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Heritability of Heat Tolerance in Dairy Cattle

Louisiana Agricultural Experiment Station, Baton Rouge

ABSTRACT

Tests of Jerseys and Holsteins involving 52 cows by 7 sires in 1944 and 68 cows by 8 sires in 1945 with respect to the heritability of heat tolerance as indicated by variations in body temperature and respiration rate gave results as follows:

1. Ranking of sire progeny by years showed a great similarity for the 2 years, although there was discrepancy between rank on basis of body temperature and that for respiration rate. Some sire groups ranked high on one basis and low on the other and vice versa.
   
2. Twenty-one cows included in study for both years showed correlations between average records for 8 warmer days (on an intra-herd-breed basis) of 0.37 for body temperature and 0.64 for respiration rate.
   
3. Using records for 8 warmer days gave a repeatability for individual body temperature records of same cow of 15.2 per cent for 1944 and 38.5 per cent for 1945, as compared to 8 per cent and 6.7 per cent for the 2 years when all observation days were used. In like manner, respiration rates were more highly repeatable using only the warmer days.
   
4. Estimates of heritability of individual records based on sire-progeny differences were for body temperature 15.1 per cent and 30.9 per cent for the 2 years, and for respiration 76.6 per cent and 84.3 per cent. Figures for respiration appear out of line, as they greatly exceed the estimates of repeatability and the reverse condition was expected.
   
5. Body temperature appears to be a safer measuring stick for heat tolerance than does respiration rate.
   
6. The estimate of heritability of body temperature (15 to 30 per cent) 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 per cent of the advantage that the parents had over the average for the herd or breed.
   
7. Using more than one record greatly increases progress through selection. If heritable portion of variance between single heat tolerance record is 20 per cent, then progress through selection would increase by 29 per cent using two records, 58 per cent with four, and 83 per cent using eight records.
   

¹ Dr. J. L. Lush of Iowa State College gave much help and many suggestions on how best to conduct this study and aided in analyzing the data secured. Dr. G. E. Dickerson of the Regional Swine Breeding Laboratory, Ames, Iowa, also assisted with statistical analyses. G. D. Miller and Dr. L. L. Rusoff of the Dairy Research Department, Louisiana State University, assisted in gathering the data and gave valuable suggestions in preparation of manuscript.

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