One hundred and forty-four weanling Duroc barrows were individually fed to study effects of season (summer and winter), energy concentration of the diet (high and low), protein concentration (high and low), and orally administered hormones (none, diethylstilboestrol, and mefhyltestosterone) on performance and carcass characteristics of growing-finishing swine.
In summer the rate of gain of pigs was 6% slower and yielded carcasses with 19% smaller loin-eye areas but 3% more lean cuts than in winter.
Pigs fed on high-energy diets gained 13% faster, consumed 8% less feed daily and required 18% less feed per unit of weight gain than pigs fed on low-energy diets. Pigs fed on high-energy diets also had a dressing percentage 2·5 units higher and yielded carcasses 2% shorter, with 15% more backfat, than those fed on low-energy diets.
Pigs fed on low-protein diets had a dressing percentage 1·6% higher and yielded carcasses with about 7% thicker backfat than pigs fed on high-protein diets. Dietary energy and protein concentrations interacted significantly in their effect on rate of gain. Pigs fed on low-energy, low-protein diets gained weight about 7% faster than pigs fed on low-energy, high-protein diets; however, pigs fed on high-energy, high-protein diets gained weight about 3% faster than pigs fed on high-energy, low-protein diets.
The only significant effect of giving each pig about 2 mg of diethylstilboestrol (DES) per day was a 2% increase in weight of lean cuts. An average daily consumption of 20 mg methyltestosterone (MT) per pig decreased rate of gain, daily feed intake, dressing percentage and backfat thickness, but increased carcass length, area of the loin-eye and weight of lean cuts. Hormones and dietary energy levels interacted in their effects on rate of gain and feed efficiency. Pigs fed on low-energy diets with or without hormones gained weight at about the same rate, but high-energy diets increased rate of gain in pigs receiving no hormone or DES by about 17% and 21%, respectively, while having no effect in pigs receiving MT. Pigs fed on low-energy diets with MT required about 7% and 6% less feed per unit of gain than pigs fed on low-energy diets without hormone or with DES, respectively; whereas pigs fed on high-energy diets containing MT required about 10% and 14% more feed per unit of gain than did pigs fed on high-energy diets without hormone or with DES, respectively.