Dairy Heat Stress THI Chart

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Key Heat Stress Issues that Impair Dairy Milk Production

To fully help a heat stressed cow, it is important for the dairyman to understand how her body is reacting and what nutritional requirements need to be met during this period. To help with this understanding, what follows are 4 key outcomes of a cow experiencing heat stress. Note that these are not necessarily isolated challenges, but are interrelated. 

Key Heat Stress Issues that Impair Milk Production
Dehydration
• 66% of body fluid is contained within cells, 34% in the soft tissue compartments,
digestive, and vascular systems.
• Water is required to carry and distribute nutrients and provide fluid for the fetus.
• Water will be utilized for managing heat stress first, then maintenance systems
• Dehydration reduces fluids and nutrients transferred to the cells
• 7 - 8 percent dehydration levels show impaired immune response
Circulatory and Nutrient Disruption
• Blood flow is directed to the surface layers of the body to cool, putting the
digestive system at risk of becoming hypoxic (low oxygen)
• Intestinal toxins can enter circulation and cause inflammation and poor health
• Cells and tissues are deficient in required nutrients and don’t function properly
• Potassium depletes due to low feed intake, sweating, and dehydration
educed Feed Intake
• Digestion increases body heat so cows will eat less in an attempt to manage
• Heat stress accounts for a 50% drop in feed intake while decreased feed intake
accounts for 50% of the drop in milk yield
• Bout feeding during cooler periods of the day creates risk of acidosis
• Low intake of magnesium leaves cow at risk of hypocalcemia
productive Compromises
• A nutrition deficit contributes to prolonged postpartum anestrus and
impairs follicular maturity
• Inadequate nutrient intake reduces body condition score and finally cessation
of estrus cycles.
• Vitamin E and selenium work to reduce the incidence of retained placenta
• Cows with retained placenta can have longer postpartum intervals to first
ovulation and more uterine infections
• Cumulative effects of heat stress events can compromise the success of
gestation
Dehydration
• 66% of body fluid is contained within cells, 34% in the soft tissue compartments, digestive, and vascular systems.
• Water is required to carry and distribute nutrients and provide fluid for the fetus.
• Water will be utilized for managing heat stress first, then maintenance systems
• Dehydration reduces fluids and nutrients transferred to the cells
• 7 - 8 percent dehydration levels show impaired immune response

Circulatory and Nutrient Disruption
• Blood flow is directed to the surface layers of the body to cool, putting the digestive system at risk of becoming hypoxic (low oxygen)
• Intestinal toxins can enter circulation and cause inflammation and poor health
• Cells and tissues are deficient in required nutrients and don’t function properly
• Potassium depletes due to low feed intake, sweating, and dehydration

Reduced Feed Intake
• Digestion increases body heat so cows will eat less in an attempt to manage
• Heat stress accounts for a 50% drop in feed intake while decreased feed intake accounts for 50% of the drop in milk yield
• Bout feeding during cooler periods of the day creates risk of acidosis
• Low intake of magnesium leaves cow at risk of hypocalcemia

Reproductive Compromises
• A nutrition deficit contributes to prolonged postpartum anestrus and impairs follicular maturity
• Inadequate nutrient intake reduces body condition score and finally cessation of estrus cycles.
• Vitamin E and selenium work to reduce the incidence of retained placenta
• Cows with retained placenta can have longer postpartum intervals to first ovulation and more uterine infections
• Cumulative effects of heat stress events can compromise the success of gestation year round

Yellow = Stress Threshold Respiration rate exceeds 60 BPM. Milk yield losses begin. Repro losses detectable. Rectal Temperature exceeds 38.5°C (101.3°F)
Orange = Mild-Moderate Stress Respiration Rate Exceeds 75 BPM. Rectal Temperature exceeds 39°C (102.2°F)
Red = Moderate-Severe Stress Respiration Rate Exceeds 85 BPM Rectal Temperature exceeds 40 °C (104°F)
Purple = Severe Stress. Respiration Rate 120-140 BPM. Rectal Temperature exceeds 41 °C (106°F)

Chart Source: Collier and Zimbleman

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