Estimating nutrient requirements
Nutrient requirements are often estimated under different conditions. The approach to estimate nutrient requirements is similar, but some particularities of amino acids, calcium, and phosphorus are detailed here.
Determining lysine requirements
From research data
Lysine requirements can be adapted from published research data from universities (Figure 1, KSU Lysine Recommendations), genetic suppliers, or feed companies. The lysine recommendations provided from research are usually derived from dose-response experiments and are often the best data available. But it is important to take into consideration the genetic line, level of growth rate and feed intake, health status, and housing conditions adopted in the experiment before applying the recommendations to a production system.
Figure 1. KSU lysine recommendations for grow-finish pigs (KSU Lysine Recommendations).
From research within the production system
Lysine requirements can be derived from dose-response experiments conducted in research facilities within the production system. The lysine recommendations are most accurately determined by using this method because it better reflects the production system conditions. But it also requires more investment in research facilities and personnel expertise to conduct experiments.
From protein accretion curves
Lysine requirements can be determined from curves of protein accretion derived from weights and ultrasound measurements of pigs within a production system (Smith et al., 1999). This method provides good estimates of farm-specific lysine recommendations because it reflects the conditions found in a commercial production system. The weighing and ultrasound scanning start earlier in the grower period and are collected past the normal market weights to ensure the lysine requirements at the beginning and end of the period are accurately estimated. Although this method is conducted under commercial facilities within the production system and does not require investment in research facilities, an ultrasound and a skilled technician are required to conduct the measurements and perform the modeling with precision.
From growth rate and feed intake data
Lysine requirements can be estimated using growth rate and feed intake data. A simple rule of thumb is that grow-finish pigs require approximately 20 g of standardized ileal digestible (SID) lysine per kg of daily gain or 9 g per lb of daily gain (Main et al., 2008). Considering the growth rate and the feed intake of pigs in the production system, the lysine recommendations can be estimated to provide 20 g of SID lysine per kg of daily gain in diet formulation. This method is not as precise, but it provides a reasonable estimate of the lysine requirement.
Determining other amino acids requirements
The amino acid requirements are usually estimated by dose-response studies and the most common approach is to express the requirements as an amino acid ratio (Table 1). Amino acid ratio is a means of expressing the requirements for amino acids relative to the requirement for lysine. Lysine is used as a reference because it is typically the first-limiting amino acid in most swine diets and the proper concentration of lysine and other amino acids is essential for protein synthesis.
In dose-response experiments, the first limiting amino acid in the diet must be the amino acid for which the requirement is being estimated and the second limiting amino acid must be lysine. The supply of other amino acids and nutrients should meet or slightly exceed the requirements to avoid being a limiting factor. Using this approach, the requirement is determined at the point which both the tested amino acid and lysine are equally limiting and can, therefore, be expressed relative to lysine (Simongiovanni et al., 2012).
The requirements of amino acids relative to lysine can be often depicted as a diminishing returns model. This model can be used to determine which ratio provides 95 to 99% of the maximum performance and indicates the most economical amino acid ratio. Thus, the optimum amino acid ratio should be set by balancing the value accrued in performance to the incremental cost to increase the ratio.
The requirements of calcium or phosphorus are usually estimated by dose-response studies (Table 2). The estimation of requirements needs to take into consideration the calcium and phosphorus concentrations as well as the calcium:phosphorus ratio because of the close association between the minerals. Typically, wide calcium:phosphorus ratios or excessive calcium and marginal phosphorus concentration interfere with phosphorus absorption.