Ventilation Fans Offset Potential Reductions in Milk Margin from Heat Stress in Wisconsin Dairy Farms

Abstract

Heat stress is becoming an increasing concern for dairy farmers due to elevated temperatures and wind shadow caused by rural development. Mechanical ventilation helps mitigate heat stress; however, transitioning from natural to mechanical ventilation increases operational costs. In this study, the number of days with no heat stress, as well as mild, moderate, and severe heat stress, was calculated for Madison, Wisconsin, over the past five years. Monthly milk margins were determined using all milk prices and feed costs from the Dairy Margin Coverage (DMC) program. The goal was to compare the potential reduction in milk margin coverage to the electricity costs of operating ventilation fans. The results indicated that while the five-year average milk margin reduction due to heat stress was USD 20,204 for a 600-head facility, the electricity cost accounted for approximately 42.6% of this amount. However, milk margins fluctuated annually due to volatility in milk and feed markets. For example, in 2021, the reduction in milk margins was estimated at USD 9804, while electricity costs reached USD 8574. It was concluded that in some years, when no severe heat stress occurs, the benefits of ventilation may be close to the expenses. Therefore, adhering to best management practices is critical for minimizing electricity costs while using ventilation fans in dairy operations.

1. Introduction

Heat stress causes economic losses annually due to lower milk yields and, in severe cases, decreased conception rates, higher disease prevalence, and even deaths of dairy cattle [1,2]. Effective ventilation and cooling strategies, such as fans and misters, are essential to mitigate these effects. However, these systems increase operational costs, affecting profit margins [3,4,5]. Dairy farmers need to balance the investment in heat abatement technologies with the potential gains in milk production to maintain profitability in a changing climate [6].

The temperature–humidity index (THI) is widely used for assessing heat stress in cows. Although it is not the only parameter taken into account, it serves as a good indicator since it combines air temperature and relative humidity, providing a measure of the thermal environment cows experience [7]. When the THI is below 68, cows typically do not show any signs of heat stress. As the THI rises to the 68–72 range, mild heat stress occurs, resulting in a slight reduction in milk production. Moderate heat stress is observed when the THI reaches 72–78, leading to a noticeable decrease in milk yields. When the THI exceeds 78, cows experience severe heat stress, resulting in a significant decline in milk yields [7,8].

One of the primary responses to heat stress is a reduction in dry matter intake (DMI). Studies have shown that cows under heat stress may reduce their DMI by 20–30%, depending on the severity and duration of exposure. The decline in feed intake, coupled with increased maintenance energy demands resulting from elevated respiration rates and panting, leads to a measurable decrease in milk production. Research indicates that milk yields can decrease by 10–25% under prolonged heat stress conditions [9]. However, this depends on several factors, including genetics and milk production rates of the cattle. In general, as milk production increases, cattle’s susceptibility to heat stress also increases due to the enhanced metabolic heat generation. It was reported that an increase in milk yield from 35 to 45 kg per day decreased the heat stress threshold by 5 °C [9,10].

Ventilation is the most frequently used method to mitigate heat stress. Even in buildings with natural ventilation, circulation fans are used to distribute heat throughout the building. Mechanically ventilated buildings, in addition to circulation fans, are equipped with supply or exhaust fans. On warm days (T > 20 °C), it is recommended to provide a minimum of 1699 m³/h (1000 cfm) to 2549 m³/h (1500 cfm) of air per cow to achieve 50 to 60 air exchanges per hour, depending on the size of the building [11].

Another factor that might affect air speed in the cow resting area is the presence of baffles. Our previous studies indicated that in cross-ventilated buildings with baffles (without circulation fans), the electricity cost for ventilation fans was about USD 9.60 per cow per month [4]. The cost was lower for tunnel-ventilated buildings, approximately USD 7.60 per cow per month [5]. However, tunnel-ventilated buildings typically include circulation fans, which can easily double the ventilation costs based on the number of fans used [5]. These costs do not include initial purchase expenses or maintenance fees. High-efficiency variable-speed ventilation fans can cost several thousand dollars, while maintenance costs vary from tens of dollars for cleaning and lubrication to hundreds of dollars for motor repairs or blade replacements [12].

