The impact of heat stress on estrous behavior, conception rates and the lactation of dairy cattle is huge. A great deal of research has been done to reduce such stress. The financial losses of dairy farmers because of the lower pregnancy rates and milk yields in dairy cattle suffering from heat stress are well known. Research suggests that an integrated approach is required in order to reduce heat stress.
This Bulletin will first focus on the engineering background related to the use of water and moist air to cool the environment. Second, it will discuss various means of reducing heat stress in dairy cattle, including some newly developed methods.
Engineering Fundamentals Related to Water and Moist Air
Psychrometric Properties of Moist Air
Psychrometric charts are tools from which engineers derive the thermodynamic properties of moist air. Such properties include dry bulb, wet bulb and dew point temperatures, absolute and relative humidity, specific volume, vapor pressure and saturated vapor pressure. At a given atmospheric pressure, other properties can be derived, given two independent properties. Charts with only three atmospheric pressures are available. They are: at sea level (one atmospheric pressure), and middle altitude and high-altitude atmospheric pressure.
Software called `Psychart', shown in Fig. 1, was developed to replace these charts. This software allows users to assign different atmospheric pressures, thus making it more accurate in practical applications. As shown in Fig. 2, users can enter either the pressure value or the altitude, in either metric or English units.
The tables below provide users with easy access to the thermodynamic properties, under regular summer conditions, of tropical and subtropical climates at 1 atmospheric pressure. Under other pressure conditions, the program must be re-run.
The pad and fan system, misting and fogging are cooling methods based on evaporative cooling. The limitation of this approach is the wet bulb depression (WBD) of the air conditions. WBD is the difference between Tdb and Twb. Table 3 shows WBD at various Tdb and RH.
The efficiency of the pad and fan system is defined as the (Tdb-outdoor-Tdb-after pad) over WBD. In Fig. 3, the assigned Tdb-outdoor equals 26oC, while RH equals 45%. The derived Twb is 17.77 oC, thus, WBD equals 8.23 oC. An 80% efficiency pad means the Tdb-after pad equals 26 -0.8 * WBD = 26-0.8 * 8.23 = 19.42oC.
Pad Efficiency at Various Facing Velocities
Trumbull et al. (1986) developed equations to predict the efficiency of three commercially available cooling pads as a function of the air velocity (V, in m/s). Although no information is available in the report about how thick the pads were, they were probably all around 10 cm thick. The equations are as follows:
Eff = 86.62 _ 20.787 * V + 2.755 * V2
for Cooling Pad A (made from cellulose paper with honey comb configuration)
Eff = 91.034- 17.91 * V +5231 * V2
for Cooling Pad B (made from cellulose paper sheets)
Eff = 76.055 + 2.909 * V _ 17.414 * V2
for Excelsior Cooling Pad C
Mannix (1981) found that the water flow rate through a pad had no effect on the evaporator pad performance, as long as the water was evenly distributed and the pad was fully saturated. In contrast, Trumbull et al. (1986) did find that the efficiency of the pads varied with different water flow rates. At low water flow rates (0.57 _ 1.53 L/s), the efficiency fell when the face velocity increased from 0.2 to 1 m/s. At high water flow rates (2.16 _ 3.33 L/s), the efficiency remained the same when face velocity increased from 0.2 to 1 m/s.
The blow-off of water from the pad occurred at 1 m/s for the excelsior pad, 1.6 m/s for cooling pad A and 2 m/s for cooling pad B (Trumbull et al. 1986).
Pad Efficiency and Pressure Drop at Various Thickness
The thicker the pad, the higher the efficiency and the greater the pressure drop at a given face velocity of air. This is shown in Fig. 4 and Fig. 5. An increase in the face velocity of air (V, in m/s) will also increase the pressure drop (in Pa) of the system. The equations to derive the following figures are listed below.
