Heat-Stress Nutrition for Dairy Cows: Adjusting Summer Rations in Vietnam

Summer in Vietnam is not only a challenge for farmers. It is also a particularly challenging physiological period for dairy cows. When both temperature and humidity rise, dairy cows — especially imported high-yielding breeds or crossbred Holstein Friesian cows with a high proportion of Holstein genetics — are more likely to experience heat stress. Common consequences include reduced feed intake, lower milk yield, poorer body condition, and negative effects on reproductive performance.

Understanding how heat stress works and adjusting rations in time are important steps that help farms limit losses during the hot season. However, heat-stress nutrition should not be understood as simply “adding minerals” or “adding so-called cooling additives.” It is a process of adjusting the entire ration, including energy, fiber, starch, fat, electrolytes, drinking water, and feeding time.

What Is Heat Stress in Dairy Cows and What Is the Situation in Vietnam?

THI and heat stress thresholds for dairy cows

The Temperature-Humidity Index (THI) combines air temperature and relative humidity. It is used to assess heat load or heat stress risk in dairy cows.

Under the traditional threshold, a THI above 72 is often considered the beginning of the heat stress zone. However, in high-yielding dairy cows, signs of heat stress may appear at lower thresholds, depending on milk yield, breed, ventilation, solar radiation, stocking density, and actual management conditions.

A commonly used THI formula is:

THI = (1.8 × temperature °C + 32) − (0.55 − 0.0055 × humidity %) × (1.8 × temperature °C − 26)

For example, at 33°C and 75% humidity, the THI calculated using this formula is around 86–87. This is a significant level of heat stress if it lasts for many hours during the day, especially for cows in high milk production.

Summer climate conditions in key dairy farming regions in Vietnam

In southern areas such as Cu Chi, Binh Duong, and Dong Nai, the hot dry season usually occurs from March to May. During peak heat waves, daytime temperatures may exceed 34°C. When combined with high humidity, THI can rise sharply. If THI stays high for several consecutive hours, cows are not only stressed during the hottest part of the day but may also accumulate heat if the night is not cool enough for them to release body heat.

In northern areas such as Ba Vi and Moc Chau, summer from June to August may not always be as hot as the South at certain times. However, high humidity and long periods of wet weather can still push THI upward. For high-yielding dairy cows, prolonged hot and humid conditions may cause mild to moderate heat stress, with clear effects on feed intake and milk yield.

It is important to note that dairy cows do not respond only to air temperature. They are affected by a combination of humidity, ventilation, solar radiation, stocking density, floor or bedding dryness, and actual drinking water intake.

Effects on feed intake, milk yield, and reproduction

When dairy cows experience heat stress, their bodies try to reduce heat production by lowering dry matter intake (DMI). In many cases, DMI can fall clearly compared with normal conditions. The actual level of reduction depends on breed, lactation stage, milk yield, barn environment, and ration quality.

Common consequences include:

• Lower milk yield: A shortage of energy, protein, and amino acids directly affects milk synthesis.
• Lower conception rates: Heat stress can disrupt reproductive hormones, reduce visible estrus signs, and reduce pregnancy success.
• Poorer body condition: Cows may mobilize body reserves to compensate for the energy shortage, leading to a lower body condition score (BCS).
• Rumen pH disturbance: Cows breathe faster to release heat, which can affect acid-base balance and reduce rumen buffering efficiency. This increases the risk of digestive disorders.
• Higher risk of postpartum health problems: Cows that lose body condition, eat less, and experience prolonged stress may be more prone to issues such as ketosis, mastitis, or reproductive disorders.

Common Dairy Cow Breeds in Vietnam and Their Sensitivity to Heat

Purebred and crossbred Holstein Friesian cows in southern Vietnam

Holstein Friesian (HF) is a dairy breed with high milk production potential, but it was primarily developed and selected under temperate climate conditions. Because of their high milk yield and strong metabolism, purebred HF cows or crossbred HF cows with a high proportion of Holstein genetics usually produce more metabolic heat and are more sensitive to heat stress.

In southern Vietnam, many commercial dairy herds are crossbred HF cows. This group has an advantage in milk yield, but it needs better nutrition, cooling, ventilation, and drinking water management to maintain performance during the hot season.

When THI remains high, a drop in milk yield may be observed after a few days. The speed and level of decline depend on the individual cow, lactation stage, barn quality, and the farm’s ability to intervene.

Heat-tolerant breeds and their production limitations

Breeds such as Sahiwal, Red Sindhi, or Zebu crossbreds usually have better heat adaptation thanks to several physiological and physical traits, such as heat dissipation ability, sweat gland function, skin traits, hair coat characteristics, and a metabolic rate that is more suitable for hot climates.

