Reducing dietary crude protein levels in growing pigs

Etienne Corrent, Development and Scientific Coordination Manager for Ajinomoto Eurolysine s.a.s.

Recent advances in the determination of amino acids (AA) requirements of pigs and increasing availability of feed-grade AA allow a reduction in the crude protein (CP) content of feed, while maintaining the supply of essential AA. Such diets markedly reduce nitrogen excretion without detrimental effects on nitrogen retention or growth performance. In piglets they significantly contribute to alleviate health (gut) disorders. In addition, moving from a formulation based on (minimum) CP to a formulation based on each essential AA leads to supply a better balanced AA profile (no lack and no excess), gives much more flexibility in the choice of feedstuffs, and results in a significant reduction of the feed costs.
Indeed, the impact of the high prices and volatility of the protein feedstuffs on the costs of the formulas can be reduced by implementing a more precise AA formulation strategy. For the successful implementation of this approach, an accurate knowledge of feedstuffs AA contents and the use of a net energy based system are a prerequisite. Then estimates of the response and the minimum requirements of each of the essential AA for pigs are needed to make choice of an adequate ideal AA profile and thus reduce dietary CP safely.

Essential amino acid requirements

Twenty AA are the building blocks of proteins. Nine of them (Lysine, Threonine, Methionine, Tryptophan, Valine, Isoleucine, Leucine, Histidine and Phenylalanine) are either not synthesised or synthesised only in small quantities by pigs (Boisen, 2003). They must therefore be supplied through the diet and are referred to as essential or indispensable AA (EAA). Feeds must provide the EAA in sufficient quantities to cover the requirements of animals, while avoiding an excessive supply. With the availability of five crystalline AA on the market it is possible to formulate low-protein diets where these AA (plus Cystine) are co-limiting at the same time, up to a point where a seventh AA becomes co-limiting. Thus, the extent to which dietary CP can be reduced depends on the limiting AA in a diet and therefore on the requirement level that is set for each of the EAA. The determination of AA requirements is generally done within the ideal protein concept (ratios to Lys), by dose-response trials but several factors canaffect the outcome of a dose-response study, starting with the protocol itself. It is therefore important to review and analyse carefully the available data and how they were generated before making decisions on requirement levels. Tryptophan (Trp), Valine (Val) and Isoleucine (Ile) are on the list of the next limiting AA to be controlled in a growing pig formula.

Tryptophan requirement

The Trp requirement and its specific effect on feed intake and health status in piglets have been extensively studied and reviewed (Le Floc’h et al., 2007; Simongiovanni et al., 2012). Tryptophan is also known to limit the growth of heavier pigs when its dietary supply is reduced. In a recent work, Simongiovanni et al. (2013) reviewed 87 trials testing different levels of dietary Trp fed to pigs from 25 to 120 kg with the aim to perform a meta-analysis of the animal response to Trp. The nutrient composition of diets was recalculated from feed ingredients and information from INRA tables. Overall, a response to Trp is described indicating that the level of this AA must be controlled in the feed since it is a predictor of performance. Among the trials, 13 were designed to express the requirement relative to Lys (i.e, Lys was the second limiting factor after Trp) while testing at least 4 levels of Trp, and these trials were considered in the meta-analysis work (Figure 1). The estimated SID Trp:Lys requirements were 20.9, 19.9 and 21.0% for ADG, ADFI and Gain to Feed (G:F) respectively, with an average value of 20.6%. The response between 17 and 21% SID Trp:Lys levels was estimated to be +6.7 and +3.6% for ADG and G:F respectively.
To validate this outcome, the model can be compared to the work of Zhang et al.  (2012) which has not been taken into account in the meta-analysis study (data collection period was till 2011). In their work, Zhang et al. (2012) set a response to Trp in Grower pigs (28 to 55 kg live weight -LW-) and refined a requirement between 20 and 23% SID Trp:Lys depending on the statistical model and the criteria to maximise.  As it is shown in Figure 2, the comparison with Simongiovanni et al. (2013) validates the model as a good predictor of the response to Trp in fattening pigs, and confirms that Trp is a limiting nutrient for the performance.

