Towards a greater understanding of calcium digestibility in fattening swine: from total Ca to digestible Ca

Introduction

Surveys in the past 10 years have shown that, on average, commercial swine diets in the EU and US contain 0.19 to 0.22 percentage units more Ca than their formulated values, respectively^1,2.

This excess can arise from a number of sources. These include limestone added directly to feed but also its use as a carrier in vitamin-mineral premixes and as a flow agent in feed raw materials such as soybean meal.

Increased dietary Ca is undesirable because it reduces phosphorus (P) digestibility³, impacts on phytase efficacy and impairs growth performance. It also leads to increased P excretion which is an environmental concern.

This article discusses the importance of understanding Ca digestibility and the interactions between Ca, P and phytase in being able to optimize dietary Ca content and avoid Ca being in excess. It also presents recent research on determining the digestible Ca requirements of swine that will pave the way for a dietary formulation system based on digestible Ca.

Requirements for Ca and P

Historically⁴, NRC recommendations for P (‘P requirements’) were based on total P in the diet. In the 1990s, when it became apparent that undigested P was an environmental concern and phytase became available for releasing additional P from the diet, the recommendations changed to available P⁵. Today (and since 2012), diets are formulated based on apparent total tract digestible (ATTD) or standardized total tract digestible (STTD) P (where STTD is ATTD corrected for basal endogenous losses of P)⁶.

Over the same period, NRC recommendations for Ca have not changed. They remain based on total Ca⁶. The data required to generate recommendations based on digestible Ca have not been available.

Ca digestibility

Two key questions need to be answered to generate robust data on the Ca digestibility of feed ingredients:

• Whether to measure at gastric, ileal or total tract level
• Whether to express digestibility as ATTD or STTD

A study of pigs cannulated in the duodenum and ileum, with feces also collected, showed that there was no net absorption of Ca in the hindgut⁷. Hence, Ca digestibility can be measured either at the ileal or total tract level. However, in practice the total tract level is preferred because it is less invasive to the animal and more cost efficient.

Separate studies have revealed that there are endogenous losses of Ca in pigs (329–659 mg/kg dry matter intake (DMI))^8,9. These originate from enzymes and as part of intestinally secreted mucin and other digestive secretions. The result is that values of STTD can vary quite substantially from those of ATTD across feed ingredients, as illustrated in Figure 1. It is therefore recommended to use values of STTD for estimating the Ca digestibility of feed ingredients.

Figure 1. Variation in values of STTD of Ca and ATTD of Ca across different feed raw materials.
Generated based on data from Lee et al. (2023)⁹.

Phytase effects on Ca digestibility

When considering the Ca digestibility of the diet, it is necessary to take account of the effects of phytase, since phytase is used almost universally in swine diets.

Phytase increases the digestibility of Ca in pigs. This is because it degrades phytate which reduces phytate availability to bind to Ca, increasing the availability of Ca for absorption.

However, the effect is highly ingredient specific, as illustrated in Figure 2. This variability needs to be considered when formulating diets.

Figure 2. The effect of phytase on STTD Ca varies in different feed ingredients.
Generated based on data from Lee et al. (2023)⁹. MCP, monocalcium phosphate; DCP, dicalcium phosphate; SBM, soybean meal; SFM, sunflower meal; RSM, rapeseed meal; M&BM, meat and bone meal.

Furthermore, when phytase is used, MCP and DCP are typically removed from the diet, leaving a much greater proportion of Ca coming from limestone (~67 % vs. ~45 % without phytase¹⁰). Limestone has higher Ca digestibility than MCP or DCP (Figure 2). In addition, the Ca in today’s limestone sources is more digestible than that of those used 7 years ago (by up to 5%¹¹) and varies geographically (lower STTD Ca in limestones from the US than Europe, by ~5%¹² ). These sources of variability can impact on the digestible Ca content of the diet if not accounted for in the dietary formulation.

The effect of phytase on endogenous losses of Ca also needs to be considered when formulating diets with phytase. Phytase reduces these losses due to reduced Ca-phytate binding.

And finally, there is an added complexity in the effect of phytase on Ca digestibility related to binding affinity. The affinity of phytate to bind with Ca is greater than the affinity of Ca to bind with phosphate. This means that in the presence of phytase, the ratio of Ca to P in the digesta is increased. Whilst not so obvious at high dose levels (≥2,000 FTU/kg; ref¹³), at lower dose levels – such as are used in finisher diets – the greater availability of Ca relative to P may need to be taken into account when generating Ca and P matrix values for phytase. (Matrix values are amounts of nutrients that are expected to be released when a given phytase is added to a given diet at a given dose level. They are based on the results of digestibility and performance trials.)

Ca effects on phytase efficacy

As well as phytase affecting Ca digestibility, Ca also has an effect on phytase efficacy. Calcium in excess can reduce phytase efficacy by:

• increasing Ca-phytate binding in the intestine¹⁴
• Increasing competition between Ca and phytase for active sites
• Increasing pH in the digesta which reduces phytase activity

Formulating the diet based on digestible rather than total Ca will reduce the risk of Ca being in excess and of a negative effect on phytase efficacy.

Towards a system for formulating diets based on digestible Ca

To enable diets to be more accurately formulated for Ca and reduce excess Ca, there is a need to move towards a system of dietary formulation based on digestible Ca.

This requires detailed knowledge of the digestible Ca content of feed ingredients (which is now available for most ingredients). It also requires knowledge of the digestible Ca requirements of pigs.

Research by the author and collaborating researchers at the University of Illinois has been addressing this topic^15,16 . This has generated and validated optimal ratios of STTD Ca: STTD P in corn-soybean meal-based diets for pigs of different ages, for maximizing growth performance and bone ash, when P is fed at the (NRC) requirement level.

These ratios are shown in Figure 3.

Figure 3. Optimal ratios of STTD Ca:STTD P in corn-soybean meal-based diets for pigs of different ages, when P is at the requirement level^15,16.

Several specific findings from this research should be noted:

• When P is below the NRC requirement, the optimal ratios will be different (to those in Figure 3). There will be an increased risk that Ca will be in excess relative to P, with potential to impair growth performance
• When Ca is above the NRC requirement (+0.2 %), especially in older pigs (>50 kg), or STTD Ca is increased from 0.29 to 0.43 %, feed intake is reduced (due to reduced P digestibility) leading to reduced weight gain (-58 g average daily gain in pigs of 50–130 kg). This highlights the importance of formulating the diet correctly for Ca to avoid Ca excess in the diets of older pigs
• However, it is important to note that the optimal STTD Ca:STTD P ratios for maximizing bone ash are different to (higher than) those for maximizing growth performance. This is particularly so in older pigs (Figure 3). Therefore, Ca must be formulated to avoid excess but also to avoid deficiency.
• Values of total rather than digestible Ca can be used to formulate diets to give similar results but only if all sources of dietary Ca are accounted for
• Further research is needed to validate STTD Ca requirements under different conditions and to determine matrix values for STTD Ca with phytase in the diet

Conclusions

Formulating diets more accurately for Ca content requires a detailed understanding of the Ca contributions from all ingredient sources. The Ca-releasing effect of phytase must also be taken into account. Digestible Ca requirements for pigs of different ages (and associated optimal ratios of STTD Ca:STTD P) have been determined but further validation is needed to enable the move to a digestible Ca based system for dietary formulation.