Models to Predict the Response of Pigs to Increasing Levels of Ractopamine

Purdue University 2001 Swine Research Report. The Food and Drug Administration approved Paylean with research completed with pigs in the late 1980s and early 1990s that had lower percent lean, lower lean accretion, higher feed intakes and poorer feed efficiency compared to todays pigs. Also, recent data conducted at Purdue University indicates that a four-week duration of feeding with high lysine diets result in the most profitable use of Paylean. The objective of these analyses was to model the response of current U.S. high lean gain terminal cross pigs to alternative approved levels of Paylean (4.5 to 18 g/ton; 5 to 20 ppm). The details of the research trial are presented by Herr et al. (2001.) Three hundred gilts, with an average weight of 184 lbs., were allotted to 60 pens by weight in a 3 4 factorial arrangement of treatments in a randomized complete block design (n = 5) with three genotypes (g) and four ractopamine levels (RL): 1) control, 0 ppm; 2) 5 ppm; 3) 10 ppm; and 4) 20 ppm ractopamine (RAC). All pigs were fed an 18.6% CP, 1.1% lysine diet for the fourweek trial. The weekly pen data were fitted to numerous linear, nonlinear and biphasic equations of ractopamine level (RL), and either duration of time (DRAC, midweek days on RAC), or weight gain on ractopamine (WTGRAC.) The values of DRAC and WTGRAC were set to zero for the control treatment. The RAC response was also divided into two phases: (1) RL1, 0 for control and 5 for all other RAC treatments, and (2) RL2, (RL-RL1) for RAC treatments 2, 3, and 4. For ADG (kg/d), the equations included the fixed effects of block G and week on test (P < .01) and either a biphasic nonlinear [.0410 RL1 + .05996 (RL2).007] or asymptotic nonlinear function {.2491 [1 exp (-.713 RL)]} of RL level. For gain:feed (G:F), the models included the fixed effects of week (P < .01) and either a biphasic linear (.0084 RL1 + .0013 RL2) or asymptotic nonlinear function {.0513 [1 exp (-.717 RL)]}. The partial sums of squares accounted for by the biphasic functions were 4.4 and 8.9% greater than the nonlinear functions for ADG and G:F, respectively. For daily feed intake (ADFI, kg/d), the equation with the lowest RSD included the fixed effects of block, G, week, and .0150 RL2 (P < .01); RL1, the change from 0 to 5 ppm ractopamine was non-significant (P > .40). The change in the RAC response with weight gain on ractopamine was not significant (P > .10) for any variable. The predicted linear change in RAC response to DRAC were small for ADG (-.008, P = .19), G:F (-.0008, P = .49), and ADFI (-.0057, P = .31).