Effects of Standardized Ileal Digestible (SID) Threonine:Lysine Ratio on Nursery Pig Performance

The objective of this study was to determine the Standardized Ileal Digestible (SID) threonine:lysine ratio on nursery pigs performance under commercial conditions. A total of 2,262 pigs (PIC 359×Camborough^®, 5.7 ± 0.1 kg) were used in a 39-d trial. Pens were assigned to dietary treatments in a randomized complete block based on initial BW and room (13 pens/treatment). Lysine was kept below the requirement to ensure that the Thr:Lys ratio was not underestimated (1.30, 1.30, 1.25, and 1.15% SID Lys for 5.7 to 6.4, 6.4 to 9.1, 9.1 to 12.2, and 12.2 to 18.8 kg, respectively). Dietary treatments were 53, 56, 59, 62, 65, and 68% SID Thr:Lys and were fed throughout the entire study. The levels of zinc oxide in the first and second diet were 1600 and 800 mg/kg, respectively. Diet analysis matched formulated values. Data was analyzed using the MIXED procedure of SAS. Linear and quadratic polynomial contrasts were used to evaluate the dose response effect. BW and room were considered random effects in the models. The NLMIXED procedure was used to evaluate competing models: linear (LN), quadratic polynomial (QP), broken-line linear (BLL), and broken-line quadratic (BLQ). From 0 to 21 d, ADG and G:F increased (linear, P = 0.016) with the increase in SID Thr:Lys, whereas ADFI and BW were not statistically affected (P = 0.236). From d 21 to 39, G:F increased (quadratic, P = 0.006) with the increase in SID Thr:Lys. However, ADG, ADFI, and BW were not statistically different (P = 0.179) between treatments. Overall, G:F increased significantly (quadratic, P = 0.001) and ADG was not statistically different (P = 0.103) as SID Thr:Lys increased, whereas ADFI was not statistically different (P = 0.236). Based on the dose-response estimation models with the best fit, the levels that maximized performance were >68% for ADG [LN; (ADG=0.64686+0.0014932×SID Thr:Lys)/2.2046] and 65% for G:F [QP; G:F=1/(3.56512–0.0661676×SID Thr:Lys+0.0005089×(SID Thr:Lys)²)]. In conclusion, formulating diets to 65% SID Thr:Lys should maximize G:F while capturing 99.4% of ADG.

	SID Threonine:Lysine
		53	56	59	62	65	68	SEM
d 0 to 39
ADGa, g		327	336	331	340	331	345	10.4
ADFI, g		490	481	476	481	472	485	16.8
G:Fb		0.667	0.685	0.694	0.699	0.699	0.699	0.008
BW, kg
d 0		5.6	5.7	5.6	5.7	5.7	5.6	0.13
d 39		18.5	18.7	18.7	19	18.6	19.1	0.45
aLinear,P=0.103.
bQuadratic, P=0.001.