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Repeatability of Heat Tolerance Observations

Dairy Research Department, Louisiana Agricultural Experiment Station, Baton, Rouge

ABSTRACT

Data gathered from observations made for 14 days on 20 milking Holstein cows were used in this study. Mean air temperatures at 2: 45 p.m. during the experimental period averaged 88° F. and relative humidity 60 per cent. Respiration rates were determined on cows while they were in the shade within pasture and just prior to entering milking barn. Shortly after entering milking barn both respiration rates and rectal body temperatures were determined.

Results of this study show the following:

1. The 14-day mean respiration rates for cows while in pasture averaged 78.5 times per minute and increased to an average of 80.4 times per minute for counts made in barn. Thirteen of the 20 cows had higher average rates after entering the barn, while seven had higher average rates while in pasture.
   
2. Relative ranking of the ten highest cows on the basis of pasture observations included seven also ranking highest when barn observations were used. The highest (1st) and lowest (20th) ranking cows were the same in both eases.
   
3. Computation of the repeatability of a single respiration record of the same cow resulted in 0.366 for pasture observations and 0.446 for barn observations.
   
4. Repeatability for body temperature observations made in the barn was 0.288. The mean body temperature for the 20 Holstein cows during the 14-day period was 103.4° F.
   
5. Computations are presented which show how the use of several observations increases the variance due to permanent differences between cows, much of which is genetic. With six respiration records this portion of the variance increased from an average of 0.4 to 0.8, and with seven body temperature observations the increase was from the average of 0.25 to 0.7.
   It is concluded that respiration observations taken in the barn are as reliable as and perhaps slightly more reliable than those taken in the pasture. The use of averages for observations plus those for body temperature made on approximately 6 or 7 relatively warm days should result in measurements that are relatively accurate for use in selecting cattle for a high degree of tolerance to heat.
   

¹ Dairy husbandman and assistant in dairy husbandry, respectively. Acknowledgment is made to Cecil Branton, assistant dairy husbandman, for suggested changes made in the final manuscript.