D will be investigated.

Table 2. Summary of Bird Performance and Mineral Availability

There was a significant improvement in FCR by the addition of phytase to low Ca/avP diets. This positive effect of exogenous phytase is of great economic advantage because FCR is one of the most critically important values in commercial broiler production. The addition of phytase to the low density diet also improved both Ca and P retention which is interpreted as an improvement in phytate-P digestibility and a reduction in the antinutritive effect of dietary phytate on cation solubility. This result is in line with many researchers who have clearly demonstrated that exogenous phytase improves Ca and P retention (Denbow, Ravindran et al. 1995).

Reduced dietary Ca/avP concentrations were associated with increased efficiency of Ca retention as compared to the four high Ca/avP diets which no doubt indicates a physiological response by the chicken to overcome a Ca deficiency by up-regulating its nutrient transfer and deposition infrastructure. Sr retention was also significantly improved when Ca and available P levels were reduced. The improved retention of Sr was directly proportional to Ca, presumably because these minerals are elementally similar. To the authors knowledge this is the first time that this relationship has been demonstrated. It maybe hypothesized that Sr uses similar metabolic pathways to Ca for intestinal absorption and bone mineralization. There is no current data on the strontium content of feed ingredients or on the effects of small amounts of strontium on basic parameters such as BW, FCR and toe ash. There is a need for future research into strontium in broiler nutrition in lieu of its close interrelationship with calcium and vitamin D. 

This study was supported by Poultry CRC and DSM Nutritional Products. Special thanks to Joy Gill and Melinda Hayter for their invaluable assistance. 

Ameenuddin S, Sunde M and Cook M (1985) World's Poultry Science Journal 41, 52-63.

Corradino R, Wasserman R (1970) Experimental Biology and Medicine 133, 960.

Denbow D, Ravindran V, Kornegay, E, Yi Z, Hulet R. (1995) Poultry Science 74, 1831.

Edwards H M. (1993) The Journal of Nutrition 123, 567-577.

Mellanby E (1950) A Story of Nutritional Research. The Effect of some Dietary Factors on Bones and the Nervous System, Williams & Wilkins, Baltimore.

Meunier PJ, Roux C, Seeman E, Ortolani S, Badurski J, Spector T, Cannata J, Balogh A, Lemmel E, Pors-Nielsen S (2004) New England Journal of Medicine 350, 459-468.

Mohammed A, Gibney M, Taylor T (1991) British Journal of Nutrition 66, 251-259.

Philipps P, Aureli R, Fru F, Weber G (2007) 16th European Symposium on Poultry Nutrition., Strasbourg, France.

Steenbock H, Herting DC (1955) The Journal of Nutrition 57, 449.

Vandepopuliere J, Ammerman C, Harms R (1961) Poultry Science 40, 951.

Wasserman R, Taylor A (1966) Science 152, 791.

Whitehead C, McCormack H, McTeir L, Fleming R (2004) British Poultry Science 45, 425-436.