Efficiency for 20 cm pad =
Efficiency for 10 cm pad =
Pressure Drop for 20 cm pad =
Pressure Drop for 10 cm pad = 1.5084*V3+3.6479*V2+6.2665*V-0.3838
Efficiency of the Nozzle in a Misting/Fogging System
The efficiency of a misting or fogging system depends on the number of nozzles used in the system, and the rate of water sprayed out of each nozzle. Bottcher et al. (1991) developed an equation to estimate the efficiency (b) of nozzles used for misting (large droplets because of low pressure, or large holes in the nozzle) or fogging (small droplets because of high pressure and small holes in the nozzle), with respect to water pressure (P, in kPa). The equation is listed below.
b = 0.124 + 1.35 * 10-4*P
At 35 atmospheric pressure, the efficiency is around 60%. When P equals 64.888 atmospheric pressure (64.888 * 100 kPa), the nozzle efficiency (b) reaches 100%, assuming 1 atmospheric pressure equals 0.1 MPa. Considering some friction loss, an atmospheric pressure of 70 is recommended for high-pressure fogging.
Temperature Humidity Index (Thi)
Both the rectal temperature and milk production of dairy cows are in direct proportion to the THI (Igono et al. 1985; Knapp and Grummer 1991). Dairy cattle are considered to be suffering from heat stress at a THI higher than 70-72oC (Ingraham et al. 1974; Johnson 1985, Stott 1981). Conception rates fall when the monthly average THI is higher than 62 (du Preez et al. 1991). Below are two equations to calculate the THI.
THI = T (in oF) -0.55 * (100-RH%)/100 *
(T-58) (Ingraham et al. 1974)
THI = Tdb (in oC) + 0.36 * Tdp (in oC) + 41.2 (Armstrong 1994)
Both equations required two environ-mental factors. The first equation requires dry bulb temperature (in degrees Fahrenheit) and relative humidity (as a percentage). The second equation requires dry bulb and dew point temperatures, both in degrees Centigrade. Please note that the THI should not be expressed as a unit, whether oF or oC. The results of the above equations are not always consistent, as listed below. The THI values of Ingraham's equation are always larger than the values calculated using Armstrong's equation. The difference is more marked as the THI values become larger, as shown in Table 4.
Linvill and Pardue (1992) developed an equation to predict milk production, based on the previous four days' THI information, as shown below.
MP (in kg/day/cow) = 21.48 _ 0.051 * 0.0099 * HA80S
where, 21.48 = regular milk production in kg per day per cow;
HD74: total hours of THI>74 for the last 4 days;
HA80S = square of total hours of THI > 80 for previous day.
A different equation, developed by Berry (1964), and listed in ASAE standards (1988), is listed below:
MPD = 1.08 _ 1.736 NL + 0.02474 (NL) (THI)
where, MPD: decrease in milk production per cow per day, in kg/day/cow;
NL: daily milk yield under no heat stress, in kg/day/cow;
THI: Temperature humidity index
Table 5 shows the MPD values for three levels of NL values, assuming daily milk yields are 20, 25 and 30 kg/day/cow. It is quite obvious that at a fixed THI, the daily milk increases with an increase in NL. This indicates that heat stress affects high-yielding cows the most, in terms of lower milk production (MPD). Furthermore, daily milk yields increase when the temperature humidity index (THI) rises, at the same level of NL.
Black Globe Temperature (BGT)
The black globe temperature (BGT) represents the combined effect of dry bulb temperature, average radiation and average wind velocity. It is normally used to quantify the effect of shading. The black globe temperature does not consider the effect of humidity. When the BGT is less than 25oC, forced ventilation has no effect in reducing the body and rectal temperatures of cattle. When the temperature of the surrounding environment (the 1.8 m above ground level inside the dairy barn) of cattle reaches 36oC, forced ventilation can reduce the rate of increase in rectal temperature by half as BGT increases (Berman 1985).
Wet Bulb Globe Temperature (WBGT)
The wet bulb globe temperature (WBGT) is a more useful index than THI or BGT. It takes into consideration the dry bulb temperature, humidity, radiation and wind velocity. A heat stress monitor ( Fig. 5) was used to calculate WBGT, using separate equations depending on whether the device was in the shade or exposed to sunlight. The equations are listed below.
WBGT indoors = 0.7 * Twb + 0.3 * BGT
WBGT outdoors = 0.7 * Twb + 0.2 * BGT + 0.1 * Tdb
where, WBGT, Twb, Tdb, BGT are in oC.
The WBGT was used in studies of "stay-time" for an individual performing various tasks in various levels of heat (PHEL). It was also used in studies of permissible heat exposure threshold limit values by the Heat Stress Division at the Naval Medical Research Institute of the US Navy.
An Integrated Approach
There is no single method of reducing heat stress in cattle and other animals. The only successful approach is an integrated approach.
