Heritability of Heat Tolerance in Dairy Cattle

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Abstract
Estimates of heritability of heat tolerance were made from body temp. and respiration rate data on Jersey and Holstein milking cows collected during summer months of 1944 from 52 cows by 7 sires and in 1945 from 68 cows by 8 sires. Sire-progeny groups had similar rank for the 2 yrs., but there was discrepancy between rank on basis of body temp. as compared to that for respiration rate. Some sire groups ranked high on body temp. but low on respiration rate, and vice versa. Records for body temp. when taken on the 8 warmer days had a repeatability of 15.2% in 1944 and 38.5% in 1945, as compared to 8% and 6.7% when all observation days were used. In like manner, respiration rates were more highly repeatable using only the warmer days. Estimates of herit-ability of individual records based on sire-progeny differences were for body temp. 15.1% and 30.9% for the 2 yrs., and for respiration 76.6% and 84.3%. Figures for respiration appear out of line, as they greatly exceed the estimates of repeatability and the reverse condition was expected. Body temp. appears to be a safer measuring stick for heat tolerance than does respiration rate. The estimate of heritability of body temp. (15-30%) is in line with that found for individual production records of cows. In practice this would mean that the offspring from parents selected because of their tolerance to heat would be expected to retain from 15 to 30% of the advantage that the parents had over the average for the herd or breed.