These are then subdivided into four types: natural, tunnel (where the air flows parallel to feedlane), hybrid (typically a tunnel-barn design with the option to naturally ventilate in the winter) and cross-ventilation (where the air flows perpendicular to the feedlane).
Each system has its benefits and management differences. While roughly 85 percent of freestall facilities in Wisconsin are naturally ventilated, more producers are transitioning to mechanical ventilation because they are seeking year-to-year consistency. Most mechanical ventilation systems are designed using three measurements: ventilation rate (typically in air changes per hour, or ACH), average barn airspeed and air flow per cow.
Ventilation and cooling
To understand the importance of ventilation, it is necessary to understand the natural heat pathways cows use to fight heat stress. Heat is transferred through either sensible or latent processes. For our purposes, think of sensible heat as temperature differences and latent heat as humidity differences. These processes are gradient-driven, meaning the larger the difference in temperature and humidity between the environment and the cow, the more efficiently the cow can cool itself.
While ventilation does not directly cool the cow, it ensures the differences between the cow and its microclimate stay at optimal levels, allowing the cow to naturally cool itself as efficiently as possible.
With this in mind, we have defined three ventilation priorities for any mechanically ventilated system:
- Provide the minimum cooling airspeed of 200 feet per minute, as evenly distributed as possible in the resting space
- Provide an adequate ventilation rate
- Provide transition methodology between summer and winter so the system operates well across all seasons
To help with system selection, we evaluated the capital and operating costs of seven ventilation systems (two natural-, two tunnel-, a hybrid- and two cross-ventilated barns) (Figure 1).
The cost of comfort
The facilities we evaluated are representative of those typically found in the midwestern U.S. While the specifics vary among farms, in general, natural system operating costs are about half those of mechanical systems, and the operation of mechanical systems in mild-weather regions (such as the Upper Midwest) costs about half those of the same systems operating in hotter climates (such as the Southeast).
For a herd size of around 1,000 cows, there was little difference between operating costs of tunnel- and cross-ventilation systems. However, there were potential cost savings operating a cross-ventilation system at larger herd sizes, and it was notable the hybrid ventilation system, which was the most flexible system for variable climates, was the most expensive to build and operate.
When comparing the operating costs of the seven systems, we used a computer model which would select random fans from available performance databases. Fan selection had the potential to double or sometimes triple the total ventilation costs if poor-quality fans were selected (Figure 2).
For example, a natural system with fans spaced every 24 feet over the stalls for a 1,000-milking-cow herd could cost the same as a tunnel system for a similar-sized herd if the selected fans have poor performance. When deciding between fans, the relationship between the fan capacity (the total air the fan moves) and the ventilation efficiency rating (VER) should be requested. However, fan performance is also tied to maintenance.
Poorly maintained fans can operate at 24 to 50 percent less efficiency than their rating, so we recommend a minimum biannual maintenance schedule for fans that operate year-round.
For example, if we take a naturally ventilated barn and retrofit it into a hybrid system, the cost per cow per year to operate the new ventilation system increases by $100 per cow per year. At a milk price of $15 per hundredweight (cwt), this amount is covered by 1.8 pounds of additional milk per cow per day.
Based on our analysis, we developed a tool to estimate the ventilation operating costs and the marginal cost of milk losses due to heat stress. The tool estimates milk loss due to heat stress and accounts for the reduced feed consumption that would accompany the loss of production. This is overshadowed by the consequences of heat stress, which are well-known (Figure 3), and lost milk production can range from as little as 2 to 3 pounds of milk loss per cow per day in mild heat stress or up to 10 to 15 pounds of milk loss per cow per day for severe heat stress.
Given the days each year spent under conditions of heat stress (THI greater than 68) across the country, it is easy to see the relative costs of providing cows an excellent ventilation system are modest compared to the magnitude of the losses from failing to cool cows adequately (Table 1).
Ventilation will always be a sound investment, and it is important to incorporate ventilation design early in the barn planning process. Keep in mind the three ventilation design numbers (ACH, average airspeed and air flow per cow), as well as the three priorities for the cow (minimum cooling airspeed of 200 feet per minute in the cow’s resting space, adequate ventilation rates and a transition methodology between summer and winter, with management plans as the weather changes).
The cost of ventilation is significant, but the cost of poor ventilation is disastrous. Adequate design, good fan selection and proper maintenance will keep costs down and cow comfort optimal.
ILLUSTRATION: Illustration by Kristen Phillips.
References omitted but are available upon request. Click here to email an editor.
- University of Wisconsin – Madison
- Email Mario Mondaca