and relative humidity.
Because the primary nonevaporative means of cooling
for the cow (radiation, conduction, convection) become
less effective with rising ambient temperature, the cow
becomes increasingly reliant upon evaporative cooling
in the form of sweating and panting. High relative humidity
compromises evaporative cooling, so that under
hot, humid conditions common to the Southeast in summer
the dairy cow cannot dissipate sufficient body heat
to prevent a rise in body temperature. Increasing air
temperature, temperature-humidity index and rising
rectal temperature above critical thresholds are related
to decreased dry matter intake (DMI) and milk yield
and to reduced efficiency of milk yield. Modifications
including shade, barns which enhance passive ventilation,
and the addition of fans and sprinklers increase
body heat loss, lowering body temperature and improving
DMI. New technologies including tunnel ventilation
are being investigated to determine if they offer cooling
advantages. Genetic selection for heat tolerance may
be possible, but continued selection for greater performance
in the absence of consideration for heat tolerance
will result in greater susceptibility to heat stress. The
nutritional needs of the cow change during heat stress,
and ration reformulation to account for decreased DMI,
the need to increase nutrient density, changing nutrient
requirements, avoiding nutrient excesses and maintenance
of normal rumen function is necessary. Maintaining
cow performance in hot, humid climatic conditions
in the future will likely require improved cooling
capability, continued advances in nutritional formulation,
and the need for genetic advancement which includes
selection for heat tolerance or the identification
of genetic traits which enhance heat tolerance.