Efficiency of the indirect cooling system in a free-stall barn to minimize heat stressABSTRACT
This study evaluated the effectiveness of an indirect cooling system using negative-pressure tunnel ventilation in a fully enclosed free-stall barn located in Rio Grande do Sul, Brazil. Over a 12-month period (January to December 2020), 27 data collections were conducted to measure internal and external temperature and humidity between 10:00 a.m. and 7:00p.m. The average external temperature (27.85°C) was higher than the internal temperature (21.78°C), showing an average air temperature reduction of 6.07°C. The thermal sensation temperature was 19.56°C, representing an additional decrease of 2.22°C. The system’s efficiency increased as external humidity decreased. In 77.77% of the evaluated period, the temperature-humidity index (THI) indicated adequate thermal comfort (THI<72), with no productive losses. The annual average conception rate was 47.83%, ranging from 48.66% (summer), 58% (autumn), 41.33% (winter), and 43.33% (spring), showing greater consistency compared with non-climatized systems. It was concluded that the evaporative cooling system with negative-pressure tunnel ventilation was effective in reducing heat stress and maintaining satisfactory reproductive performance throughout the year, thereby improving animal comfort and productivity.
Keywords:
indirect cooling system; heat stress; dairy cattle; climate control
RESUMO
Este estudo avaliou a eficácia de um sistema de resfriamento indireto, por ventilação negativa em túnel de vento, em um pavilhão free stall fechado, no Rio Grande do Sul, Brasil. Ao longo de 12 meses (janeiro a dezembro de 2020), foram realizadas 27 coletas de dados de temperatura e umidade internas e externas, entre 10h e 19h. A temperatura média externa (27,85°C) foi superior à interna (21,78°C), com redução média de 6,07°C. A temperatura de sensação térmica foi de 19,56°C, representando diminuição adicional de 2,22°C. Observou-se que a eficiência do sistema aumentou com a menor umidade externa. Em 77,77% do período, o índice de temperatura e umidade (ITU) indicou conforto térmico adequado (ITU<72), sem perdas produtivas. A taxa média anual de concepção foi de 47,83%, variando entre 48,66% (verão), 58% (outono), 41,33% (inverno) e 43,33% (primavera), com maior estabilidade em relação a sistemas não climatizados. Conclui-se que o sistema de resfriamento evaporativo com ventilação negativa foi eficaz para reduzir o estresse térmico e manter bons índices reprodutivos ao longo do ano, favorecendo o conforto e a produtividade dos animais.
Palavras-chave:
sistema de resfriamento indireto; estresse térmico; gado leiteiro; climatização
INTRODUCTION
Hot and humid environments are common in tropical and subtropical regions of Brazil, negatively affecting the productive and reproductive performance of high-yield dairy cows housed in barns. This impact results from periods of high temperature and humidity combined with the animals’ metabolic heat, which limits their ability to dissipate body heat. This leads to heat stress, impairing milk production, conception rate (CR), and animal development. Therefore, environmental modification measures are essential to improve animal comfort, reduce milk and reproductive losses, and mitigate economic damage (Pires and Campos, 2004).
From a reproductive standpoint, most losses occur during the embryonic period, within the first 35 days of gestation. Studies indicate that 43.2% of culling in dairy farms is involuntary, with 22.1% due to reproductive problems (Bergamaschi et al., 2010). Hansen (2005) emphasizes that heat stress affects the ovary, oviduct, uterus, and embryo, altering reproductive cells. Elevated temperatures during embryonic development may reduce survival, as heat can change steroid hormone concentrations due to alterations in the uterine environment.
Over recent decades, numerous studies have evaluated the performance of high-producing dairy cows in hot environments, leading to the development of efficient cooling systems to mitigate heat stress. The two main systems are: direct cooling, where cows are cooled by water sprinkling and direct ventilation; and indirect cooling, where ambient temperature is reduced (Lacetera et al., 2006; West et al., 2003).
In addition to directly affecting productivity and animal welfare, heat stress also reduces dry matter intake, leading to lower milk yield and quality. Irregular feed intake due to heat-induced stress delays the reproductive cycle and weakens the immune system, increasing susceptibility to infections and pathogens. Thus, improving cow comfort is key to increasing profitability and herd productivity - an investment to be viewed as a long-term benefit, not an additional expense (Toledo, 2018).
