Calcium and phosphorus in nursery diets
Calcium and phosphorus are essential for growth performance of nursery pigs. These minerals are involved in skeletal structure development and maintenance, lean tissue deposition, muscle contraction, and many other physiological functions. Phosphorus levels in nursery diets have typically a low safety margin because of environmental and economic concerns. Calcium levels, on the other hand, are typically high in nursery diets due to the unaccounted-for contribution of calcium from carriers, release from phytase, variability of calcium estimation in feed ingredients, and no environmental and economic concerns regarding calcium.
The accurate estimation of calcium and phosphorus requirements of nursery pigs is important to maximize growth performance, minimize phosphorus excretion in swine waste to the environment, and make savings in diet cost. Current statistical modelling techniques have been applied to determine the dose-response to calcium and phosphorus, as well as the ratio of calcium relative to phosphorus.
Calcium requirement estimates (Table 1) are typically expressed as total calcium. Total calcium accounts for the analyzed calcium content of ingredients. Recently, values for calcium digestibility in feed ingredients have been determined (González-Vega et al., 2015a,b; Merriman et al., 2016), allowing the requirements for digestible calcium to be estimated (González-Vega et al., 2016).
Nursery diets with excessive calcium levels have a severe negative impact on growth performance (González-Vega et al., 2016). The negative impact of excessive dietary calcium on growth performance is even more evident in diets with marginal or deficient phosphorus levels (González-Vega et al., 2016; Wu et al., 2018a).
The NRC (2012) calcium requirements for nursery pigs are very high, which may lead to an impact on growth performance when formulating diets to meet the calcium requirement estimates. A practical approach consists of maintaining adequate phosphorus levels and setting calcium levels relative to phosphorus, targeting between 1.10:1 and 1.25:1 total calcium to total phosphorus ratio or between 1:20 to 1.40:1 digestible calcium to digestible phosphorus ratio (González-Vega et al., 2016). By taking this approach, the NRC (2012) calcium requirement estimates can rather be used as an indication of maximum calcium levels in nursery diets.
Phosphorus requirement estimates (Table 1) are typically expressed as digestible phosphorus. Recently, the phosphorus requirements for nursery pigs have been determined by dose-response models, allowing for a more precise estimation of phosphorus levels to maximize growth performance and optimize economics while minimizing phosphorus excretion (Vier et al., 2017).
The phosphorus requirements of nursery pigs appear to be greater than the NRC (2012) recommendations of digestible phosphorus as a percentage of the diet (Vier et al., 2017; Wu et al., 2018b). The variation on phosphorus requirements depends on the goal, but typically the phosphorus requirements to optimize phosphorus retention is greater than to maximize growth (Vier et al., 2017; Wu et al., 2018b). A practical approach consists of maintaining phosphorus levels at approximately 140% and 130% of the NRC (2012) recommendations of digestible phosphorus for nursery pigs between 12 to 25 lb and 25 to 50 lb, respectively.
The calcium:phosphorus ratio greatly influences growth performance of nursery pigs and can even be more important than the absolute concentration of calcium and phosphorus. The ideal calcium:phosphorus ratio seems to be between 1.10:1 and 1.25:1 total calcium to total phosphorus ratio (Wu et al., 2018a) or between 1:20 to 1.40:1 digestible calcium to digestible phosphorus ratio (González-Vega et al., 2016).
Nursery diets with wide calcium:phosphorus ratios or excessive calcium and marginal or deficient phosphorus concentrations interfere with phosphorus absorption (Reinhardt and Mahan, 1986). Consequently, growth performance of nursery pigs is negatively affected by wide calcium:phosphorus ratios (González-Vega et al., 2016; Wu et al., 2018a). Diets with adequate phosphorus levels allow the calcium:phosphorus ratio to be on the upper range, with a decrease in growth performance around 1.9:1 to 2:1. However, diets with marginal phosphorus levels require a narrow calcium:phosphorus ratio (Reinhardt and Mahan, 1986; Qian et al., 1996; Wu et al., 2018a).
Phytase is an enzyme that catalyzes the release of phosphorus from phytate. The addition of exogenous microbial phytase to nursery diets is a common practice to efficiently and economically enhance phosphorus utilization. Moreover, the use of phytase above conventional levels (500 to 1,000 FTU/kg) seems to have the potential to improve growth performance of nursery pigs beyond what is expected with adequate phosphorus levels (Zeng et al., 2014). The use of high levels of phytase is also becoming more common in nursery diets (Gourley et al., 2018; Laird et al., 2018).
More information about phytase is available at: Phytase in Swine Diets.