Feed Formulation and Soybean Meal (SBM) Inclusion Recommendations

When replacement of SBM by distillers’ dried grains with solubles (DDGS) and/or synthetic amino acids in swine diets is too extreme, growth and feed conversion decline and thus profit is impaired. This was not recognized as a concern until tryptophan, the fourth essential amino acid, entered commercial use in 2013. The combination of tryptophan and DDGS caused dietary SBM volume to plummet by an additional 250–300 lbs/ton in growing and finishing diets.

The primary benefit of using higher SBM levels occurs during the summer months (May–September, depending on location). This timing minimizes the predictable reduction in carcass weight that results from suppressed feed intake due to heat stress (see Section 3). This effect (commonly referred to as the “summer dip”) is well known and has typically been considered a persistent problem and a cost of doing business.

Higher SBM levels may also prove beneficial during winter and early spring (Jan–April) when swine respiratory disease (SRD) tends to be most active (see Section 2, Q2). SRD has a profound effect on gain and feed conversion, and optimum SBM levels can mitigate this problem. Respiratory disease growth suppression may also account for a premature carcass weight decline ahead of the summer months.

It is recognized that NE values for SBM published in international references are too low, and not aligned with more recent animal growth experiments. More reliable estimates of both NE and ME for swine have been derived from growth assays and recently published in the June 2023 edition of Feedstuffs, Amino acid levels and energy specifications in SBM for poultry and pigs.

Given that SBM NE increases as SBM crude protein content increases, it is best to express NE as a percentage of the corn NE value, where the NE value is typically 85%–95% of corn NE. There is also evidence that the NE value expressed may be greater under commercial conditions when pig health is challenged. A conservative approach is to use the lower NE values published in Feedstuffs, and let the pig express the value through improved feed conversion if there is a health challenge. This simplifies formulation while also taking advantage of the SBM benefit on feed conversion (FCR).

The maximum level of synthetic L-lysine that should be used is the same, whether or not a minimum SBM level is set. These have been determined in practice for each phase of growth, and are shown in the table below. The number of feeding phases is greater than some systems use but can be reduced according to need.

The maximum limits for synthetic lysine are often exceeded, especially in pig weights of 200 lbs. or more. Feed invoice is a driving force, but skilled nutritionists have studied the consequences of exceeding the limits on both growth and feed conversion. With the new understanding, the suggested minimum SBM levels should only be applied under conditions of (1) summer heat stress or (2) when respiratory disease stress is a persistent problem. They do not apply to fall diets unless respiratory disease is a problem (e.g., influenza).

Minimum levels of SBM for heat stress were established by a large commercial system, since this normally represents the biggest profit opportunity (see Section 3, Q1). A specific and lower recommendation has also been provided for when respiratory stress is an issue (see Section 2, Q2). In each case, there is a significant benefit to carcass weight gain; FCR is also markedly improved during respiratory stress. Typical diets use too much DDGS and/or synthetic lysine; they displace SBM and the natural growth- and health-promoting functional bioactive compounds that it contains.

Maximum Synthetic L-Lysine Levels by Phase of Growth to Prevent Performance Loss

With the economic competitiveness of synthetic amino acids over the past decade, significantly higher levels have been used in swine diets, resulting in less predictable performance results. Exceeding maximum recommended levels of L-lysine HCl (and consequently not meeting recommended soybean meal/crude protein levels), particularly in late-finishing pigs (e.g., 180–300 lbs), can severely compromise growth and feed efficiency. SBM provides important functional bioactive compounds, as well as the proper balance of essential amino acids. Recent evaluations in a large production system indicated performance improvements when using minimum SBM levels in the summer months, which reduced the summer weight dip. Functional bioactive compounds contained in SBM have also provided performance improvements during health-challenged periods of the year (e.g., Jan–April).

Lysine:energy ratios are helpful and should be used for extreme energy level changes. However, the lysine:energy ratio must meet the needs of the specific pigs involved, because the pigs have a great propensity to “eat around” energy differences in their diet. The cost of maintaining energy in the diet, while allowing the use of lower energy ingredients, is much greater today because of higher fat and oil prices. The lysine:energy ratio is commonly used in the formulation process, but formulators must verify that their software settings allow the least-cost process to lower energy slightly to achieve least cost per unit of dietary energy. If the ratio is fixed, then in some cases the ratio will increase feed costs without performance benefits. If a formulator is not familiar with this issue, we recommend using NE and SID-lysine levels as the formulation pressure points (for a given production phase and genetic source) and routinely monitoring the ratio of SID Lysine to NE.

The preferred method is to set maximum L-lysine limits, and then set appropriate ratios of the most limiting amino acids to the diet lysine specification. A second step is to set a minimum protein limit, to increase the confidence in obtaining more consistent performance. It is a current practice to set a minimum level of crude protein and maximum level of L-lysine HCl. However, when elevated SBM content is required to deliver health benefits, a crude protein minimum constraint is not recommended because alternative protein sources could meet a minimum protein constraint but with insufficient SBM. In this case, a minimum SBM constraint should be used.

BCAA are a minor concern, and formulating with a minimum level of SBM actually helps, as compared to using excessive levels of corn protein as DDGS. BCAA are less of a concern in finishing diets than in nursery diets. The primary formulation pressure should be on lysine, threonine, tryptophan, and methionine + cysteine. The BCAA imbalance that is created by using high DDGS levels becomes significant because it displaces SBM. SBM is important to preventing imbalance and the adverse effect an imbalance can cause.

It is a well-established fact that different genotypes have different protein deposition rates (see graph). Animal nutritionists need to work with the genetic supplier to tailor nutrient levels to protein accretion rates. Currently, our understanding of SBM level in supporting performance during stressful periods (heat stress, health challenge) is not differentiated across genetic lines. If there is a difference, it is expected to be small and hard to determine.

Genetic Differences in Protein Accretion Rate