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Porcine reproductive and respiratory syndrome virus (PRRSV) reduces grower pig performance. The amino acid (AA) requirements and lysine:metabolizable energy ratio (Lys:ME) of health-challenged pigs for optimum performance are poorly understood. Two experiments were conducted to evaluate the effect of increasing standardized ileal digestible (SID) Lys:ME (g SID Lys per Mcal ME) on growth performance during a PRRSV challenge. In Exp. 1, a total of 379 barrows (51.3 ± 0.3 kg body weight [BW]) were allotted to one of six diets (1.87 to 3.41 Lys:ME) for a 35-d growth study. In Exp. 2, a total of 389 barrows (29.2 ± 0.23 kg BW) were allotted to one of six diets (2.39 to 3.91 Lys:ME) for a 49-d growth study. These isocaloric diets represented 80% to 130% of National Research Council (NRC) SID Lys requirement. For each experiment, pigs were randomly allotted across two barns of 24 pens each with seven to nine pigs per pen (four pens per diet per health status). On day 0, one barn was inoculated with live PRRSV, one barn sham inoculated (control), and all pigs were started on experimental diets. Pen growth performance and feed intake were recorded weekly and gain-to-feed ratio (G:F) was calculated. Breakpoint analysis was used to determine the Lys:ME that maximized average daily gain (ADG) and G:F over the 35 or 49-d test periods for Exp. 1 and 2, respectively. In Exp. 1, increasing Lys:ME increased ADG (quadratic P = 0.01) and G:F (linear and quadratic P = 0.04) in control pigs over 35 d. In PRRSV-infected pigs, ADG and G:F increased linearly with increasing Lys:ME (P &amp;lt; 0.01). The Lys:ME for optimum ADG and G:F during PRRSV challenge was 2.83 and 3.17, respectively, compared to 2.24 and 2.83, respectively, in control pigs using a one-slope broken-line model. In Exp. 2, pigs in the control barn became naturally infected after 21 days post inoculation. Before infection, ADG and G:F increased with increasing Lys:ME in control and PRRSV-infected pigs (linear and quadratic P &amp;lt; 0.05), and optimum ADG and G:F were achieved at 3.02 and 2.92 Lys:ME, respectively, in PRRSV-infected pigs compared to 2.82 and 3.22 Lys:ME, respectively, in control pigs. Over the 49-d period, increasing Lys:ME improved ADG (P &amp;lt; 0.01, linear and quadratic) and G:F (linear P &amp;lt; 0.01) in naturally infected pigs. The response was similar in experimental infection for ADG (P &amp;lt; 0.01, linear and quadratic) and G:F (linear P = 0.01). The optimum ratio for ADG (2.86 vs. 3.12 Lys:ME) and G:F (3.18 vs. 3.08 Lys:ME) were similar between natural and experimental infection. In summary, increasing Lys:ME by 10% to 20% above NRC requirements improved performance and feed efficiency during an experimental and natural PRRSV challenge.

Increased lysine: metabolizable energy ratio improves grower pig performance during a porcine reproductive and respiratory syndrome virus challenge