Ensuring animal feed formulas meet all nutritional requirements is particularly challenging for vital mineral elements such as calcium. Essential for bone development and a key player in several metabolic paths, calcium is often added to diets with large safety margins to avoid deficiencies. But if calcium deficit poses a threat to animal growth, high calcium concentrations that are highly soluble can also severely impair productivity. – Dr Ceinwen Evans, Danisco Animal Nutrition (IFF)
Animal production strongly depends on healthy animal growth. In turn, growth and maturation rely on the normal development of the skeleton. Bone mineralization is dependent on a variety of elements with phosphorus and calcium being the two most important. In poultry, for instance, calcium is the highest concentration mineral in the body, representing more than one third of the total mineral body content of adult birds and one third of eggshell components.
Calcium supplementation: a tricky balance
To fulfil the nutritional needs required by the expanding livestock production sector, diets have been formulated using different types of cereal-based ingredients. Although providing high amounts of energy, cereals are poor in several elements including calcium, most presenting concentrations as low as 0.02% and 0.06%. To meet calcium requirements, diet formulations usually include inorganic sources of this nutrient such as limestone and dicalcium phosphate. As an example, in a typical corn-soy based starter diet for broilers, more than 80% of the calcium comes from inorganic sources. Unlike phosphorus addition, calcium supplementation is low-cost, and limestone is easily accessible.
An unexpected anti-nutrient
High calcium concentrations can be as harmful as low ones: calcium deficiencies have a negative impact on growth performance, while increased concentrations have anti-nutritional effects on other important key nutrients. Of major importance is the negative effects of excessive calcium on phosphorous availability and digestibility. Calcium has the ability to form complexes with phosphate groups in the gut, which interfere with phosphorus availability. Phosphorus is crucial for energy mobilization and protein synthesis but also bone development and overall maintenance. Its primary storage form in plant tissues is phytate, which is broken down by phytase to release phosphorus. For animals that lack phytase, like poultry and swine, the enzyme is added to feed formulas. High dietary calcium content affects phytase effectiveness due to the formation of calcium-phytate complexes. The high prevalence of calcium also triggers phytate to bind with protein molecules more easily, further reducing phytate availability to be hydrolyzed by phytase. Proteins usually form complexes with phytate at the low pH (<4) of the upper gastrointestinal tract but not at higher pH levels. This process is challenged in the presence of calcium that promotes protein-phytate chelation at pH >4, thereby acting as a cation bridge to promote the indirect binding of proteins and phytate. This anti-nutritional effect of calcium depends largely on its solubility, which is very much influenced by the calcium source and particle size. Limestone, the major inorganic calcium source used in poultry diets, is marketed under different particle sizes that influence calcium’s solubility. Particle size impacts the calcium-phytate binding and its consequences. Research has shown that the smaller the limestone particles, the faster they solubilize in the upper gastrointestinal tract resulting in more available calcium.
Fighting the anti-nutritional battle with phytase
Although all marketed phytase enzymes break down phytate, not all phytases are the same. Enzymes with a superior pH profile will hydrolyze phytate more quickly in the upper part of the digestive tract making it unavailable to form the anti-nutritional calcium-phytate complexes. Axtra PHY is a Buttiauxella phytase with high bio-efficacy and activity at low pH. This phytase acts fast in the intestinal tract, hydrolyzing phytate during the early stages of digestion. The enzyme revealed an improved pH profile when compared to its competitors, performing even at very low pH levels (>2.5). By cleaving phosphate groups rapidly, phytase decreases calcium-phytate binding and thereby mitigates its anti-nutritional effect on both proteins and minerals. But phytase efficiency largely depends on the size of the calcium particles found in different calcium supplementation sources. For instance, phytase addition may occur at different doses when overcoming the negative impact of highly soluble calcium (HSC) on nutrient digestibility or the effects of limestone.
The benefits of a fast-acting phytase and the right calcium source
A dual-action strategy can be employed to alleviate the detrimental effects of calcium over-supplementation on nutrient digestibility and growth performance. First, the addition of a fast-acting phytase promotes a timely and successful hydrolysis of phytate and avoids chelation. What is important to note here is that a strong pH profile phytase ensures the phytate is hydrolyzed swiftly in the very acidic first portion of the gut. Secondly, adjusting the amount of calcium included in diets (better calcium-to-phosphorous rates) and observing the solubility of the selected calcium source will aid improving feed quality and animal health. One central consideration here is the grind size of the calcium supplementation. Diets containing higher calcium inclusion levels and/or more soluble calcium sources, such as HSC or coarse limestone, benefit from using a tailored phytase dose to completely remove phytate and avoid the antinutritional effect of phytate-calcium complexes.
