Genetic and heat stress interaction effects on finishing swine

Swine of two genetic composites (moderate-growth and high-lean-growth) were each subjected to three environmental temperatures: control treatment at thermoneutral (18 degrees C), with two elevated air temperature treatments to reduce ad-lib feed intake by 13 or 26%, based on control animals of each breed. The genetic composites were penned individually, three animals per chamber, and chamber temperatures were adjusted daily to obtain the desired feed intakes based on treatment averages. Feed intakes of the controls were similar between the genetic composites but the growth rate of the lean pigs was 10% greater. Growth rates of the heat-stressed moderate-growth composites were generally reduced in proportion to feed intake and there was little effect on feed conversion. However, growth rates of the high-lean-growth composites were drastically reduced by 34 and 25% for the 26 and 13% reduced feed treatments, respectively, causing much poorer feed conversion for the heat-stressed animals. Ultrasound measured backfat was similar among the three treatments for the moderate-growth pigs but backfat and intramuscular leaf fat were increased by 10 to 25% for the heat-stressed lean pigs, respectively. All animals were fed the same ration, therefore, there were apparently differences in utilization of that ration by the two genetic composites.