EFFECT OF PORCINE SOMATOTROPIN (PST) TREATMENT AND WITHDRAWAL ON PERFORMANCE AND ADIPOSE-TISSUE CELLULARITY IN FINISHING SWINE

A 10-wk trial was conducted with 60 barrows and 60 gilts (initial weig ht = 47 kg) to investigate the effect of pST treatment (3 mg/d) for wk 0 to 5, 5 to 10, or 0 to 10 on performance, carcass traits, and adipo se tissue cellularity of finishing hogs. The primary objective of the study was to determine the effect of 5 wk of pST treatment followed by 5 wk of withdrawal on performance and carcass traits relative to a co ntrol or to pigs treated for the entire period. After 5 wk, pST-treate d pigs exhibited improved (P < .001) gain (11%) and efficiency (22%) a nd decreased (P < .001) feed intake (-22%) and 10th rib backfat accret ion (-93%, P < .001). There was an increase in adipose cell number/gra m of tissue (control, 2.57 vs pST, 3.36 x 10(6)/g; P < .005) and a dec rease in cell diameter (control, 72.4 vs PST, 67.2 mu m; P < .005). Ba ckfat thickness and cell diameter increased in control pigs from wk 5 to 10. During the last 5-wk period, pST-treated animals continued to e xhibit improved performance and maintained adipose cell size distribut ion and number similar to the pST-treated pigs at wk 5. Pigs treated f or the first 5 wk and withdrawn from treatment for the second 5 wk had decreased efficiency (-12%, P < .05) and a twofold increase in backfa t accretion (P < .001) relative to control. The increase in backfat th ickness was accompanied by an increase in adipocyte diameter, indicati ve of lipid filling. Most of the benefits of PST observed during the f irst 5 wk were lost during the withdrawal period. Reductions in carcas s fat in response to pST treatment are at least partially accounted fo r by inhibition of lipid filling in adipose tissue, an effect that is reversed upon cessation of treatment.