This short communication focused on the Madison area in Wisconsin. The objectives of the study were (1) to calculate potential reductions in milk margins due to heat stress based on the weather data in the last five years, (2) compare the reductions in milk margin to ventilation electricity costs to better understand if mechanical ventilation pays off, (3) and make recommendations to dairy producers to increase their profitability by lowering ventilation electricity costs. While there is a growing concern about heat stress, which is leading to increased fan use in dairy barns, more ventilation is not always better. If ventilation fans are not selected and maintained properly, the added electricity costs can sometimes reduce overall farm profitability.

2. Materials and Methods

2.1. Historical Weather Data

The daily average temperature and relative humidity data were collected over the past five years from the Dane County Regional Truax Field Station, where an Automated Surface Observing System (ASOS) gathers weather data, providing continuous real-time measurements [13]. The temperature–humidity index was calculated using Equation (1) [14,15]:

THI = 1.8 × T + 32 − (0.55 − 0.0055 × RH) × (1.8 × T + 32 − 58)

(1)

In addition to calculating the temperature–humidity index, the number of days with an average temperature above 20 °C was noted, assuming that the ventilation fans operated at full capacity during those days, while during cooler days, natural ventilation was adequate for bringing fresh air into buildings [11].

2.2. Income over Feed Cost

Monthly all-milk prices (USD/cwt) and final feed costs for Dairy Margin Coverage (DMC) were gathered from the official website of the USDA Farm Service Agency [16], with milk assumed to have a standard composition of 3.5% butterfat and 3.0% protein. Milk margins were calculated using Equation (2).

Milk Margin Above Feed Costs for DMC = All Milk Price − Feed Costs for DMC

(2)

All terms are in USD/cwt, where cwt stands for hundredweight, which is equivalent to 100 pounds of milk. This unit was not converted to an SI unit (1 cwt ≈ 45.36 kg) because, in the United States, milk is traditionally priced per hundredweight, and this measurement is embedded in federal DMC reports.

Equation (2) yielded national milk margins from May to September. To calculate milk margins for Wisconsin, the production estimates of annual milk costs were collected from the USDA Economic Research Service’s website [17]. Wisconsin’s milk margins were estimated using the Wisconsin-to-National margins ratio on an annual basis.

Wisconsin’s milk production rates were gathered from the USDA National Agricultural Statistics Service [18]. The adjusted milk margins were converted from dollars per hundredweight (cwt) to dollars per cow per day by multiplying the daily average milk production rates.

Milk margin reductions were estimated by assuming a 0, 5, 10, and 15% reduction during no, mild, moderate, and severe heat stress days, respectively [8]. The impact of heat stress depends on the duration as much as it depends on the THI, so it is difficult to estimate the actual impact. However, in the Madison area, there were only two days of severe heat stress in the last two years and no severe heat stress conditions in the preceding three years (data shown in Section 3). Based on this information, it was assumed that the primary impact of heat stress was on milk production and feed intake. The effect of heat stress on reproduction or other health issues was not considered, and a modest approach was taken in calculating the economic losses.

2.3. Estimate Ventilation Fan Electricity Costs

Ventilation electricity costs were estimated as explained in our previous study using Equation (3) [19].

$$\mathrm{E}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{C}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{s}={\displaystyle \frac{\mathrm{A}\mathrm{i}\mathrm{r}\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}\mathrm{R}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{C}\mathrm{o}\mathrm{w}\times \#\mathrm{o}\mathrm{f}\mathrm{H}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{s}}{\mathrm{F}\mathrm{a}\mathrm{n}\mathrm{E}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}}}\times \mathrm{E}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{R}\mathrm{a}\mathrm{t}\mathrm{e}$$

(3)

where energy costs are in USD/head, airflow rate per cow is in cfm, # of hours is the total hours of operation, fan efficiency is in cfm/Watt, and electricity rate is in USD/Watt.

When ambient temperature exceeds 20 °C, the recommended airflow rate for heat abatement is between 1000 and 1500 cfm per cow [20]. Ventilation fan efficiencies are measured by the Bioenvironmental and Structural Systems Laboratory (BESS Lab) at the University of Illinois at Urbana-Champaign [21]. Although there are various types of fans, in this study, calculations were made for an average high-efficiency fan (18.9 cfm/Watt) and a low-efficiency fan (16.0 cfm/Watt, 15% reduced efficiency).

