Evaluation of Standardized Ileal Digestible Tryptophan: Lysine Ratio with and without Ractopamine HCl on Growth Performance and Carcass Characteristics of Finishing Pigs

Previous research has reported that increasing SID Trp:Lys ratio to 24.5% in finishing pigs fed Ractopamine HCl (RAC) during summer months improved ADG by 70 and 37 g/d in comparison with ratios of 18 and 21%, respectively. The objective of this experiment was to determine the effects of feeding higher SID Trp:Lys ratios with and without RAC on growth performance and carcass characteristics of finishing pigs during summer months. In August 2016 (mean outside temperature 23.03°C), a total of 1,101 pigs (PIC 1050 × 327, initial BW 99.3 kg) were used in a 30-d trial. Pens of 26 or 27 pigs were randomly assigned to 6 dietary treatments arranged in a 2 × 3 factorial with main effects of RAC (0 or 10 ppm) and SID Trp:Lys (20, 24, and 28%) with 7 replications per treatment. Diets with and without RAC were formulated to 0.90 and 0.66% SID lysine, respectively. Overall (d 0 to 30), a RAC × SID Trp:Lys interaction was observed (linear, P<0.05) where increasing SID Trp:Lys improved BW, ADG, and G:F when diets contained RAC, but decreased these criteria when diets did not contain RAC. Similarly, RAC × SID Trp:Lys interactions were observed (linear, P<0.05) for carcass criteria with improvements in carcass ADG, carcass G:F, and HCW when pigs were fed increasing SID Trp:Lys in diets containing RAC. A linear decrease (P<0.05) was observed in carcass ADG, and carcass G:F in pigs fed higher SID Trp:Lys when diets did not contain RAC. In summary, increasing SID Trp:Lys ratio above 20% improved growth and carcass performance when diets contained RAC, whereas pigs fed higher SID Trp: Lys ratio above 20% had reduced performance when diets did not contain RAC.

									Probability, P <
Ractopamine HCl, ppm		0				10			R × T1
SID Trp:Lys, %	20	24	28		20	24	28	SEM	Linear	RAC
Item
ADG, kg	0.88	0.84	0.82		0.98	1.03	1.03	0.049	0.012	0.001
ADFI, kg	2.51	2.44	2.48		2.39	2.36	2.42	0.075	0.351	0.003
G:F	0.35	0.34	0.33		0.41	0.44	0.42	0.007	0.010	0.001
Final BW, kg	125.1	124.1	123.3		128.0	130.2	129.3	2.75	0.030	0.001
Carcass ADG, kg	0.64	0.61	0.60		0.72	0.75	0.75	0.036	0.009	0.001
HCW, kg	90.6	90.2	89.5		93.3	95.0	94.9	2.14	0.057	0.001
Carcass yield, %	72.4	72.7	72.6		72.9	73.0	73.4	0.20	0.490	0.001

¹ RAC × SID Trp:Lys