The longer side of the dairy barn should have an east-west orientation. This reduces the amount of direct sunlight shining on the side walls or entering the house, as shown in Fig. 6.
Painting the roof white may increase the level of sunlight reflected, thus reducing the amount of absorbed solar energy (All heat at the same wave length must be either reflected or absorbed, so increasing the amount of reflected heat reduces the amount of heat absorbed).
An open type dairy barn should be shielded from direct sunlight as much as possible by means of side curtains. As shown in Fig. 7 taller sheds allow more direct sunlight to enter the house.
Side curtains are the cheapest way of preventing sunlight from entering the house. Installing the side curtains on the support posts ( Fig. 8) gives little protection when the curtains are rolled up. However, the eaves can be extended with shading material ( Fig. 9), and the vertical shading moved to the outside of the eaves. This gives much better protection from the sun. The west side of the barn can also be fitted with side and vertical curtains.
Tall structures (with eaves more than 3.5 meters high) are not economically viable. There are other ways of removing the hot air trapped under the roof inside the building. Outer coverings and shading, perhaps combined with a roof-spray, are popular greenhouse technologies which can be applied in the structural design of dairy barns to reduce the height of the eaves. There are also various roof systems which give improved natural ventilation by means of roof openings, enhanced solar chimney effect, etc.
For a closed type dairy barn, pads and fans may be installed at both ends. An alternative when no pads or fans are installed is to install an extra wall layer at either end. A fixed nontransparent curtain can be used as the outer layer. The outer layer should be 10 cm away from the inside wall, with vertical openings at both ends. The bottom opening allows cold air to enter, and the upper opening allows hot air to exit. The ten-centimeter of air between the two walls provides a thermal barrier to prevent conductive thermal energy from entering the house. This is the cheapest double wall approach, and is a proven technology in structural design ( Fig. 10, Fig. 11 and Fig. 12).
For an open type dairy barn, a roof vent is required to allow heat to escape from the upper part of the barn. With the traditional open-roof structure, the key factors for natural ventilation are: the size of the roof opening in relation to the floor area, and the vertical distance between the air inlet and outlet. For example, a house six meters wide requires a roof vent at least 30 cm wide. The vent should be 5 cm wider for every additional three meters in house width, as shown in the following equation:
Wro = 30+5 * (Wh _ 6)/3
where, Wro: width of roof opening, in centimeters;
Wh: width of dairy house, in meters.
A simple model exists to predict thermally-induced natural ventilation where there is only one inlet and one outlet. However, use of this model should be limited to making initial or field estimates. If the inlet and outlet are equal in size and there is no wind, airflow can be estimated by the equation listed below (Albright 1990).
V = 2 * A * (C/0.65) * [g * h * (Ti _ To)/Ti]1/2
where V is in m3/5, g is the gravitational constant, A is the area of one of the openings, C is the coefficient of discharge of each opening, h is the distance, m, between the two openings, and Ti and To are indoor and outdoor air temperature, K, respectively. If the two openings are not equal, the smaller of the two is used in the above equation. V is adjusted by multiplying (1 + % increase in flow). The % increase in flow can be calculated using the regression equation shown in Fig. 13.
Various kinds of roof vent are available. These include a double roof with tiny holes ( Fig. 14) and a wind-driven rotating roof vent ( Fig. 15). Rotating roof vents are quite popular in Taiwan. However, some manufacturers do not understand the principle of the solar chimney, so performance of the system varies from one brand to another.
In an open type dairy barn, fans can be installed the length of the house and tilted at no more than 30 degrees. It should be noted that the air temperature inside a barn is higher in the upper part than down near the ground, because hot air rises. Air density decreases when the temperature increases. If the fans have too much tilt, the heated air in the upper layer will be brought down around the dairy cattle. One way to prevent this, while still using large fans tilted at an angle, is to install a high-pressure fogging system under the roof. The foggers should be facing downwards or horizontally ( Fig. 16). The fog will evaporate, thus reducing the temperature of the air in the upper part of the barn without increasing the humidity of the air around the cattle.
A movable fan system was developed to provide breeze to the cattle, sometimes just at meal times and sometimes all the time. Two fans per set were installed, with 10 meters between each set. One fan in the set faced the feed, and the other fan was trained on the necks of the cattle, as shown in Fig. 17. One motor was used to drive four sets of fans, the number suited to the dimensions of the dairy barn. In other applications, the same motor can drive up to ten sets of fans. The system has been working since 1995, and no failure has been found up to now (2002).
