Experimental approaches and nutritional insights
Authors: Dr. Laura Merriman, Swine Technical Service at AB Vista
Carrie Walk, Director of Research and Development
Gustavo Cordero, Global Swine Technical Manager
Spenser Becker, Swine Technical Service
This paper discusses the current understanding of calcium in growing pigs and sows. The key parameters are discussed as well as the next steps in completing our understanding of calcium requirements. The role phytase plays is also included, emphasizing the need to utilize a matrix for calcium in feed formulations.
Introduction
Bone mineralization is critical in both growing animals where rapid bone growth is occurring and in lactating sows where mineral demands increase to support milk production, all while trying to preserve maternal bone health. If dietary intake of calcium (or the calcium to phosphorus ratio) is insufficient, sows rely on body reserves from their bones to supply it, potentially compromising long-term skeletal health and productivity over successive parities. Inadequate calcium can also result in lameness and reduce sow longevity.
The role calcium plays on phosphorus
Calcium is a relatively inexpensive nutrient, typically supplied by limestone, which is also used as a carrier for feed additives and premixes or a diluent in soybean meal. Consequently, analyzed dietary calcium levels often exceed targeted concentrations. Unfortunately, that can come at a cost when not properly balanced with other minerals, particularly phosphorus. Excess calcium binds to phosphorus as it is eliminated via the urine, which causes a decrease in phosphorus digestibility. Thus the requirements for phosphorus may not be adequately supplied for the pig if calcium is oversupplied or at a wide calcium to phosphorus ratio.
Experimental approaches
Considerable progress has been made in understanding calcium digestibility of ingredients in growing pigs. This information allows nutritionists to more precisely formulate calcium in diets based on their target response. Multiple response parameters have been assessed to estimate the utilization of calcium in various ingredients and subsequent calcium requirements. These response parameters include:
- Growth performance
- Digestibility of ingredients from fecal or digesta samples
- Plasma mineral levels
- Bone mineral content
- Bone biomechanical and physical characteristics
- Urinary mineral losses
- Bone mineral content from living animals via dual-energy X-ray absorptiometry
These measurements are captured in controlled studies, generally using a titration of deficient to surplus calcium levels and may include a factorial design to consider interactions between calcium and phosphorus. In growing animals, these methodologies are easily executed to determine the optimal dietary concentrations to maximize growth performance and bone mineralization.
A comprehensive evaluation of calcium and phosphorus digestibility in raw materials containing appreciable levels of calcium has been conducted at the University of Illinois by Dr. Hans Stein’s laboratory. By employing consistent methodologies, their work has established calcium digestibility coefficients and requirements for each growth phase, providing a thorough understanding of this area in swine nutrition. The final step in our calcium understanding for growing animals is to validate these responses in commercial conditions under various health statuses and stocking densities.
However, translating these findings from young and growing pigs to gestating and lactating sows is challenging. Key parameters in growing pigs, such as growth performance, are less relevant in sows given their differences in utilization and partitioning of nutrients. Instead, other parameters may be a better indicator. For instance, bone mineral content is a sensitive measurement and key to understanding shifts in bone mineralization and understanding the requirements of the sow. Extracting bone samples from sows is quite expensive and logistically complex. Instead, non-invasive methods such as DEXA (dual-energy X-ray absorptiometry) can be a viable option to use on living animals to assess mineral content. Another option to evaluate sow requirements could be the use of mineral analysis via the urine. The laboratory of Dr. Tom Crenshaw showed promising work on urinary ratio of calcium to phosphorus, suggesting that this could be a strategy employed not only by universities but also at the farm level to indicate whether the animal was provided an adequate diet for calcium and phosphorus. Other parameters of interest may include mineral concentrations in milk and blood as well as biomarkers of bone formation and resorption like blood serum C-terminal propeptide of type I collagen and C-terminal telopeptide of type I collagen, respectively, providing additional in vivo options to understand mineral status (Heurtault et al., 2024).
Practical approaches to formulating for calcium and phosphorus
One thing to keep in mind is that the nutrient levels required to optimize bone parameters typically exceed those needed to optimize growth performance. For growing animals, an average response to growth (i.e., gain) and bone parameters may be a compromise between these parameters, which could be used as the requirement for practical formulation.
In contrast to growing animals, very little is known about the calcium requirements for sows. It is clear, however, that sows digest calcium differently from growing animals. Gestating sows digest calcium in the diet at a much lower level (26%) compared to growing animals (45% to 78%; Lee et al., 2021). Recent changes in nutritional guidelines have recommended wider ratios between calcium and phosphorus be used for gestating and lactating sows, similar to European strategies. In addition, previous work in 100 to 130 kg growing pigs found that wider calcium to phosphorus ratios formulated at or below the phosphorus requirement effectively reduced growth rate (due to lower feed intake) – yet bone continued to be deposited (Merriman et al., 2016). This could be a useful strategy for gilts and sows to manage weight while also supporting bone health. In fact, in contrast to growing animals, it may be advantageous to consider levels of calcium and phosphorus required to optimize bone as key requirement concentrations. This strategy would support skeletal development and sow longevity. Further research is warranted to understand these effects long-term in breeding pigs.
Phytase: A key player to calcium and phosphorus availability in diets containing phytate
Phytase enzymes have revolutionized swine nutrition by unlocking phosphorus tied up in plant-based feed ingredients. They not only improve phosphorus availability but also enhance the absorption of other minerals such as calcium by reducing the anti-nutritional effects of phytate. As such, it is important to consider credit for calcium by the phytase via the feed formulation. Applying a matrix only to the phosphorus could potentially lead to an imbalance in the calcium and phosphorus supplied to the pig.
Phytases are not equally effective at liberating calcium and phosphorus bound in the phytate form. Thus, when incorporating phytase, it is important to consider dietary constraints and phytase efficacy to ensure the enzyme is being used effectively within the overall diet formulation.
Conclusion
Calcium is an essential nutrient for bone development and growth. Bone mineralization in sows and growing pigs is a complex process influenced by dietary calcium, phosphorus, and phytase, as well as by physiological state. Recent work has generated a good understanding of the calcium digestibility in feedstuffs and estimated the digestible calcium requirement in growing animals. Unfortunately, there remains a substantial knowledge gap in breeding animals. However, recent research using urinary or milk concentrations of calcium and phosphorus from sows may provide opportunities for on-farm assessments to build on the knowledge available for sows today. Research on calcium digestibility, digestible calcium requirements, and impact of phytase in lactating sows, gestating sows, and developing gilts will enhance our understanding and support the development of nutritionally optimized, sustainable swine production systems.