Electricity rates over the past five years were gathered from the US Energy Information Administration [22]. While these rates differ by farm, the commercial average monthly rates for Wisconsin were used in the calculations.

3. Results

3.1. Heat Stress Days in Madison, WI

Table 1. Temperature–humidity indices (THI) and the number of heat stress days during the warm season (May–September) from 2020 to 2024.

THI	Heat Stress Levels	Effects on Cows	Months	Days Recorded in Madison, WI
				2020	2021	2022	2023	2024
<68	No heat stress	Normal behavior, no impact on feed intake	May	28	25	23	29	30
			June	14	8	16	15	13
			July	1	12	7	8	8
			August	13	9	11	13	12
			September	29	26	21	23	23
			Total	85	80	78	88	86
68–72	Mild heat stress	Slight decrease in milk production	May	1	6	3	2	1
			June	13	15	8	13	13
			July	14	10	14	14	18
			August	9	11	16	12	11
			September	1	4	8	4	7
			Total	38	46	49	45	50
72–78	Moderate heat stress	Noticeable drop in milk production	May	2	0	5	0	0
			June	3	7	6	2	4
			July	16	9	10	9	5
			August	9	11	4	4	6
			September	0	0	1	3	0
			Total	30	27	26	18	15
>78	Severe heat stress	Significant drop in milk yield	May	0	0	0	0	0
			June	0	0	0	0	0
			July	0	0	0	0	0
			August	0	0	0	2	2
			September	0	0	0	0	0
			Total	0	0	0	2	2

lists the number of days with THI values of <68, 68–72, 72–78, and >78 in Madison, WI. Over the past five years, “no heat stress days” accounted for the highest number of days across all years, ranging from 78 to 88 days from May to September. June and August contributed to mild heat stress days, and July was the month with the highest frequency of moderate heat stress. Between 2020 and 2022, no severe heat stress days were recorded. However, two severe heat stress days were observed in August and September of 2023 and 2024, possibly indicating a changing trend, but this would depend on conditions in the coming years.

3.2. Milk Margin Reduction

Table 2. National milk margins, adjusted margins for Wisconsin, and estimated economic losses.

Year	Month	All Milk Price (USD/cwt)	Final Feed Costs for DMC (USD/cwt)	Income Over Feed Costs (USD/cwt)	Adjusted Milk Margin for Wisconsin (USD/cwt)	Wisconsin Milk Production (lbs per Cow per Day)	Milk Margin (USD per Cow per Day)	Estimated Economic Losses (USD per Cow)
2024	May	22.0	11.48	10.52	11.26	74.0	8.33	0.42
	June	22.8	11.14	11.66	12.48	70.7	8.82	9.26
	July	22.8	10.47	12.33	13.19	69.8	9.21	12.89
	August	23.6	9.88	13.72	14.68	69.0	10.13	14.69
	September	25.5	9.93	15.57	16.66	69.2	11.53	4.04
2023	May	19.3	14.47	4.83	5.17	72.8	3.76	0.38
	June	17.9	14.25	3.65	3.91	71.0	2.77	2.36
	July	17.4	13.88	3.52	3.77	70.3	2.65	4.24
	August	19.7	13.24	6.46	6.91	69.5	4.80	6.25
	September	21.0	12.56	8.44	9.03	69.2	6.25	3.12
2022	May	27.3	14.79	12.51	13.64	72.0	9.82	6.38
	June	26.9	14.98	11.92	12.99	70.0	9.09	9.09
	July	25.7	15.78	9.92	10.81	69.4	7.50	12.76
	August	24.3	16.22	8.08	8.81	69.2	6.09	7.31
	September	24.4	15.78	8.62	9.40	68.7	6.45	3.23
2021	May	19.2	12.53	6.67	8.27	71.5	5.91	1.77
	June	18.4	12.37	6.03	7.48	69.0	5.16	7.48
	July	17.9	12.43	5.47	6.78	68.9	4.67	6.54
	August	17.7	12.67	5.03	6.24	68.1	4.25	7.01
	September	18.4	11.71	6.69	8.30	67.8	5.62	1.12
2020	May	13.6	8.44	5.16	6.86	68.7	4.71	1.18
	June	18.1	8.27	9.83	13.07	68.0	8.89	8.45
	July	20.5	8.21	12.29	16.35	66.8	10.92	25.11
	August	18.8	8.11	10.69	14.22	67.1	9.54	12.88
	September	17.9	8.64	9.26	12.32	66.8	8.23	0.41

