DISCUSSION
The ability of homeotherms to successfully maintain
a constant body temperature can be compromised when
environmental conditions limit the loss of metabolic
heat or contribute to the heat load of the animal (i.e.,
when surrounding temperature is greater than surface
temperature or heat is gained from other objects by
radiation). These environmental conditions include not
only Tdb, which affects sensible heat loss via conduction
and convection, but also humidity, because it affects
the amount of latent heat loss; wind speed, which affects
rate of sensible and latent heat loss; and radiation.
Estimates of the degree of heat stress affecting
animal regulation of body temperature were made by
developing mathematical formulae that combine one or
more meteorological variables. Here we show, that for
lactating dairy cows in a subtropical environment, the
most commonly used indices, collectively termed THI,
explain much of the variation between cows in rectal
temperature. At a practical level, the predictive value
of THI is only slightly better than Tdb alone, because
there were very high correlations between Tdb and THI1
to THI8. Thus, little is to be gained in terms of evaluating
an environment by using THI instead of Tdb.
Data for the current study were collected in a subtropical
environment in which high humidity was a
prevailing characteristic (Figure 1). It is possible that
the effectiveness of various heat stress indices will vary
DISCUSSIONThe ability of homeotherms to successfully maintaina constant body temperature can be compromised whenenvironmental conditions limit the loss of metabolicheat or contribute to the heat load of the animal (i.e.,when surrounding temperature is greater than surfacetemperature or heat is gained from other objects byradiation). These environmental conditions include notonly Tdb, which affects sensible heat loss via conductionand convection, but also humidity, because it affectsthe amount of latent heat loss; wind speed, which affectsrate of sensible and latent heat loss; and radiation.Estimates of the degree of heat stress affectinganimal regulation of body temperature were made bydeveloping mathematical formulae that combine one ormore meteorological variables. Here we show, that forlactating dairy cows in a subtropical environment, themost commonly used indices, collectively termed THI,explain much of the variation between cows in rectaltemperature. At a practical level, the predictive valueof THI is only slightly better than Tdb alone, becausethere were very high correlations between Tdb and THI1to THI8. Thus, little is to be gained in terms of evaluatingan environment by using THI instead of Tdb.Data for the current study were collected in a subtropicalenvironment in which high humidity was aprevailing characteristic (Figure 1). It is possible thatthe effectiveness of various heat stress indices will vary
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