Valine and isoleucine requirements

Valine is a limiting essential AA in current piglet diets and potentially limiting in fattening pig diets containing very low levels of CP. In comparison to piglets, the Val requirement of fattening pigs has not been extensively studied since, considering the current CP levels, it is a less potentially limiting AA for heavier pigs than Trp for instance. Van Milgen et al. (2013) recently published a review and meta-analysis of the existing Val dose-response trials for growing pigs and concluded that, on average, a dietary level of 69% SID Val:Lys ratio is necessary to achieve the best ADG in the considered data set. This supports the practical usual recommendation of 70% SID Val:Lys implemented in piglet (<25 kg LW) but the question of the optimum Val level in fattening pigs (>25 kg LW) remains to be confirmed since only few studies have been performed (i.e., one study only is considered in van Milgen et al., 2013 work). An optimum value is therefore difficult to assess but it is at least recommended to apply a minimum level of 65% SID Val:Lys for pigs above 25 kg when dietary CP is reduced in fattening pigs.
Isoleucine requirement in growing pigs used to be controversial due to published values varying from less than 50% to more than 60% SID Ile:Lys for the same weight category. One of the main reasons of such variability is now explained and lies into the usage of Spray Dried Blood Cells (SDBC) in the dose-response experiments. The SDBC are poor in Ile and so are a very good material to use in basal diet dedicated to perform Ile dose responses. But, at the same time, the SDBC have an imbalanced global AA content (low level of Ile but excessive amount of Val, Leu, His & Phe) which leads to singular interactions. This results in an increasing catabolism of Ile and so in an apparent increase of the Ile requirement. The work of Wiltafsky et al. (2009) and a meta-analysis work of van Milgen et al. (2012) clearly indicate that without usage of SDBC in pig diets the Ile requirement is between 50% and 54% SID Ile:Lys.

Ideal amino acid profile

These recent and published works allow to assess a dietary ideal AA profile for growing pigs. Recommendation of an AA profile must take into account several factors and is not just an average of published values. As a basis, meta-analyses must be performed to efficiently review a data set and determine a response and a “requirement” value. The methodological aspects must be analysed to understand from where comes the apparent variability in the different trials (protocols, statistical models). It must be also reminded that a requirement estimate is firstly linked to the feedstuffs table or the analyses used in the trial. Then, at the time of making a choice on the AA level, the accuracy of the user’s matrix must be considered since an AA profile is also a tool of nutritional risk management. The economic issue is also influencing the decision but the economic optimum varies at every update of the feedstuffs price list and does not change the animals requirement per se. In Table 1, an AA profile is given based on literature review. The efficiency of this AA profile has been tested and confirmed in low CP trials in piglets or fattening pigs (Figure 3).

Impact of dietary crude protein reduction on performance and feed costs

The reduction of the dietary CP level has no impact on performance as soon as the AA and energy levels are set properly. As Figure 3 shows, dietary CP is not a predictor of performance and so managing risk with this constraint is costly and inefficient.
By using minimum constraints on each EAA, it is possible to optimise a formula without controlling protein per se. The limiting EAA will determine the protein level and dietary CP will be reduced with confidence.
By using at least the minimum constraints recommended in Table 1, and formulating without a minimum CP constraint, substantial costs savings can be realised. The impact on costs savings of formulating by using either a minimum CP constraint or a minimum on each EAA is presented in Figure 4, based on raw materials prices from January 2012 to July 2013.
In this exercise, a Grower formula has been considered (16% minimum CP, 0.85% SID Lys, 9.7 MJ/ Kg Net Energy). When the AA profile (Table 1) is implemented instead of the minimum CP constraint, the feed price is always reduced, from -2 to -13 €/T depending on the month. The CP level decreased from 16% to about 14.5% insuring that the minimum EAA needs were covered. In these formulas, soybean meal was substituted by cereals, rapeseed and/or sunflower meals and feed-use AAs.

Reference available on request.

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