However, the milk yield of these breeds or crossbreds is usually lower than that of high-yielding HF cows. For this reason, many commercial farms still choose crossbred HF cows and compensate with nutrition, ventilation, cooling, and environmental management instead of completely switching to heat-tolerant breeds.

Adjusted Nutritional Needs for Dairy Cows in Tropical Summer Conditions

Nutritional requirements by lactation stage when THI exceeds the threshold

When DMI decreases because of heat stress, the ration needs higher nutrient density so cows can still receive enough energy, protein, and minerals from each kilogram of dry matter consumed. The table below is a reference guide and does not replace a detailed ration formulation for each farm.

Nutritional indicator	Cool-season ration	Guideline when THI is high	Notes
Net energy for lactation (NEL)	1.55–1.60 Mcal/kg DM	Around 1.65–1.72 Mcal/kg DM	Increase energy density because DMI decreases
Crude protein (CP)	16–17% DM	Around 17–18% DM	Prioritize protein quality and appropriate RUP
Total NDF	28–32% DM	Around 30–33% DM	Do not increase low-quality fiber too much
Forage NDF	Depends on the ration	Around 22–25% DM	Monitor DMI, rumination, manure, and milk fat
NFC/starch, non-fiber carbohydrates/starch	38–42% DM	Around 35–38% DM	Reduce rapidly fermentable starch if acidosis risk is high
Potassium (K)	1.0–1.2% DM	Around 1.5–1.8% DM	Calculate based on the total ration, not only supplementation
Sodium (Na)	0.18–0.22% DM	Around 0.35–0.45% DM	Balance with salt, premix, and drinking water
Magnesium (Mg)	0.25–0.30% DM	Around 0.30–0.35% DM	Monitor total dietary Mg and herd response

These indicators should be adjusted based on milk yield, DMI, forage type, starch level, milk fat, manure, rumination status, and the barn’s cooling capacity.

Changes in energy, protein, and mineral needs when temperature rises

A common misunderstanding is that heat-stressed cows do not necessarily need more total energy. However, because they eat less, each kilogram of dry matter needs to have higher energy density. The energy source also needs to be selected carefully. Farms should not simply increase starch to compensate for energy, because rapidly fermentable starch may increase the risk of acidosis and fermentation heat in the rumen.

For protein, farms should not only increase crude protein. They should prioritize protein quality and balance RUP (rumen undegradable protein) and suitable amino acids to make up for the shortage without increasing the fermentation burden in the rumen.

For electrolytes, heat-stressed cows may lose more potassium, sodium, and magnesium through sweating and saliva losses, as well as changes in acid-base balance. These minerals should be balanced based on the total ration DM, including minerals already present in grass, concentrate feed, premix, and drinking water.

Comparison Between a Regular Ration and a Heat-Stress-Adjusted Ration

The table below illustrates the general adjustment between a regular TMR ration and a heat-stress-adjusted ration for lactating crossbred HF cows. The figures are for reference and should be adjusted based on each farm’s real data.

Ingredient / Indicator	Regular ration	Heat-stress adjustment guideline	Notes
Fresh grass / silage / quality hay	Based on the base ration	May increase if quality is good	Do not increase poorly digestible fiber too much
Straw / treated straw	Used in limited amounts	Use only to support effective fiber	Avoid high levels if it reduces DMI
Mixed concentrate feed	Based on milk yield	May reduce rapidly fermentable starch	Do not cut suddenly
Rumen-protected fat	None or low amount	Around 200–300 g/cow/day	Adjust based on DMI, milk yield, and manure
Sodium bicarbonate	None or low amount	Around 150–200 g/cow/day	Suitable when acidosis risk is high
Potassium from a suitable source	Based on the base ration	Balance to reach around 1.5–1.8% K in DM	Do not use fertilizer-grade KCl unless feed-grade suitability is confirmed
Salt (NaCl)	Based on the base ration	Adjust to reach the target Na level	Calculate together with premix and drinking water
MgO	Based on the base ration	Adjust to reach the target Mg level	Do not supplement mechanically without calculation
Free access to drinking water	Must be ensured	Must be strictly controlled	Water shortage reduces DMI and milk yield

The core point is that a heat-stress-adjusted ration is not simply about adding minerals. It means adjusting the energy structure, reducing acidosis risk, maintaining effective fiber, increasing energy density with energy sources that generate less fermentation heat, and ensuring unlimited clean drinking water.

