Dairy farmers have always worked closely with the weather, but it has recently become clear that changing seasonal patterns are raising the cost of keeping dairy buildings ventilated. As summers become longer and warmer, the energy needed to keep cows comfortable is growing. We compared average seasonal data from the past five years (2020 to 2024) to data from 20 years earlier (2000 to 2004) and estimated dairy ventilation energy needs for both periods. For these analyses, we assumed that when temperatures rise above 68°F, the ventilation rate is set at 60 air changes per hour (ACH), and when temperatures fall below 40°F, it is set at 4 ACH. During the transitional periods in spring and fall, the ventilation rate changes in five incremental steps between these two levels. While we recognize that ventilation on farms is not controlled this precisely, we followed a standardized approach to highlight the effects of changing seasonal patterns.

Pattern changes

According to the Köppen Classification, as expected, southern Wisconsin generally experiences warmer temperatures than the central and northern regions. Table 1 summarizes the change in seasonal temperatures in southern Wisconsin (Madison) and northern/central Wisconsin (Wausau) in the last 20 years. The data show a shift to an elevated number of warm days in both locations. In Madison, the number of summer days (68°F and higher) bumped up from 67 to 85 days, while winter days (less than 40°F) declined slightly from 135 to 131 days. Similarly, Wausau experienced a jump in summer days from 57 to 71 days, along with a reduction in winter days from 151 to 150 days.

Transitional days declined at both locations, dropping from 163 to 149 days in Madison and from 157 to 144 days in Wausau. These changes suggest a trend, with longer summers and shorter transitional seasons becoming more common in Wisconsin.

Energy requirements

Taking changes in seasonal temperatures into account, we calculated the energy requirements required to maintain the recommended air exchange rates in dairy buildings throughout the year. While in our previous studies, our focus was on summer ventilation, here, calculated for year-round operating conditions, assuming a fan efficiency of 16 cubic feet per minute per watt (cfm/watt).

As shown in Table 2, annual energy consumption for ventilation spiked between 2004 and 2024 for both Madison and Wausau. Using average electricity rates of 7.24 cents per kWh in 2004 and 12.85 cents per kWh in 2024, we estimated the annual ventilation energy cost per cow. We used the Consumer Price Index (CPI) to adjust the cost from 2004 to 2024 to make a fair comparison that accounts for inflation.

In 2024, the annual ventilation cost per cow was $57 in Madison and $52 in Wausau. These values may be lower than those observed in practice, as they are based on the assumption that ventilation was precisely managed to meet seasonal air exchange requirements. In comparison to the 2024 costs, the inflation-adjusted 2004 costs were $50 per cow in Madison and $47 per cow in Wausau. This represents a $7 rise per cow in Madison (14%) and a $5 jump in Wausau (11%). These spikes reflect both higher electricity rates and greater ventilation demands over time due to longer or more frequent warm periods. Factors such as genetic changes in cows, can also contribute to growing ventilation requirements; however, they were not considered in this study.

Staying profitable

This quick analysis reveals that energy consumption for ventilation is rising on Wisconsin farms. Even if the rise may not be as steep as in some other states, improving ventilation efficiency can help reduce energy costs — especially important when considering fluctuations in milk margins over the past five years. In Wisconsin, while 2024 recorded milk margins were on the higher side at $12.33 per hundredweight (cwt.), lower values were seen in 2021 at $8.09 per cwt. and 2023 was $7.13 per cwt. Cutting ventilation-related energy costs can help stabilize income during low-margin years.

In a previous study, we examined the number of days in Wisconsin over the last five years with conditions that may have caused mild, moderate, and severe heat stress. The data showed that, on average, there were about 46 days per year with conditions for mild heat stress, with a temperature-humidity index (THI) of 68 to 72, 23 days with the potential for moderate stress at 72 to 78 THI, and approximately half a day conducive to severe heat stress with a THI greater than 78. Assuming nearly 70 days in a year may cause a 5% to 15% reduction in milk production, efficient ventilation becomes even more critical.

While we do not have control over the seasons, there are practical strategies that can support profitability:

• Select energy-efficient ventilation fans. While the initial investment may be higher, they typically pay for themselves in a short period through reduced energy use.
• Size air inlets correctly. Proper inlet sizing is critical for maintaining consistent airflow and achieving effective ventilation.
• Maintain equipment regularly. Dirty or poorly maintained fans and shutters can reduce airflow efficiency and inflate energy use.
• Use natural ventilation when possible. In mild weather conditions, natural airflow can reduce the need for mechanical ventilation.
• Match ventilation rates to seasonal needs. Adjusting ventilation fans based on temperature improves cow comfort while minimizing energy use.
• Use the University of Wisconsin-Madison’s CoolCows tool, accessible at go.wisc.edu/coolcows, to assess mechanical ventilation design.

Farmers are constantly adjusting to changing conditions. Ventilation is another opportunity to do the same by making good choices that support long-term profitability.