INTERACTIVE EFFECTS AMONG PORCINE SOMATOTROPIN, THE BETA-ADRENERGIC AGONIST SALBUTAMOL, AND DIETARY LYSINE ON GROWTH-PERFORMANCE AND NITROGEN-BALANCE OF FINISHING SWINE

An experiment was conducted to evaluate the interactive effects among porcine somatotropin (pST), salbutamol, and dietary lysine on growth performance, nitrogen balance, and carcass characteristics of finishing barrows (n = 32; 62.8 kg initially). Two replicate 32-d studies were set up in a split-plot design to evaluate singular and combined use of pST (0 or 4 mg/d) and salbutamol (0 or 2.75 ppm of the diet) as whole-plot treatments and diets containing .8, 1.2, 1.6, or 2.0% lysine as subplot treatments. Dietary lysine levels were administered within subplots in a 4 x 4 Latin square with pigs allowed 4 d of adjustment to diets followed by 4 d of urine and feces collection for determination of N retention and apparent digestibility of DM and N. Interactions between lysine and salbutamol were not found (P > .16). A pST x lysine interaction (P < .05) resulted in ADG being maximized at 1.2% lysine for pST-treated pigs (lysine quadratic, P < .02) but decreased linearly (P < .02) with increasing lysine for pigs receiving buffer. Pigs injected with 4 mg/d of pST had improved gain:feed (G:F) up to 1.2% lysine (lysine quadratic, P < .05), but no further increase to 2% lysine, whereas pigs injected with buffer showed no benefit from increased lysine (pST x lysine, P < .06). Pigs receiving buffer had greater increases in plasma urea N (PUN) as lysine increased than those receiving pST (pST x lysine, P < .002), but PUN was reduced by pST (P < .001) regardless of lysine level. Digestibility of N and DM was similar among pST treatments up to 1.6% lysine, but both were greater for pST-treated pigs at 2% dietary lysine (pST x lysine, P < .03). Salbutamol increased (P < .01) ADG, G:F, and digestibility of DM and N independently of lysine or pST treatment. Both pST and salbutamol improved (P < .001) N retention relative to intake and digestible N, but the magnitude of increase was greater for pST than for salbutamol. Percentage of N retained decreased linearly (P < .001) as dietary lysine increased independently of pST or salbutamol treatment. Carcass fat depth tended to be increased by salbutamol, whereas pST caused dramatic reductions and negated the effects of salbutamol. Conversely, pST and salbutamol additively increased (P < .05) longissimus muscle area and percentage of muscle. These data suggest that, regardless of salbutamol treatment, pigs injected with 4 mg/d of pST need approximately 29 g/d of lysine (1.2% of the diet) to maximize G:F and ADG.