Step-by-Step Guide to Adjusting Summer Rations Against Heat Stress

Step 1: Reduce rapidly fermentable starch, maintain effective fiber, and prioritize high-quality forage

During the hot season, farms should not increase starch just to maintain milk yield, because rapidly fermentable starch may increase the risk of acidosis and heat production in the rumen. However, concentrate feed should not be reduced mechanically, and fiber should not be increased mechanically either.

A safer approach is to adjust the ration by reducing rapidly fermentable starch, maintaining enough effective fiber, prioritizing high-quality forage, and increasing energy density with energy sources that generate less fermentation heat, such as rumen-protected fat when suitable.

Forage NDF can be guided at around 22–25% DM, but it should be adjusted based on forage quality, DMI, milk fat, manure, and rumination activity in the herd. If straw or low-quality fiber sources are increased too much, cows may eat less, the ration may become lower in energy, and milk yield may drop more sharply.

Step 2: Add rumen-protected fat to maintain dietary energy

When starch is reduced, dietary energy density may also decline. One common solution during hot weather is to add rumen-protected fat, or by-pass fat, often in the form of calcium salts of fatty acids or prilled fat.

A reference dose may be around 200–300 g/cow/day, depending on DMI, milk yield, milk fat, manure quality, and the supplier’s recommendation. Farms should not increase the dose too much on their own, because excessive added fat may affect fiber digestion or reduce palatability in some herds.

By-pass fat has the advantage of providing high energy density while producing less fermentation heat than rapidly fermentable starch. However, actual effectiveness still depends on the base ration and the cow’s ability to maintain feed intake.

Step 3: Adjust electrolytes — potassium, sodium, and magnesium based on the total ration

Potassium, sodium, and magnesium should be balanced based on the total ration DM, including minerals already present in grass, concentrate feed, premix, and drinking water. Farms should not simply add fixed doses of KCl, salt, or MgO without knowing the base ration.

Common guidelines during hot weather include:

• Potassium in ration DM: around 1.5–1.8%
• Sodium in ration DM: around 0.35–0.45%
• Magnesium in ration DM: around 0.30–0.35%

KCl or K₂CO₃ may be used as a suitable potassium source if they meet feed-grade standards. K₂CO₃ supplies potassium and has a mild alkalizing effect, which may be more suitable in some rations with acidosis risk. However, it is usually more expensive and less common.

Electrolyte supplementation levels should be calculated with a nutritionist whenever possible, especially for high-yielding herds, herds with a history of acidosis, or when forage sources change seasonally.

Step 4: Add rumen buffers and antioxidants

Sodium bicarbonate can be added directly to TMR at a reference dose of around 150–200 g/cow/day, depending on starch level, DMI, and acidosis risk. It is a commonly used ingredient that supports rumen pH buffering, especially in rations with a high concentrate ratio or when cows show signs of reduced rumination.

For antioxidants, vitamin E and selenium are often considered under heat stress conditions because heat-stressed cows may experience higher oxidative stress. Supplementation should be done through a mineral-vitamin premix or according to a nutritionist’s guidance. Farms should avoid using high doses on their own for long periods.

Step 5: Split feed delivery and adjust feeding time according to daily temperature

Adjusting feeding time is a low-cost measure with high practical value. The main principle is to shift most of the TMR to cooler times of the day to encourage cows to eat more.

Farms can apply the following practices:

• Provide around 60–70% of the TMR at night or in the early morning, for example before 7:00 a.m. and after 7:00 p.m.
• Split feeding into at least 2–3 times per day and avoid leaving feed in the trough for too long under hot conditions, which can cause spoilage.
• Push feed closer to the cows more often during the day to stimulate intake.
• Check TMR temperature and feed freshness, especially in fermented or high-moisture rations.

Heat-Stress Nutrition Ingredient Checklist for Dairy Cows in Vietnam

Reference ingredients, dosage, and usage notes

Ingredient	Role	Reference dose	Notes
Sodium bicarbonate	Supports rumen pH buffering	Around 150–200 g/cow/day	Adjust based on DMI, starch level, and acidosis risk
Salt (NaCl)	Adds sodium and supports electrolyte balance	Based on the target dietary Na level	Do not supplement too much at once
KCl or suitable potassium source	Supplies potassium	Calculate to reach around 1.5–1.8% K in DM	Use only sources that meet feed-grade standards
MgO	Supplies magnesium	Based on total dietary Mg	Avoid using a fixed dose without calculation
By-pass fat	Increases energy density	Around 200–300 g/cow/day	Do not exceed supplier or nutritionist recommendations
Vitamin E + selenium	Supports antioxidant function	Based on premix/nutritionist guidance	Avoid using high doses on your own
High-quality forage	Maintains effective fiber and rumination	Depends on the ration	Prioritize quality over quantity
Straw / treated straw	Adds long fiber when needed	Use with caution	Avoid reducing DMI and ration energy density

Ingredient supply in the Vietnamese market

Sodium bicarbonate and salt are usually easy to find through agricultural supply stores or feed distributors. However, ingredients used in dairy cow rations need to meet suitable feed standards, have clear origins, and be stored properly.

