Porcine somatotropin (pST) administered using a commercial delivery system improves growth performance of rapidly growing, group-housed finisher pigs

Abstract

Porcine somatotropin (pST) treatment of pigs consistently improves average daily gain (ADG) and feed conversion ratio (FCR). However, most studies have been conducted with individually housed pigs, of unimproved genotype and over a lighter finisher phase than current practice. The present study was designed to determine whether a commercial pST (Reporcin) treatment regime would improve growth performance in heavy boars and gilts (initial weight 79 kg) of an improved genotype and housed under commercial conditions. The 2 2 factorial experiment involved 160, 19-week-old Large White Landrace pigs (80 males and 80 females) in 20 pens of 8 pigs/pen. The respective factors were sex (boars and gilts) and dose of pST (0 and 5 mg/day). Pigs were fed a wheat-based diet formulated to contain 200 g crude protein, 10.2 g available lysine, and 14.6 MJ DE/kg to ensure that responses to pST and sex were expressed. Injections of pST were given daily for 28 days prior to slaughter using a commercial applicator designed for this purpose. Pigs were bled by venipuncture on days 7 and 28 of treatment and the plasma samples analysed for plasma urea nitrogen (PUN) and glucose. The degree of stomach ulceration at slaughter was assessed by a veterinary pathologist. Daily pST treatment increased ADG (P = 0.003), particularly in gilts, as indicated by the interaction (P = 0.015) between sex and pST. Thus, gilts treated with pST grew 23% faster than control gilts (1093 v. 1273 g/day), whereas the pST-treated boars grew only slightly faster (+2.5%) than control boars (1261 v. 1291 g/day). Feed intake was similar for boars and gilts and was decreased in both sexes by 10% during pST treatment. FCR was higher in control gilts than in boars and was improved by pST treatment. This was particularly evident in the gilts such that there was no difference in the FCR of pST-treated gilts and boars. Back fat at slaughter was reduced by 2.3 and 3.2 mm in boars and gilts treated with pST, respectively. The only stomach lesions observed were very minor in severity and there was no effect of pST on the proportion of pigs exhibiting stomach ulcers (7/80 and 9/79 for control and pST-treated pigs, respectively; χ² = 0.31, P = 0.58) or visible possible injection site lesions (0/80 and 1/79; χ² = 1.01, P = 0.31). The PUN response mirrored the effects of sex and pST on FCR. Indeed, there was a high correlation (R = +0.84, P < 0.001) between FCR and PUN. In conclusion, pST treatment of finisher pigs of an improved genotype and housed under simulated commercial conditions improved growth performance, decreased back fat, and negated sex differences.