lists all milk price, final feed costs for DMC, and the daily income over feed costs per cow. As expected, milk prices and feed costs significantly fluctuated over the past five years, affecting milk margins. For example, in July 2023, the milk price was USD 17.40 per cwt, while feed costs were USD 13.88 per cwt, resulting in a margin of USD 3.52 per cwt. In September 2024, the milk price reached its highest level in five years at USD 25.50 per cwt, while feed costs remained relatively low at USD 9.93 per cwt. Consequently, the margin rose significantly to USD 15.57 per cwt. This pattern of fluctuating margins highlights the volatility in milk markets, emphasizing the need to reduce operating costs to remain competitive [5].

3.3. Ventilation Fan Electricity Costs

Table 3. Electricity rates and estimated ventilation fan electricity costs.

Year	Month	Number of Days Ambient T Above 20 °C	Electricity Rates in WI (USD/kWh)	Estimated Electricity Costs (USD per Cow)
				@18.9 cfm/Watt	@16.0 cfm/Watt
2024	May	2	USD 0.1285	0.33	0.39
	June	19		3.10	3.66
	July	26		4.24	5.01
	August	22		3.59	4.24
	September	11		1.79	2.12
2023	May	4	USD 0.1276	0.65	0.77
	June	21		3.40	4.02
	July	26		4.21	4.98
	August	19		3.08	3.64
	September	10		1.62	1.91
2022	May	8	USD 0.1185	1.20	1.42
	June	16		2.41	2.84
	July	28		4.21	4.98
	August	16		2.41	2.84
	September	11		1.66	1.96
2021	May	8	USD 0.1095	1.11	1.31
	June	24		3.34	3.94
	July	24		3.34	3.94
	August	25		3.48	4.11
	September	6		0.83	0.99
2020	May	4	USD 0.1075	0.55	0.65
	June	20		2.73	3.23
	July	30		4.10	4.84
	August	23		3.14	3.71
	September	2		0.27	0.32

lists a comparison of electricity rates and estimated ventilation fan electricity costs. The average electricity rates in Wisconsin increased over the years, ranging from USD 0.1075/kWh in 2020 to USD 0.1075/kWh in 2024. The number of hours during which the ambient temperature exceeded 20 °C ranged from 48 to 192 in May, 384 to 576 in June, 576 to 720 in July, 384 to 600 in August, and 48 to 264 in September. July had the highest number of warm days, leading to the highest electricity costs, followed by August and June. For example, in July 2024, with 26 days above 20 °C, the estimated ventilation costs per cow reached USD 4.24 (18.9 cfm/Watt) and USD 5.01 (16.0 cfm/Watt). Similar patterns appeared in previous years, though 2022 had a notably high number of warm days in July (28 days), matching the peak electricity costs seen in 2023 and 2024. September consistently showed the lowest ventilation costs due to fewer warm days, with costs dropping below USD 1 per cow in some years (e.g., USD 0.83 in September 2021 at 18.9 cfm/Watt). May also had lower costs, reflecting fewer warm days early in the summer.

3.4. Comparison of Margin Reductions to Electricity Costs

Table 4. Estimated losses due to milk margin reduction and fan electricity costs for a 600-head farm at 16 cfm/Watt fan efficiency.

Months	Estimated Milk Margin Reduction for a 600-Head Farm (USD)
	2020	2021	2022	2023	2024
May	707	1064	3829	226	250
June	5067	4489	5457	1414	5557
July	15,068	3926	7654	2542	7735
August	7727	4205	4388	3747	8813
September	247	675	1936	1875	2421
Total	24,776	9804	23,265	14,359	28,817
Months	Estimated Fan Electricity Costs for a 600-Head Farm (USD)
	2020	2021	2022	2023	2024
May	387	788	853	459	231
June	1935	2365	1706	2412	2197
July	2903	2365	2986	2986	3007
August	2225	2464	1706	2182	2544
September	194	591	1173	1148	1272
Total	7643	8574	8425	9187	9252

summarizes the estimated economic losses due to the potential reductions in milk margins from heat stress and the corresponding ventilation fan electricity costs for a 600-head facility. The top section highlights the milk margin reductions, showing significant variability across years and months. The highest total loss occurred in 2024 (USD 28,817), with July and August consistently being the months with the greatest economic impact. Conversely, 2021 had the lowest total loss (USD 9804). The bottom section shows the electricity costs for ventilation fans, which range from USD 7643 in 2020 to USD 9252 in 2024. Electricity costs tend to peak in July and August, aligning with the warmest months.

