Interrelationships between dietary ractopamine,energy intake, and sex in pigs

Abstract

A major constraint to protein deposition in the pig is energy intake. Ractopamine (RAC) isa b-agonist which has been shown to increase protein deposition under both ad libitum and restrictive feeding regimes. To assess the interactions between energy intake, sex, and dietary RAC, 104 crossbred pigs (52 boars and 52 gilts) were used in a slaughter-balance experiment conducted over the growth phase of 60-90 kg liveweight. To obtain initial body composition, 4 pigs of each sex were slaughtered at 60 kg. The remaining 96 pigs were allocated to a 2 6 2 factorial experiment. The respective factors were sex (boar or gilt), dietary digestible energy (DE) intake (21·2, 24·7, 28·2, 32·7, 36·7MJ DE/day and ad libitum), and dietary RAC (0 and 20 mg/kg of ractopamine.HCl). Average daily gain increased with DE intake and was faster for boars than gilts. Dietary RAC increased averagedaily gain in both boars and gilts independent of DE intake. Neither dietary RAC nor sex had any effect on ad libitum feed intakes while effects on feed : gain ratio reciprocated growth rates. Protein deposition increased with DE intake and was higher in boars than in gilts. Although protein deposition was increased by dietary RAC in both boars and gilts across the range of DE intakes investigated,ad libitum feed intakes were necessary to maximise protein deposition. Fat deposition increased with DE intake and was greater in gilts than in boars. Whereas dietary RAC had no effect on the rateof fat deposition or backfat depths, the fat content of the empty body was lower due to increased protein deposition and lower fat : protein ratio. Dietary RAC improves growth performance and carcass composition in both boars and gilts independent of DE intake. However, ad libitum feed intakes maybe necessary if responses are to be maximised.