-  All cattle are at risk on becoming hyperthermia, and the problem is greater on lactating dairy cattle populations.

- The ability to detect estrus declines during periods of heat stress due to:

-- A reduction in the duration of estrous behavior. 

-- Reduced locomotors activity.

-- Reduce peripheral concentrations of estradiol-17B,

-- Decrease gonadotropin responses to injection of GnRH in cows with low circulating levels of estradiol-17B

- Heat is the major stressful environmental factor due to:

-- lowered conception rates in the summer.

-- Disrupted ovulation or early pregnancy.

--Altered the normal embryonic development.

- Fertilization failure has been associated with heat stress due to:

-- Effects on the oocyte during follicular development,

-- Effects follicular growth and dominance,

-- Alters the quality of the oocyte.

- Embryos become more resistant to effects of heat stress as pregnancy proceeds due to:

-- Greater cellular resistance to elevated temperatures.

-- Heat-shock protein synthesis in embryos,

-- Other biochemical changes may also be important.

- Hyperthermia disrupts endometrial function through

 -- The inductions of endometrial synthesis of two heat shock proteins (HSP-90 and HSP-70) that are the components of the progesterone receptor complex. In addition, the increased synthesis of HSP-70 and HSP-90 in endometrium inhibits endometrial responsiveness to progesterone.

-- It reduces secretion of the anti-luteolytic hormone, Interferone-7 in conceptus by day 17.

-- Increased release of PGF_2∞ and PGE₂ from the endometrium.

-- Increased uterine production of PGF_2∞in response to oxytocin.

- Abortion caused by heat –stress is rare,

- Placental function is influenced by heat stress in the last third of pregnancy during the summer or experimental heat stress, through:

-- Reduced secretion of placental estrone-sulfate hormone.

-- Reduced placental size and calf birth weight.

-- Reduced blood flow to the placenta.

-- Reduced subsequent milk yield.

The effect of Heat stress on reproduction can be prevented by:

1-  Increase estrous detection in summer ovulation through the application of estrus synchronization protocols

2-  Cooling cows around the time of anticipated estrus might also increase estrous detection.

3-  Altering the environment to reduce the magnitude of heat stress improves fertility in Lactating cows.

4-  Reproductive biotechnology can eliminate infertility caused by heat stress:

- By A.I., semen collected in cold environments can be stored in a frozen state until later use in hot weather.

- By E.T., embryos that develop to the blastocyst stage with acceptable morphologic characteristics are transferred. Embryos produced by super-ovulation are more likely to survive in heat-stressed recipients than are embryos produced by in-vitro fertilization.

How can you manage a cow in heat stress?

1.    Provide shade. The sun's radiant energy adds to the heat load. However, don't cut out ventilation using an enclosed building to provide shade.

2.    Control horn flies. Flies cause the cows to "bunch-up".

3.    Don't let dry cows to lose weight. If more ration energy is needed, it is probably better to feed higher quality forages than more grain or supplementing fat particularly for far-off dry cows.

4.    Don't feed excessive protein (>12-13%). Metabolizing the extra protein adds to the heat load.

5.    Consider supplemental cooling especially around calving time.

6.    Don't eliminate salt. Sodium and potassium add to a positive cation: anion balance which is undesirable ahead of calving and salt (sodium) has been implicated with udder edema. However, the cow's need for these cations increases with heat stress.