It is therefore important to adopt measures that ensure thermal comfort for dairy cows, considering barn characteristics, regional climate, herd traits, and the cost-benefit ratio of the chosen cooling system (Lopes et al., 2020; Porcionato et al., 2009). Thermal comfort can be achieved using free-stall systems, which improve production efficiency and offer favorable cost-effectiveness (Souza et al., 2004).
In a study comparing dairy cows housed in a non-climatized free-stall barn and those in a free-stall barn equipped with fans and sprinklers, the latter group showed an average increase of 4.2kg of milk per day, demonstrating that investing in climate control systems provides a favorable cost-benefit ratio (Souza et al., 2004).
In Brazil, new climatization systems have been implemented in dairy production, where free-stall barns have enclosed sides and use evaporative cooling with cross ventilation (low-profile cross-ventilated free stall - LPCV). Compared with conventional free-stall barns using natural ventilation, fans, and sprinklers (FVA), LPCV systems offer better microclimate control, lower temperature fluctuations, and reduced exposure time to thermal stress, improving animal welfare and herd performance (Garcia, 2017).
It is worth noting that most studies in the literature were conducted in Europe and the United States, where climate conditions differ greatly from Brazil. Therefore, this study aimed to evaluate thermal comfort using calculated indices, as well as reproductive, economic, and milk production indicators in a free-stall barn equipped with an indirect cooling system using negative-pressure tunnel ventilation, located on a rural property in Rio Grande do Sul, Brazil. The study analyzed reproductive parameters such as herd conception rate and the cooling system’s efficiency in relation to potential milk yield losses, by comparing internal and external temperature-humidity index (THI) values throughout the study period.
ETHICAL ASPECTS
This research was not submitted to the Ethics Committee on Animal Use.
MATERIAL AND METHODS
The study was conducted on a dairy farm located in the municipality of Estrela, Rio Grande do Sul, Brazil (latitude 29°30′07″ S, longitude 51°57′57″ W). The region’s climate is classified as subtropical according to Köppen.
Cows were housed in a free-stall confinement barn measuring 55.0×26.0m, with 128 individual stalls equipped with ground rubber mattresses (1.20×2.6m). The central feeding alley included both headlocks and open sections, where a partially mixed diet was provided. Part of the concentrate was supplied during robotic milking. Primiparous cows were preferably housed on one side of the barn and multiparous cows on the other. The barn floor was made of concrete, and the alleys were automatically cleaned by scrapers. The barn housed 112 lactating Holstein cows, divided into two groups, milked by two free-flow robotic systems.
The free-stall barn was fully enclosed and equipped with a tunnel-ventilated indirect cooling system consisting of evaporative panels made of corrugated cellulose sheets, kept constantly moistened to allow air passage. Sixteen exhaust fans located on the opposite end of the panels pulled air through the barn at an average speed of 4m/s.
To evaluate and compare thermal comfort, 27 measurements of internal and external air temperature and humidity were taken using a dry-bulb thermometer between January 16 and December 31, 2020, at random days and times between 10:00 a.m. and 7:00 p.m.
Thermal Sensation (TS) was calculated according to Vilela Franco (INMET) using the equation: TS=33+(10×VV+10.45-VV)×((T-33)/22) where TS = thermal sensation (°C); VV = wind speed (m/s); T= air temperature (°C). The Temperature-Humidity Index (THI) was calculated according to Buffington et al. (1982 apud Turco et al., 2006):
where THI = temperature-humidity index; TDB = dry-bulb temperature (°C); UR = relative humidity (%).
To estimate the relationship between climatic conditions and cow performance under controlled environments, the decline in milk production for Holstein cows at a given THI was estimated using the equation proposed by Pires and Campos (2003): DMP = -1.075-1.736×NP+0.02474×(NP×THI) where DMP = absolute decline in milk yield (kg/cow/day); NP = normal production level (kg/cow/day); THI = daily mean temperature-humidity index.
According to Bergamaschi et al. (2010), the conception rate (CR) is obtained by dividing the number of pregnant cows by the total number of inseminations. The CR influenced by THI was estimated using the empirical equation proposed by Pires and Campos (2003): CR=388.3-4.62×(THI) where CR = conception rate; THI = average daily temperature-humidity index two days before insemination. Monthly milk yield (kg/cow/day) and reproductive efficiency were analyzed through monthly and seasonal (by season) conception rates.