For KCl, farms should use sources suitable for use in animal feed/rations or sources confirmed by the supplier as safe for animal rations. Fertilizer-grade KCl should not be used unless the origin, purity, and impurities have been checked.

By-pass fat is now available through many imported products and some products distributed by feed companies. When choosing a product, farms should check the fat content, product form, ability to mix into TMR, and the supplier’s recommended dosage.

Monitoring Effectiveness, Common Mistakes, and Practical Notes

Weekly indicators to monitor: milk yield, BCS, and rectal temperature

After adjusting the ration, farms should monitor at least the following indicators:

• Milk yield per cow per day: This reflects the short-term effect of nutrition and environmental interventions.
• DMI: Farms need to measure or estimate dry matter intake, not only look at the amount of feed delivered.
• BCS: This should be monitored to detect loss of body condition.
• Rectal temperature: Normal rectal temperature is usually around 38.5–39.5°C. If it frequently exceeds 39.5°C during the day, heat stress and barn conditions should be reviewed.
• Drinking water intake: Both unusual increases and sudden decreases should be investigated.
• Milk fat and manure: These help evaluate acidosis risk or a lack of effective fiber.
• Conception rate and estrus signs: These should be monitored monthly to assess long-term impacts.

Common mistakes: insufficient drinking water, too much starch, and incorrect electrolyte supplementation

Mistake 1 — Insufficient drinking water

This is a common mistake that causes major losses. Dairy cows need significantly more water in summer than in the cool season. In high-yielding cows, water demand can become very high, even around 100 liters/cow/day or more, depending on milk yield, DMI, environmental temperature, ration, and the amount of salt and minerals supplemented.

Water troughs, flow rate, and trough hygiene should be checked at least twice per day. During peak heat waves, they should be checked more often, especially after milking and after feeding.

Mistake 2 — Increasing starch to compensate for lower milk yield

Many farms try to compensate for lower milk yield by increasing concentrate or corn. This can backfire if it increases rapidly fermentable starch, raises heat production in the rumen, lowers rumen pH, and worsens heat stress.

Mistake 3 — Incorrect electrolyte supplementation or uncontrolled dosage

Potassium, sodium, and magnesium should be evenly distributed in the TMR ration. They should not be given all at once or mixed into water if water intake cannot be controlled. Excessive MgO supplementation may reduce palatability or cause digestive issues in some herds. The dose should be balanced based on total dietary magnesium and actual herd response.

Mistake 4 — Sudden ration changes

Ration adjustments should be made gradually over 5–7 days. Farms should not change the entire ration suddenly, as this may disturb the rumen microbial population. When changing forage sources, changing starch levels, or adding fat, farms need to monitor DMI, manure, rumination, and milk yield.

Estimated cost of ration adjustment and economic considerations

The additional cost of a heat-stress-adjusted ration can vary widely depending on the ingredients used, market prices, by-pass fat level, minerals, premix, and herd size. The range of VND 5,000–15,000/cow/day should only be seen as a reference example, not a fixed number.

If only sodium bicarbonate and basic electrolytes are used, the cost can be significantly lower than a plan that includes by-pass fat. However, economic effectiveness should be considered in the context of potential losses from lower milk yield, lower conception rate, loss of BCS, and higher risk of postpartum health problems.

During prolonged heat waves, milk yield may drop clearly in many herds. The exact level of decline should be tracked using each farm’s own data instead of relying on a fixed number. Farms should record milk yield, DMI, supplementation cost, and conception rate to evaluate actual effectiveness after each heat wave.

FAQ

How does heat stress affect milk yield, and how long does recovery take?

Heat stress reduces feed intake, which reduces the supply of energy and protein to the mammary gland. A drop in milk yield is usually noticeable after a few days from the beginning of a heat wave, depending on stress level, lactation stage, and individual cow differences.

After ration adjustment and barn environment improvement, milk yield may need several weeks to partially recover. The level of recovery depends on how long the cows were under heat stress, their health status, BCS, and the farm’s ability to control the environment. Not every herd returns completely to its pre-stress milk yield if heat stress lasts for a long time.

How much potassium and sodium should a summer dairy cow ration contain?

Under heat stress conditions, potassium in ration DM can be guided at around 1.5–1.8%, while sodium can be around 0.35–0.45% DM. However, farms should not mechanically convert these numbers into grams of KCl or salt per day without knowing DMI and the mineral content of the base ration.