While the average annual loss of income over five years was estimated at USD 20,204, the average electricity cost stood at USD 8616. Although ventilation electricity costs remain relatively stable, milk margin reductions due to fluctuations in milk and feed costs can vary significantly, sometimes making it necessary to reduce electricity costs to help offset losses. For example, in 2021, severe heat stress days were not observed, and milk prices dropped; as a result, the total milk margin reduction (USD 9804) was close to the electricity costs (USD 8574). This highlights the importance of minimizing ventilation fan operational costs to maintain profitability.

4. Discussion

Proper ventilation management is critical for maintaining energy efficiency and controlling operational costs in dairy facilities. The data showed that while most summer days in recent years were not classified as heat stress days, the appearance of severe heat stress days in the last two years, especially later in the summer, raises concerns about the potential need to run fans for longer periods, leading to higher energy use and increased electricity costs. One of the study’s limitations was that THI values were calculated based on weather station data, but the conditions inside a barn might be different. It was also assumed that the THI was the main factor affecting milk production during heat stress.

Milk and feed prices remain the main sources of uncertainty for dairy producers. These prices are influenced by a variety of factors, including supply and demand, global trade conditions, weather events, input and transportation costs, and government policies. Because they often fluctuate independently of one another, milk margins can shift rapidly, making it difficult for farms to plan. For example, in a previous study, 105 dairy farms in New York were evaluated from 2010 to 2019, and the data showed that while long-term financial stability remained steady during poor financial years, the farms’ ability to cover short-term expenses and repay debt declined [23]. Compared to these unpredictable market forces, electricity costs are generally more stable and offer more opportunities for control through improved equipment and better management.

Several factors affect ventilation performance, including air inlet size, fan selection, and maintenance practices [19]. Ensuring adequate air inlet size is essential for minimizing static pressure within the facility. A small air inlet increases resistance to airflow, raising static pressure and reducing fan efficiency. Even a modest increase in static pressure (e.g., 25 Pascals) can lower fan efficiency by approximately 15%, leading to increased energy consumption and higher operational costs [5,19]. Fan efficiency should be carefully evaluated before purchase to optimize long-term energy use. Efficiency ratings vary significantly across models, making initial selection an important decision. For example, a 15% decline in fan efficiency (from 18.9 cfm/Watt to 16 cfm/Watt) can increase summer ventilation costs by approximately USD 1063, amounting to over USD 10,000 over 10 years for a 600-head dairy operation. Selecting high-efficiency fans can mitigate these added expenses. The use of larger fans can further improve efficiency. When using larger fans, variable-speed models are preferable, as they allow for seasonal adjustments in air exchange rates, improving overall ventilation performance and energy savings [19]. Regular fan cleaning and maintenance are also vital to sustaining efficiency. Dust accumulation on fan blades and housings can reduce efficiency by 15–30%, increasing energy consumption. Implementing routine maintenance schedules helps ensure optimal performance, prolonging equipment lifespan while minimizing operational costs [19]. This is especially important in countries with higher electricity costs than the US. In these countries, mechanical ventilation can become cost-prohibitive if not managed properly.

5. Conclusions

Managing ventilation costs is essential for maintaining profitability in dairy operations amid fluctuating milk margins and climate variability. Over the past five years, mild and moderate heat stress days were common, with severe heat stress emerging in 2023 and 2024. Milk margins varied significantly, with 2024 experiencing the highest reductions resulting from heat stress (USD 28,817) and 2021 the lowest (USD 9804). In contrast, ventilation electricity costs remained relatively stable, averaging USD 8616 annually, but peaked in July and August.

Efficient ventilation strategies can help offset economic losses. Ensuring adequate air inlet size, selecting high-efficiency fans, using larger variable-speed models, and performing routine maintenance can significantly reduce energy use and costs. As climate patterns shift, cost-effective ventilation management will be crucial for sustaining dairy operations. Future studies can include a more comprehensive economic analysis, including reproduction losses and initial purchase costs of the ventilation fans.