that heat stress commences at temperatures of 25 8C
[28]. Heat stress may be defined as any combination of
environmental variables that give rise to conditions that
are higher than those of the temperature range of the
animal’s thermal neutral zone (5–20 8C [29]). Heat
stress before AI has been associated with decreased
fertility in countries with warm weather [30]. High
producing dairy herds are usually subjected to more
complete management practices than other herds, and it
could be that the high producing cow’s environment is
more comfortable than that of lower producers because
of the use of fans, shades, air-conditioning and other
management measures. The four herds of the study have
routine hygiene measures such as provision of clean and
frequent removal of bedding, appropriate stocking
densities in buildings, good drainage, adequate ventilation
and appropriate building design. Moreover, all
cows are in the shade, and fans opened at 25 8C. More
studies are needed to control real farm climate
conditions and to establish the impact of stress on
cows