A safer approach is to calculate based on the total ration DM, including grass, concentrate feed, premix, salt, KCl/K₂CO₃, and drinking water. If the farm does not have an on-site nutritionist, it is better to use low to moderate supplementation levels and monitor herd response instead of applying high doses from the beginning.

Which feed ingredients are most useful in heat-stress nutrition for dairy cows in tropical regions?

No feed can directly “cool” the cow in the sense of immediately lowering body temperature. Heat-stress nutrition in dairy cows should be understood as a ration strategy that helps reduce metabolic heat production, reduce acidosis risk, maintain DMI, and support electrolyte balance.

In this direction, commonly prioritized groups include:

• High-quality forage that is digestible and provides enough effective fiber
• Rumen-protected fat to increase energy density without adding too much starch
• Sodium bicarbonate to support rumen buffering when needed
• Potassium, sodium, and magnesium balanced based on the total ration
• Clean, cool water with sufficient flow rate

Rapidly fermentable starch sources such as finely ground corn or cassava should be tightly controlled during hot weather, especially in herds at risk of acidosis or reduced rumination.

How can farms adjust dairy cow rations against heat stress without an on-site nutritionist?

Farms can start with simple, lower-risk steps:

• Ensure clean, sufficient, and easy-to-access drinking water 24/7.
• Check water flow rate at troughs or drinkers.
• Shift most feeding time to early morning and late afternoon/evening.
• Push feed closer to the cows several times per day.
• Add sodium bicarbonate at a reference level if the ration has acidosis risk.
• Do not mechanically increase starch to compensate for lower milk yield.
• Prioritize high-quality forage instead of increasing poor-quality straw or grass.
• Monitor DMI, milk yield, manure, and rumination for 1–2 weeks after adjustment.

If the herd shows serious milk yield reduction, deep feed intake decline, abnormal manure, or a clear decline in conception rate, farms should consult a nutritionist or veterinarian.

What is the estimated cost of adjusting summer rations against heat stress per cow?

The cost can vary widely depending on ingredients, market prices, milk yield, herd size, and the level of by-pass fat supplementation. If only sodium bicarbonate and basic electrolytes are used, the cost is usually lower than a plan that includes rumen-protected fat.

The range of VND 5,000–15,000/cow/day can be used as a reference example to estimate costs, but it should be recalculated based on actual ingredient prices at the time of purchase. When evaluating effectiveness, farms should compare supplementation cost with their own milk yield, DMI, BCS, and conception rate data.

Heat-Stress Nutrition Must Be Combined With Barn Environment Management

Heat-stress nutrition is not a complete solution if used alone. A properly adjusted ration needs to go together with ventilation, cooling, shade, dry bedding or flooring, clean drinking water, and feeding time management.

However, ration adjustment is an important foundation that farmers can proactively implement from the beginning of the hot season. When done correctly, farms can limit DMI reduction, better maintain milk yield, reduce the risk of rumen disorders, and support reproductive health during difficult weather conditions.

The dosages and thresholds in this article are practical guidelines. Farmers should consult a nutritionist or veterinarian to adjust them based on the specific conditions of each farm.

Explore Dairy Cow Nutrition and Hot-Season Management Solutions at VIETSTOCK 2026

VIETSTOCK 2026 – Vietnam’s Premier International Feed, Livestock & Meat Industry Show – is expected to bring together more than 300 brands and 13,000 trade visitors from many countries, including feed suppliers, feed additive suppliers, barn equipment suppliers, and dairy farm management solution providers. This is an opportunity to:

• Directly connect with suppliers in Vietnam and across the region that provide ingredients, additives, and specialized dairy nutrition products
• Discuss summer ration adjustment, body condition management, and milk yield maintenance in hot and humid conditions with nutrition and technical experts
• Explore ventilation, cooling, and barn management equipment being used on dairy farms to reduce the impact of heat stress
• Connect with businesses and experts across the dairy value chain to update trends in high-yield dairy herd management and increasingly strict market quality standards

Time: October 21–23, 2026

Venue: Saigon Exhibition and Convention Center (SECC), 799 Nguyen Van Linh, Ho Chi Minh City.

Register now to seize opportunities for business growth and networking in the livestock industry:

Visitor registration: https://www.vietstock.org/en/online-registration-2/

Event website: https://www.vietstock.org/en/

Contact information:

• Exhibiting: Ms. Sophie Nguyen – [email protected]
• Group Delegation Support: Ms. Phuong – [email protected]
• Marcom Support: Ms. Anita Pham – [email protected]