Pad and Fan System
In a closed type dairy barn, an evaporative cooling system is required in tropical and subtropical climates. The pad and fan system is one possibility. The usefulness of the pad system depends on the local climate and the efficiency of the pad. If expensive imported pads are used, the efficiency is 80% for a ten-centimeter pad at 1.5 m/s suggested face velocity, and about 90% for 15-centimeter pad at 2.5 m/s suggested face velocity.
The potential of the pad system in Taiwan was investigated by the author, based on local climate (using 10 years of hourly weather data) (Fang 1994).
High humidity is not necessarily accompanied by temperatures. This is true in other places besides Taiwan. This is the reason why evaporative cooling systems are still useful, even in hot, humid areas such as Taiwan.
However, how good is the evaporative cooling system? What can we expect in terms of reducing air temperature by means of a pad? Fig. 18 attempts to show this in a diagram, which is rather misleading. First, in Taiwan, we don't have humidity as high as 30%, while our highest temperature is more than 35oC. Thus, it is reasonable to expect a temperature drop of 5oC at 100% `pad efficiency'. For example, in Tainan (south Taiwan), for 92% of the year one should not expect the fall in temperature of air passing the pad to exceed 4oC, assuming 80% efficiency.
Lowering the temperature of the water used in the pad system has little effect on the temperature of the air passing through the pad.
Fogging Multi-Layer Net and Fan System
In Taiwan, we have tried different materials to replace expensive imported pads. One promising system, a fogging multi-layer net and fan system developed by the author, was patented in 2001.
The efficiency of the system reaches 92.5%, which is higher than either the ten-centimeter or 15-centimeter pad system. The fogging multi-layer net and fan system can be used with either negative pressure or positive pressure. The positive pressure type system was installed in a dairy barn ( Fig. 19 and Fig. 20) of a company in South Taiwan. The negative pressure type systems have been successfully installed in chicken houses, greenhouses, etc. ( Fig. 21 and Fig. 22).
Berman et al. (1985) have suggested that 25-26oC is the upper critical temperature for high-yielding dairy cows. If a closed dairy barn is equipped with pads and fans, or a fogging multi-net and fan system, either system should maintain the environment of the cows at a temperature of less than 26oC throughout the day.
Intermittent Spraying and Forced Ventilation
A fogging and movable fan system may increase lactation, but it has little effect on rectal temperature. Direct spraying of water onto the body is the cheapest way of reducing the rectal temperature, thus reducing the heat stress of cattle. A considerable amount of research has shown that this system is effective in improving milk yield and the reproductive performance of dairy cows (Wang et al. 1993; Berman 1995).
Fig. 23 and Fig. 24 show the infrared image of the cattle before and after spraying. An intermittent timer control system was installed to control the on/off of the sprayer and fans. The best cooling effect can be achieved by setting the system at 1:9 intervals per cycle and 5 to 6 cycles per treatment. Berman (1985; 1995) and Wang et al. (1993) suggested that spraying for 0.5 minute should be followed by turning on the fan for 4.5 minutes. In total, five minutes per cycle and 30 minutes per treatment can have 6 cycles.
The author used one minute of spraying and nine minutes of fan per period, the treatment lasting for 50 minutes (five cycles) using different types of nozzle and fan (Fang 1998). The exact setting should depend on the local situation and the water flow rate, the volumetric flow rate of the fan and the number of fans installed. The key is to wet the cattle thoroughly and dry them again completely. The major cooling effect lies in the period during which the wet cattle become dry.
Using Berman's approach, the cooling effect lasts for 30-45 minutes, (0.5 min spray, 4.5 min forced ventilation, 6 cycles, 30 minutes). The cooling effect lasts for 60-90 minutes using the author's approach (1 min spraying, 9 min forced ventilation, 5 cycles lasting a total of 50 minutes).
Berman (1985) suggested that spraying plus forced ventilation treatment should be conducted every two hours for 30 minutes, thus requiring 7 to 8 treatments per day. The improvement in performance of cows receiving this cooling treatment is significant. Conception rate increased from 20% to 30%, and the steady estrous behavior increased from 45% to 70%, without estrous behavior decrease from 33% to 12%, length of estrous period increase from 11.5 to 16 hours during the summer time.
