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