Descriptive analysis was performed to summarize and visualize the essential characteristics of the collected data, using statistical measures such as mean, coefficient of variation, standard deviation, and variance, as well as graphical tools like histograms to highlight relevant patterns and distributions.
RESULTS AND DISCUSSION
In Table 1, the average external ambient temperature recorded over the 27 days of analysis was 27.85°C, with temperatures exceeding 25°C in 70.37% of the evaluated period. The average external relative humidity was 57.55%. These data were collected between 10:00a.m. and 7:00p.m., with 92.6% of the temperature measurements taken between 10:00a.m. and 4:00p.m., the period during which the highest regional temperatures are typically observed.
Considering the external air humidity over the 27 days analyzed throughout 12 months, it was observed that during periods when external humidity was below 40%, the difference between the maximum external temperature (MET=32.92°C) and the minimum internal temperature (MIT=24.95°C) was 7.97°C. In contrast, throughout the entire study period, with an average external humidity of 57.55%, the difference between MET and MIT was 6.07°C, indicating a greater temperature reduction during periods of lower humidity.
When analyzing data with external humidity below 50%, the difference between MET and MIT was 6.98°C. For humidity levels between 50-60%, this difference was 6.14°C. However, when considering only data collected with external humidity above 50%, the difference between MET (27.02°C) and MIT (21.48°C) was 5.54°C, reinforcing the hypothesis that the system performs more efficiently under lower external humidity conditions.
These results indicate that the indirect evaporative cooling system with negative-pressure tunnel ventilation is most effective in climates where external air humidity remains close to 50% for most of the year. It is also noteworthy that only 37% of the data collected on the property showed external humidity levels below 50%. In this study, the average external air humidity during spring (October to December) was 60.33%, while in summer (January to March) it averaged 51.75%.
Descriptive statistics of internal environmental conditions (internal temperature and internal humidity) recorded during the experimental period, measured inside the climatized barn in Estrela, Rio Grande do Sul. Data are presented as number of observations (N), mean, minimum, maximum, coefficient of variation (CV), variance, and standard deviation (SD)
In addition, in the present study, the average internal air humidity in the barn (82.40%) was higher than the external air humidity (57.55%). One reason for the high humidity inside the free-stall in this indirect cooling system is the correlation between external and internal humidity, as the air entering the system is already humidified. Therefore, high external humidity and temperature also reduce the efficiency of the negative-pressure tunnel ventilation evaporative cooling system.
These results are consistent with those reported by Martello et al. (2004), who compared a direct cooling system with sprinklers and ventilation (different from the present study) to a control group, where cows only had shading from the feed alley roof. They found that the average dry-bulb temperatures at 11:00 and 13:00 hours, which were 29.3°C and 30.8°C, were reduced to 26.9°C and 25.5°C, respectively. The relative humidity at the same times, which was 62.7% and 56.5%, increased to 73.7% and 71.2%. These results indicated that direct cooling with sprinklers and ventilation can lead to excessive air humidity inside the facility.
Furthermore, according to Martello et al. (2004), visible signs of heat stress-such as increased respiratory rate, shallow breathing, reductions of up to 10% in milk production, and decreased feed intake-can occur when ambient temperature ranges from 26 to 32°C and relative humidity is between 50 and 90%. With the same humidity but temperatures between 32°C and 37.8°C, cows show severe reductions in milk production and feed intake, along with an increase in body temperature.
Descriptive statistics of internal environmental conditions (thermal sensation and internal humidity) recorded during the experimental period, measured inside the climatized barn in Estrela, Rio Grande do Sul. Data are presented as number of observations (N), mean, minimum, maximum, coefficient of variation (CV), variance, and standard deviation (SD)
When analyzing thermal sensation (TS), based on air passing over the animal at a specific speed and creating a perceived temperature lower than that measured by the dry-bulb thermometer (DBT), it was observed that, with the same internal air humidity and an airspeed of 4 m/s (measured with an anemometer), the average thermal sensation was 19.56°C. This represents a 10.2% reduction compared to the internal temperature (21.78°C), or 2.22°C lower than the temperature measured by the DBT.
Regarding critical temperatures for heat stress in high-producing cows, the literature shows significant variations, particularly concerning the thermoneutral range. In this study, the minimum thermal sensation recorded was 11.84°C and the maximum was 25.19°C, both within the range considered neutral for high-yield cows. This criterion is supported by Garcia (2017), who defined suitable ranges for dry-bulb temperature, relative humidity, and air speed in confinement barns for lactating cows.
