Álvaro García
Rising global temperatures continue to intensify extreme weather events such as heat waves, droughts, and erratic rainfall, placing growing pressure on livestock production and animal well-being. These effects are particularly severe in South and Southeast Asia, the Arabian Peninsula, North and East Africa, and parts of Central and South America, where the temperature–humidity index (THI) values frequently exceed 78–80, surpassing the thermal comfort zone for dairy cows. Countries such as India, Pakistan, Saudi Arabia, Nigeria, and Brazil experience these stressful conditions for months each year. In southern Europe and the southeastern United States, heat waves are also becoming longer and more intense. Under such environmental pressure, dairy cows struggle to dissipate body heat, leading to reduced feed intake, lower milk yield, impaired fertility, and higher disease risk.
Enteric methane (CH₄) is a key component of dairy’s greenhouse gas footprint and serves as both an environmental and efficiency indicator. Methane results from microbial fermentation in the rumen, and its production is typically expressed as methane yield (MEY: grams of CH₄ per kilogram of feed intake) or methane intensity (MPM: grams of CH₄ per kilogram of milk produced). As feed intake declines under heat stress, total methane emissions per cow tend to drop. However, milk output decreases even more sharply, increasing methane intensity. This makes heat-stressed cows less efficient in converting feed energy into milk solids, increasing the climate impact per unit of product.
In a recent publication we emphasized the same principle: methane yield reflects feed efficiency, while methane intensity reflects production efficiency (García, A. 2025). Both are important for sustainability, but methane intensity offers a clearer link to farm profitability since it connects emissions directly to milk output and income over feed costs.
From a practical standpoint, we contend that improving feed utilization, milk yield, and cow comfort simultaneously lowers methane intensity and boosts profitability. Thus, in thermally stressed herds, management strategies that sustain milk yield also protect climate efficiency.
At the herd level, methane emissions come not only from lactating cows but also from dry and replacement animals, which can account for 20–30% of total CH₄ output. In tropical systems, poor fertility and higher culling rates expand this non-productive group, raising the overall methane footprint per unit of milk. Because of this complex interaction between milk yield, reproduction, and herd structure, it is essential to use whole-herd simulation models to evaluate how heat stress alters productivity, economics, and environmental performance.
Recent research
A recent controlled-environment experiment (Chen et al., 2025) showed that when the THI exceeded 75, high-producing Holstein cows experienced significant drops in feed intake, milk yield, and fertility. Building on those results, this study applied a herd-level simulation approach to assess how prolonged heat exposure affects herd productivity, economic returns, and methane emissions. The analysis used the SimHerd dynamic bioeconomic model, which simulates weekly changes in milk production, reproduction, health, and culling across individual cows in a herd producing about 11,000 kg (24,000 lb.) of energy-corrected milk annually. The model projected herd performance and enteric methane emissions under different heat exposure scenarios.
To explore real-world impacts, three types of heat stress responses were modeled:
- Reduced milk production due to decreased feed intake.
- Reduced milk and fertility, adding reproductive losses.
- Reduced milk, fertility, and health, incorporating higher risk of disease such as mastitis or lameness.
Each scenario was simulated for 1, 2, or 4 months of heat per year, representing mild, moderate, or prolonged heat stress typical of many dairy regions of the world.
Methane and economic calculations
Methane output was estimated based on feed intake and diet composition. Emissions were expressed as grams of CH₄ per kilogram of milk, aligning with our emphasis on methane intensity as the most relevant indicator for linking climate performance and profitability.
Economic performance was measured using gross margin (milk income minus feed, breeding, and health costs). This allowed direct comparison between biological losses (milk and fertility) and their financial implications.
Results
Across all simulations, heat stress reduces milk yield and profitability. Cows under heat stress produced 2–7% less milk, depending on the length of exposure, and up to 9% less when fertility and health losses were also considered. Fertility declines increased culling and replacement needs, cutting available heifers from 15 to only 2 per year and raising herd replacement from 36% to 40%. Profit per cow dropped from €2,700 to €2,200 annually, confirming that heat stress directly erodes farm income.
