The leaner your pigs, the lower their heat tolerance threshold. Data from studies at USDA’s Meat Animal Research Center in Clay Center, Neb., back up that conclusion.

John Nienaber, research leader at MARC, ran trials on two genotypes of pigs from 110 pounds to 220 pounds.

The first genotype averaged 1.9 pounds of gain per day and had a 10th-rib backfat average of 0.9 to 1 inch. The second genotype tested gained 2.27 pounds per day with backfat averaging 0.7 to 0.8 inch.

The first line was defined as a moderate lean-growth genotype, says Larry Turner, University of Kentucky agricultural engineer. The second group was classified a high lean-growth genotype.

Nienaber’s trials show significant differences in the threshold at which heat stress affects performance of the two genotypes.

At 110 pounds, pigs of both genotypes suffered heat stress around 73° F. At 125 pounds, the moderate lean-growth pigs could tolerate temperatures up to about 84° F. High lean-growth pigs were affected at 81° F.

But as pigs grew, heat had more impact on the high lean-growth animals. From 145 to nearly 190 pounds, high lean-growth pigs were affected any time the temperature reached 75° F. Moderate lean-growth pigs weren’t stressed until temperature climbed to 82° F or higher.

From 190 pounds to 220 pounds, the high lean-growth hogs suffered if temperatures climbed past 70° F. The moderate lean-growth pigs weren’t stressed until temperatures surpassed 85° F.

Turner says this shows you have to treat different genotypes differently to avoid heat stress problems.

University of Kentucky graduate student Tami Brown-Brandl is working on a related trial. She’s feeding diets with high amino acid levels to pigs to see how it may temper heat stress effects in high lean-growth and moderate lean-growth pigs.