Additionally, evaluating and classifying the Temperature-Humidity Index (THI), a robust indicator of thermal comfort derived from air temperature and humidity, is highly relevant to milk production. This index helps determine whether animals are in comfort or experiencing heat stress, guiding the need for and effectiveness of cooling systems.
According to Pires and Campos (2004), THI categories are as follows: ≤70 = normal (ideal temperature and humidity for production); 70-72 = alert (conditions at the limit for maintaining performance); 72-78 = alert (conditions above the critical threshold, leading to compromised productivity); 78-82 = danger (all bodily functions are affected); >82 = emergency (immediate action required).
Dalcin (2013) reported that Holstein cows housed with THI above 72 showed physiological stress, suggesting that stress can occur even below 72. Zimbelman et al. (2009) indicated that cows with THI of 68 already exhibit heat stress.
Of the 27 temperature and humidity samples obtained from the DBT and used to calculate TS THI, 6 THI values (22.22%) exceeded 72, with an average of 73.86 (minimum 73.05, maximum 74.61), remaining in the 72-78 range (above the critical threshold for milk production). These values occurred during summer (February and March) and spring (November and December).
The calculations of the external THI show that in 22.22% of the evaluated moments, the THI reached the critical limit, while in 33.33% of the period, the cows’ bodily functions would be compromised, and in 18.52% of the period, they would be in an emergency state if exposed to these THI levels. This demonstrates that heat stress in cows raised in non-climatized environments is a real issue in the region where the farm is located. Therefore, it was confirmed that climatizing the free-stall barn was a correct decision to promote animal welfare and reduce the economic impacts caused by heat stress.
On the other hand, in 77.78% of the evaluated moments, the THI of thermal sensation (TS) inside the free-stall remained below 72, and in 22.22%, it was at the critical THI range (72-78), where some production losses begin.
Using the Absolute Milk Production Decline (DMP) equation developed by Pires and Campos (2003), it was calculated that an average milk production of 40 kg at a THI of 71.25 would result in zero DMP. Above this THI, production losses occur. Considering the same productivity for the 22.22% of days when THI exceeded 72, the DMP would be 2.58kg of milk, representing a 6.45% reduction. However, if the animals were in the external environment (where THI exceeded 72 in 74.07% of the evaluated period), the average DMP would be 8.79kg/cow/day, a 21.97% reduction in productivity if no cooling system were available.
In addition to milk production, climatization also influences reproductive performance, particularly the conception rate (CR). In the study period, the average CR was 47.83%. By season, it was 48.66% in summer (January-March), 58% in autumn (April-June), 41.33% in winter (July-September), and 43.33% in spring (October-December). Thus, the summer CR was higher than the winter CR, contrary to what occurs in non-climatized systems, demonstrating a significant benefit of the cooling system (Pires and Campos, 2004).
Bergamaschi et al. (2010) consider a CR above 50% ideal. They also highlight that reproductive efficiency most strongly affects herd productivity and profitability, and an inefficient reproductive program increases involuntary culling, reduces cow longevity, decreases replacement numbers, and leads to higher costs for inseminations and medications, compromising genetic progress.
Using the empirical equation proposed by Pires and Campos (2003) to estimate CR, a THI equal to or below 71.06 could achieve an optimal CR of 60%. In the study farm, based on internal air humidity and calculated TS, 77.78% of THI values were below 72, indicating the possibility of reaching this optimal conception rate if other limiting factors are excluded. For the 22.22% of the period where THI exceeded 72, with an average of 73.86, the estimated CR would be 47.07%.
CONCLUSION
Climatization was essential for mitigating the adverse effects of climatic conditions on the dairy herd. The significant reduction in heat stress not only improved cow welfare but also had direct positive impacts on milk production and conception rate. By effectively controlling temperature and humidity within the free-stall barn, it was possible to minimize production losses during periods of high temperatures and elevated relative humidity. Moreover, the conception rate was more favorable when thermal comfort conditions were maintained, demonstrating that proper environmental management can maximize the herd’s reproductive efficiency. These findings underscore the critical importance of effective cooling systems for the sustainability and profitability of dairy farming.
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The research data are available within the article itself.
