Nutritional strategies to mitigate heat stress in dairy cows
Heat stress is one of the main factors affecting productivity and animal welfare, with dairy cattle—especially high genetic merit cows—being particularly sensitive.
According to Yousef (1985), the ideal temperature for cows varies depending on the type of animal and its condition, but it is generally considered that for dairy cows, it falls within the range of 5°C to 24°C.
When cows are kept within this range, they do not need to expend extra energy to maintain their body temperature, which maximizes comfort and productivity.
Thermal comfort is also influenced by ambient humidity, which can worsen heat stress conditions.
Under high-temperature conditions, the energy requirements of cattle increase due to the greater energy expenditure needed to dissipate body heat.
When a cow is under heat stress, the first sign is a decrease in Dry Matter Intake (DMI). In addition, the animals remain standing, rumination decreases, respiratory rate increases, and in some cases, they pant and drool excessively.
As a consequence, the cow presents:
Besides the decrease in Dry Matter Intake (DMI) and milk production, heat stress causes immune system dysfunction, making animals more susceptible to diseases.
Taking into account that production can drop by 35–40% (Ding et al., 2022) and that, as recently demonstrated, if the cow is pregnant, the future production of her offspring will also be affected, it is worthwhile to pay attention to the problem of heat stress and implement measures to prevent it.
Heat increase 
Heat increment is defined as the energy expenditure associated with the digestion and assimilation of food. Cows have several methods to dissipate heat, including reducing feed intake, which leads to a corresponding decrease in heat production linked to ruminal fermentation and metabolic processes within the body.
Other factors such as solar radiation, wind speed, diet digestibility, production level, etc., also influence the decline in Dry Matter Intake (DMI), which has a direct impact on milk production.
The provision of easily digestible feed helps to counteract the heat increment generated by ruminal fermentation, but it also reduces rumination time. This situation, combined with an increase in salivation to dissipate heat, often ends up causing Subacute Chronic Acidosis. Since heat generation varies according to different nutrients, there is the possibility of formulating diets that produce a lower heat increment.
Diet formulation
It is necessary to reformulate diets to achieve an adequate nutrient density that meets the recommended requirements for this species.
Fiber
Usually, in summer diets, the crude fiber content is reduced to avoid the heat generated by fermentation. However, this type of ration must be implemented cautiously due to the ruminants’ need for a sufficient fiber content to promote rumination and ensure ruminal health and animal welfare.
Starch
Under heat stress conditions, it is essential not to overload the ration with starch, as an excess can lead to subacute ruminal acidosis. A valid option is the combination of cereals with different fermentation rates.
Proteins
Excess protein can decrease productive efficiency, since nitrogen that exceeds requirements must be metabolized and excreted as urea in the urine, which requires additional energy. The supply of protected amino acids, which are not degraded by rumen bacteria, and bypass protein play an important role in the nutrition of cows under heat stress conditions.
Lipids
The use of fats in the diet of dairy cows subjected to heat stress is highly recommended to compensate for the reduction in starch and to increase the energy level of the diet. However, it should be noted that the total lipid percentage in the diet should not exceed 5–7%.
Minerals
As a consequence of the reduction in voluntary intake due to heat stress, the intake of essential minerals also decreases.
Additives
Some additives, such as sodium bicarbonate, live yeasts, and other buffers, can help maintain ruminal health and improve feed efficiency under heat stress conditions.
Feeding management 
The main objective of feeding management should be to stimulate feed intake, since, as mentioned earlier, it is the first sign observed in a cow suffering from heat stress. In this regard, there are several key points to consider:
Frequent feeding
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Availability of fresh water
Water sources should always provide clean, fresh water. It’s important to have water accessible in holding areas and alleys. Regular checks on waterers are crucial to guarantee a consistent and sufficient water supply.
Feeding schedules
During hot periods, cows tend to consume more feed during the cooler hours of the day (at dusk and early morning). Knowing that the rumen reaches its peak heat production 3–4 hours after feeding, it is important to schedule feeding times so that the period of highest ruminal fermentation does not coincide with high ambient temperatures.
For example, a cow fed at 8 a.m. may reach its fermentation peak at midday, when temperatures are very high. In contrast, a cow fed at 5 p.m. will reach the fermentation peak around 9 p.m., allowing it to dissipate excess body heat in a cooler environment.
Daily monitoring
Constant monitoring of the following parameters is vital to quickly detect and correct any problems that may arise:
Maintaining Dry Matter Intake (DMI) is fundamental for milk production, but it is not the only determining factor to ensure the continuity of milk volume produced.
