INFLUENCE OF GENOTYPE AND SEX ON THE RESPONSE OF GROWING PIGS TO RECOMBINANT PORCINE SOMATOTROPIN

The dose-dependent effects of porcine somatotropin [pST] on growth performance and composition of carcass gain were investigated in 150 growing pigs. The experiment involved two genotypes [barrows from the Pig Improvement Com any [PIC] and a University of Nebraska [NEB] gene pool line] and two sexes [PIC barrows and boars]. At 30 kg, pigs were randomly assigned within each genotype and sex subclass to receive daily i.m. injections of 50, 100, 150, or 200 mug of pST/kg BW or an equivalent volume of an excipient, A diet [3.5 Mcal of DE/kg] supplemented with crystalline amino acids and containing 22.5% CP was available on an ad libitum basis until pigs were slaughtered at approximately go kg live weight. Excipient-treated PIC barrows exhibited faster and more efficient growth [P < .001] and a higher capacity for carcass protein accretion [P < .001] but similar rates of lipid deposition compared to excipient-treated NEB barrows. Within the PIC genotype, control boars grew at a rate similar to that of barrows, but they were more efficient (P < .05] and deposited more carcass protein [P < .05] and less lipid (P < .001]. Carcass protein accretion rate increased [P < .001] up to approximately 150 mug of pST.kg BW-1.d-1, whereas lipid deposition decreased [P < .001] with each incremental dose of pST. Although differences between PIC boars and barrows for all criteria were negated with increasing pST dose, they were maintained between the two genotypes. Polynomial regressions suggested that a slightly higher pST dose was required to optimize the feed:gain ratio compared with rate of gain and that the dose [micrograms per kilogram BW per day] was a function of the genotype and sex [feed:gain: 185, 170, and 155; rate of gain: 155, 155, and 125 for NEB barrows, PIC barrows, and PIC boars, respectively].