Methane and environmental efficiency
Even though cows ate less feed, methane per kilogram of milk rose by 0.5–6.5%, because milk yield fell faster than methane output. This matches García’s economic framework: while feed efficiency (methane yield) may improve slightly, production efficiency (methane intensity) worsens, increasing the environmental cost per unit of milk.
Feed efficiency and fertility sensitivity
Feed efficiency, the cow’s ability to convert feed energy into milk, is one of the most critical links between biological performance, profitability, and methane emissions. When efficiency was assumed to deteriorate significantly under heat stress (a 19% drop compared with normal conditions), methane intensity increased by an additional 4–5%, and herd profitability declined accordingly. This pattern illustrates that even modest changes in feed utilization have disproportionate effects on both the carbon footprint and the economic bottom line of dairy operations.
Reduced feed efficiency under heat stress is often driven by several biological and behavioral factors. Cows eat less to limit internal heat production, but their maintenance energy requirements rise because more energy is used to cool the body through panting and sweating. The result is a double penalty, less energy available for milk production and more loss as heat. This imbalance causes a drop in feed-to-milk conversion, leading to higher methane output per kilogram of milk even if total feed intake declines.
As we have emphasized, methane yield (grams of CH₄ per kilogram of feed) and methane intensity (grams of CH₄ per kilogram of milk) move in opposite directions depending on how efficiently cows use the energy they consume. Under heat stress, yield may appear stable or even slightly improved, but intensity worsens sharply because less milk is produced for every unit of feed. This demonstrates that maintaining feed efficiency is the cornerstone of both economic and environmental sustainability.
Extending heat stress effects to young replacement heifers revealed additional challenges. When fertility in heifers was reduced, the number of animals reaching first calving declined, creating gaps in herd replacement and disrupting long-term production stability. The delayed age at first calving and reduced conception rates prolonged the nonproductive phase of the herd, which increased the proportion of methane-emitting animals that contributed no milk output. This ripple effect further amplified herd-level methane intensity and reduced profitability over multiple lactation cycles.
In practical terms, these findings underscore the need for a whole-herd strategy to protect feed efficiency and reproduction, not only in lactating cows but across all animal groups. Nutritional adjustments that support rumen health during hot periods, cooling systems that maintain intake levels, and reproductive programs designed to minimize heat stress on breeding stock are essential for maintaining efficiency. Protecting feed conversion and fertility preserves both herd resilience and methane efficiency, confirming that environmental sustainability and economic performance are deeply interconnected.
Discussion
This study confirms that heat stress amplifies methane intensity, through decreased milk yield and feed efficiency. As we demonstrated previously, methane yield and intensity must be balanced: yield represents feed-use efficiency, while intensity captures overall productivity. Under heat stress, both deteriorate, lower feed efficiency increases methane per unit of feed, and lower production spreads those emissions across less milk.
Economically, this means that the same amount of feed energy produces less saleable milk, translating to both higher emissions per kilogram of milk and lower income per cow. Chen’s et al. simulation aligned with our conclusion that maintaining high milk yield and efficient feed conversion is the most direct route to reducing methane intensity and improving profitability.
While total methane emissions per cow may decline under heat stress, the environmental and economic efficiency of the herd deteriorates. This reinforces the idea that productivity is sustainability, that is, high-yielding, efficient cows are not just more profitable but also more climate-efficient.
Practical implications
For dairy producers, reducing heat stress is both an economic and environmental strategy.
- Cooling systems and shade improve feed intake and comfort.
- Feeding management (cooler feeding times, high-quality forages, methane-inhibiting additives) supports digestion and reduces emissions at the source.
- Reproductive planning during cooler months and health monitoring during heat peaks can limit culling.
- Genetic and nutritional selection for cows with better heat and feed efficiency can sustain performance in warming climates.
Even small reductions in methane intensity yield cumulative financial gains at the herd level. For example, increasing milk yield by just 10 pounds per cow per day can cut methane intensity by 11% and save thousands annually when scaled across large herds (Garcia, A. 2025).
Ultimately, improving milk yield, feed efficiency, and cow comfort not only reduces methane emissions but also enhances farm profitability, providing a practical path toward sustainable dairy systems in hot regions of the world.
The full list of references used in this article is available upon request.
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