New research shows health-challenged pigs perform better with higher lysine to energy ratios

Increasing ratio of lysine to metabolizable energy helps pigs fight health challenges, no matter how the increase is achieved.

September 4, 2021

7 Min Read
New research shows health-challenged pigs perform better with higher lysine to energy ratios

Pigs facing a health challenge such as Porcine Reproductive & Respiratory Syndrome virus (PRRSV) show a significant loss of performance. In 2019, Schweer et al. showed that increasing the ratio of lysine to metabolizable energy to 110 to 120% of the National Research Council (NRC) recommendations improved both performance and feed efficiency in pigs challenged by PRRSV.  Building on that work, Jasper and co-workers recently validated this and compared different ways of increasing that ratio, and monitored pig performance. 

Swine nutritionists generally formulate feed based on energy, amino acids, and the ratio of grams of standardized ileal digestible lysine per Mcal metabolizable energy (SID Lys:ME).  In diets composed of corn and soybean meal (SBM), the SBM is the primary source of amino acids, including lysine, which is the first-limiting amino acid.  It may be that sick pigs eat less overall, thus reducing both their dietary energy intake and their amino acid intake.  In this study, the ratio was changed to 120% in two different ways – by increasing the amount of SID Lys (numerator) or by reducing the ME (denominator). 

Four hundred pigs were used, all 19-21 days old at the start.  They were randomly divided into two identical barns, and on day one all the pigs in one barn were vaccinated with a live modified PRRSV vaccine (Ingelvac MLV, Boehringer Ingelheim), while those in the other barn were not.  All pigs in both barns were fed the same nursery diet.  After 42 days, they were further subdivided into grower pens and fed the same corn-soybean meal diet for the next 14 days. 

At this point and within vaccine status, pigs were allotted to one of three diets:

  • Control - 2.69 g SID Lys:ME, or 100% Lys:ME based on NRC recommendations

  • High Lysine (HL) -  3.23 g SID Lys:ME, including soybean meal and synthetic amino acids

  • Low Energy (LE) - 3.22 g SID Lys:ME, dietary ME content was reduced by including sand.

Both the HL and LE diets contained 120% of the requirements for 35 to 75 kilogram (kg) pigs, and the total amounts of calcium, phosphorus, and ratios of SID threonine (Thr), tryptophan (Trp), methionine (Met), isoleucine (Ile), and valine (Val) to SID Lys were similar in all diets. Crude protein (CP) levels and soybean meal (SBM) inclusion rates were similar in both the control and LE diets, but the HL diet had an increased level of soybean meal (26.5%, as compared to 19.4% (control) and 22.0% (LE)), which probably increased intake of multiple amino acids, not only lysine. 

On day 56 post-weaning, all pigs in both barns were challenged with PRRSV (day post-inoculation [dpi] 0), and fed these diets for 42 days. 

The pigs were then allowed unrestricted access to their assigned diet and water for the next 42 days, and their health and growth monitored weekly.  By dpi 7, all pigs were confirmed viremic and positive for PRRS. All pigs become naturally, and unintentionally, infected with porcine circovirus 2 (PCV2) between dpi 7 and 14.  Due to the co-infection, the vaccinated barn experienced 11 mortalities (5.6%) and the unvaccinated barn experienced 22 mortalities (11.3%).  Mortality did not differ across dietary treatments.  All pigs were placed on water amoxicillin, to decrease the risk of additional opportunistic pathogens. 

During the first seven 7 dpi on test diets, the non-vaccinated LE pigs had greater average daily gain (ADG) and feed efficiency. However, vaccinated pigs on the LE and HL diets had only slightly higher ADG and increased average daily feed intakes (ADFI) compared to the control pigs.

The final body weight of pigs fed the HL and LE diets were 6.9 kg and 4.2 kg heavier, respectively, when compared to the control pigs. While the final body weight of pigs fed the HL and LE diets were 5.4 kg and 3.2 kg more, respectively, than the control pigs in the non-vaccinated barn.  Over the entire challenge period (dpi 0-42; Table 1), ADFI increased by 19.8% (20% and 17% in vaccinated and non-vaccinated pigs, respectively) for pigs on the LE diet, but the HL treatment was not statistically different from the control.  This shows that immune-stimulated pigs can voluntarily adjust their feed intake and will attempt to eat to their energy needs, regardless of the energy density of the feed. Irrespective of vaccine, these data agree with earlier work that increasing the SID Lys:ME ratio to 120% in growing pigs facing a viral challenge significantly mitigated the usual negative growth performance associated with PRRS.

The increased ADFI in the LE diets could explain the improved ADG. While the use of sand to dilute the energy is not a practical commercial strategy, dietary fiber could be a more practical approach. However, the degree to which fiber augments satiety and impedes appetite will need to be examined further.

Experiencing a performance-limiting disease during the growth phase is almost inevitable in the swine industry.  Increasing the ratio of SID Lys:ME is a viable strategy for improving performance during disease challenges, regardless of how that increase is achieved. 

Table 1. Effects of increasing the ratio of standardized ileal digestible (SID) lysine to metabolizable (ME) on growth performance in PRRSV infected pigs. Adopted from Jasper et al.[1]


g SID1 Lys:Mcal ME2


2.69 (control)

3.23 (HL)

3.22 (LE)


    Start BW, kg



    End BW, kg



    ADG, kg



    ADFI, kg



    ME intake/d, Mcal







    Start BW, kg



    End BW, kg



    ADG, kg



    ADFI, kg



    ME intake/d, Mcal








Increased lysine: metabolizable energy ratio improves grower pig performance during a porcine reproductive and respiratory syndrome virus challenge.  Wesley P Schweer, Omarh F Mendoza, Caleb M Shull, James Lehman, Aaron M Gaines, Kent J Schwartz, Nicholas K Gabler.  Translational Animal Science, Volume 3, Issue 1, January 2019, Pages 393–407.

National Research Council. Nutrient requirements of swine. 11th ed. Washington: Natl. Acad. Press; 2012.

Increasing the ratio of SID lysine to metabolizable energy improves pig performance during a viral challenge.  Jessica E. Jasper, Omarh F. Mendoza, Caleb M. Shull, Wesley P. Schweer, Kent J. Schwartz, and Nicholas K. Gabler, Journal of Animal Science, 2020, 1–10.  doi:10.1093/jas/skaa082


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