Wang et al. (1993) tested sprinkling followed by forced ventilation 5 times per day between 10:00 am to 4:30 pm. During the experiment, the rectal temperature of the cows was significantly reduced by 0.2oC. In Author's experiment, we found the rectal temperature reduced by 0.4oC (Fang 1998). Wang et al. (1993) also conducted same cold treatment 9 times per day between 5 am to 9 pm for 10 days and found that the amount of lactation increased by 2.6 kg/day/cow in treatment group over control group.
The intermittent spraying and forced ventilation can be conducted at the holding location before milking, as shown in Fig. 25 or in a separate location. The place should have good drainage.
Stermer et al. (1986) conduct experiments on releasing heat stress by lowering drinking water temperature. Results shown that water temperature at 22oC is significantly better than water temperature at 10, 16 and 28oC. The body temperature decrease 0.6oC and respiration rate reduced 12 times per minute.
One method proved successful in one place does not guarantee success elsewhere. One method which has proved to be unfeasible in one place, might become highly successful at another time or in another place, or sometimes when applied by another person. One must realize that there are certain conditions which must be met to guarantee success, and there are limitations to all methods from the local climate, availability of resources, personnel, etc. Even when the same method is used, there are various approaches, while the same approach may have different settings of operating conditions.
In the Appendix, engineering fundamen-tals related to moist air and water are reviewed. Equations, Tables and Figures are provided as the reference for further studies of readers.
The `Psychart' software developed by the author can be downloaded from http://ecaaser3.ecaa.nut.edu.tw/weifang/psy/cea2-5.tm.
This website contains DOS-based and WINDOWS-based programs developed by the author throughout the years. To download the software mentioned in this report, please click on the newest version.
This paper has discussed a number of methods for reducing heat stress in dairy cattle. They are all proven technologies, but some have not been applied to dairy cattle. I hope that we shall be able to test the usefulness of systems such as double walls, double roof with small holes, negative pressure type fogging multi-net and fan system, etc., in the near future.
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Index of Images
Figure 1 Digital Psychrometric Chart
Figure 2 Pop-up Window Which Allows Users to Assign Atmospheric Pressure
Figure 3 Output of Pad and Fan System in Psychart Software
Figure 4 Pad Efficiency at Various Air Velocity and Thickness
Figure 5 Heat Stress Monitor. from the Left Are DRY Bulb, Black Globe and Wet Bulb Temperature Probes
Figure 6 Proper Orientation of the Dairy Barn
Figure 7 Aller Sheds Allow More Direct Sunlight to Enter the House
Figure 8 Original Side Curtains
Figure 9 Improved Side Curtains
Figure 10 West Side of the Dairy Barn before Renovation
Figure 11 West Side of the Dairy Barn after Renovation (View 1)
Figure 12 West Side of the Dairy Barn before Renovation (View 2)
Figure 13 Percentage Increase in Flow in Relation to Ratio of Outlet-to-Inlet Area
Figure 14 One Layer of a Double Layer Roof Pierced by Small Holes
Figure 15 Wind-Driven Rotating Roof Vent
Figure 16 Movable Fan Installed with Fogging on Upper Part of the Dairy Barn
Figure 17 Moveable Fan Playing on Necks of Dairy Cattle
Figure 18 Misleading Schematic Diagram of Pad and Fan System Provided by Manufacturers of Cooling Pads
Figure 19 Fogging Multi-Layer Net and Fan System Used in Dairy Barn (1)
Figure 20 Fogging Multi-Layer Net and Fan System Used in Dairy Barn (2)
Figure 21 Misting Multi-Layer Net and Fan System Used in Greenhouse (1)
Figure 22 Misting Multi-Layer Net and Fan System Used in Greenhouse (2)
Figure 23 Body Temperatures before Spraying
Figure 24 Body Temperatures after Spraying
Figure 25 Intermittent Spraying and Forced Ventilation
Table 1 TWB (in Oc) at Various TDB (20-44oc) and RH (50-100%)
Table 2 RH (in %) at Various TDB and TWB (20-44oc)
Table 3 WBD at Various TDB (20-44oc) and RH (50-100%)
Table 4 Comparison of Thi Equations
Table 5 Various MPD for 3 Levels of NL under Various Environmental Conditions
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