Three hundred and twelve feeder pigs, six lots of 52 pigs each (52 pounds) were used to study the response of five antibiotic supplementations in a commercial hog finishing operation.

Lot one received neomycin 80 grams per ton, lot two Bacitracin MD (40 grams per ton), and lot three no antibiotic o Lots four, five and six received SP-250 for 10 days (five pounds per ton). After termination of SP-2S0, lot four received neomycin (80 grams per ton), lot five bacitracin MD (40 grams per ton) and lot six no antibiotic.

At the end of the 9l-day feeding period, the first draft of hogs) which included all hogs weighing approximately 200 pounds, was removed for slaughter. A second draft of hogs was slaughtered 14 days later, the remaining hogs in each lot were all removed 32 days later. The supplementation of SP-250 (lots 4, 5 and 6) for 10 days increased (P .01) the ADG by 0.2 pounds over control, Neomycin and Bacitracin MD did not affect the ADG. Combinations of SP-250 and Neomycin or Bacitracin MD produced gains similar to those of SP-250 alone. Carcass length was measured from the first rib to the aitch bone, also average chilled carcass weight and dressing per cent were obtained at the packing plant. Feed intake did not appear to be affected by any of the treatments. Feed efficiency appeared to be improved by SP-250 due to the greater gains.

The overall mean live weight at slaughter also had a tendency to be greater for lots 4, 5, and 6. There was essentially no difference among lots I, 2, and 3 or among lots 4, 5, and 6 in live weight at slaughter. Mean slaughter weights were 220 lbs. and 212 lbs. for those supplemented with SP-250 and those not, respectively. Chilled carcass weights of pens 4, 5, and 6 evidenced the slightly greater live weights as compared to lots 1, 2, and 3. The treatments had no effect on dressing % or carcass length. Those lots receiving SP-2S0 tended to have slightly thicker back fat.