), and it has been identified as a significant risk to the sustainability of global dairy production (
). The major and most obvious contribution to productivity loss in dairy cattle from heat stress is decreased feed intake and subsequent reduced milk yield and milk quality. The effect of heat stress on feed intake does not fully explain the reduction in milk yield. It should be noted that heat stress results in reductions in milk yield that are greater than the impact of reduced feed intake on its own (
). Less obvious effects are decreased reproductive performance and increased prevalence of animal health problems (
). Evaporative cooling is the main means by which dairy cows can reduce heat load. Evaporative cooling is achieved via sweating, panting, or evaporation of water from the coat of the cow (
). Evaporative cooling is affected by wind speed, dry bulb temperature, relative humidity, and the physical properties of the cow’s hair coat (
). Cooling infrastructure, whether by the use of shade, fans, sprinklers, or a combination of all of these, comes at a cost, and therefore it is essential that cooling strategies are cost effective (
). Moreover, cooling strategies may be more effective where they are applied to maximize the animal’s natural capacity to dissipate heat.
reported that regaining normal body temperature overnight was a critical factor in maintaining milk production during periods of heat stress. Beef feedlot studies have suggested that nighttime heat load reduction is required if beef cattle are to cope with high daytime heat loads (
). Cooling cattle after peak ambient temperature has been reached and continuing that cooling into the early evening and early morning appears to be beneficial by lowering mean rectal temperature and respiration rate and maintaining DMI compared with cattle only cooled during the day (
). During heat waves, the minimum nighttime temperature may not allow cows to dissipate sufficient heat to enable body temperature to return to normal for a sufficient period to ensure ongoing normal physiological function. This can be further exacerbated where relative humidity is high at night and there is little to no airflow. Overall, this failure to dissipate heat can result in a progressive increase in body heat load. Nighttime cooling strategies must be in addition to heat alleviation during the hottest part of the day to ensure animal well-being is maintained (
); however, there is an increasing trend to have fans and other cooling strategies running 24 h/d. It is well known that the application of water (in association with adequate air flow) will reduce body temperature and respiration rates, and will minimize reductions in feed intake and milk yield during periods of hot weather (
). The use of fans and the application of water can be costly, especially if utilized over long periods each day. In many instances, shade is the most economical option for reducing heat load. Shade will reduce the impact of solar load (
), but has little effect on air temperature, and does nothing to enhance heat loss at night. It should be noted that shade structures may reduce the radiant heat load from cows at night because they reduce radiant heat loss from the cows to the cooler night sky.
) have investigated cooling options for cows that are largely pasture based or managed on a feedpad. The current study is focused on cows housed on a feedpad.
MATERIALS AND METHODS
) and Queensland
|Item||% of DM|
Milking Parlor and Holding Yard
Feedpad and Loafing Area
Loafing Area Ducted Air Array
Mean Panting Score
) for a group of animals. The MPS is determined by observing individual panting scores of a group of cows and then averaging the individual scores to obtain a mean for the group (
). Individual scores were used to determine the percentage of cows within each treatment within panting score (PS) categories: PS = 0, PS = 1, PS ≥ 1.5 < 2.5, PS ≥ 2.5 < 3.5, and PS ≥ 3.5 (
). For dairy cows, we used an additional scoring point PS = 1.5 to account for their lower heat tolerance relative to beef breeds, from which the PS index used was developed. Cows with individual scores ≤1.5 (i.e., PS = 0 and PS = 1) were not considered heat stressed (
). Cows with PS ≥1.5 were deemed to be heat stressed; those in the range PS ≥ 1.5 < 2.5 were considered to be under moderate heat load, those with PS ≥ 2.5 < 3.5 were considered to be under high heat load, and those with PS ≥ 3.5 were considered to be under extreme heat load. The rational for using MPS rather than respiration rates was that to be close enough to determine individual animal respiration rates, the observer’s presence would change the cow’s behavior. Individual panting scores ranged from 0 = no panting; 3 = open mouth and excessive drooling, neck extended, head up tongue not extended, excessive drooling; and 4.5 = head held down, cattle “breath” from flank, drooling may cease (
; Table 2). For the present study, MPS were used to describe the following 4 stress categories: (1) no stress, MPS score between 0 and <0.4; (2) low stress, MPS ≥0.4 and <0.8; (3) high stress, MPS ≥0.8 and <1.2; and (4) severe stress, MPS ≥1.2 (
|Panting score||Breathing condition|
|1||Slight panting, mouth closed, no drool, slight chest movement|
|1.5||Fast panting, mouth closed, no drool, easy to see chest movement|
|2||Fast panting, drool present, no open mouth|
|2.5||As for 2, but occasional open-mouth panting, tongue not extended|
|3||Open mouth and excessive drooling, neck extended, head held up|
|3.5||As for 3 but with tongue out slightly and occasionally fully extended for short periods|
|4||Open mouth with tongue fully extended for prolonged periods with excessive drooling; neck extended and head up|
|4.5||As for 4 but head held down; cattle “breath” from flank; drooling may cease|
). Three temperature sensors (Hobo U12-008, Onset Computer Corp.) were placed in 3 black globes and set up under the shade structures on the west and east sides of the feedpad.
Heat Wave Definition
). For example, THI hours = 7:30 means that THI ≥79 for 7 h and 30 min in a 24-h period. A THI threshold between 80 and 89 is considered to indicate moderate to severe heat stress, and >89 is severe heat stress (
). A THI of ≥68 is considered the heat stress threshold for dairy cows (
). A THI between 68 and 71 is considered low stress and a THI below 68 indicates no stress.
|Relative humidity, %||60.39 ± 0.28||59.75 ± 0.28||69.16 ± 0.30||70.62 ± 0.27|
|Ambient temperature, °C||26.93 ± 0.07||26.18 ± 0.07||25.28 ± 0.07||21.08 ± 0.07|
|Black globe temperature, °C||30.09 ± 0.13||28.76 ± 0.13||27.79 ± 0.13||23.46 ± 0.14|
|Solar radiation, W/m2||297.56 ± 5.74||252.04 ± 5.30||195.07 ± 4.35||181.15 ± 4.55|
|Wind speed, m/s||1.99 ± 0.02||2.19 ± 0.02||1.42 ± 0.02||1.25 ± 0.02|
|THI||74.64 ± 0.06||73.66 ± 0.07||73.28 ± 0.07||67.37 ± 0.09|
Study Effects (90 d) on DMI, MY, MPS, and TRUM
|Item||EDN (n = 58)||DC (n = 51)||P-value|
|DIM||63.76 ± 1.12||60.48 ± 0.29||0.6827|
|Milk yield, kg/cow per day||28.74 ± 0.18||26.69 ± 0.24||<0.0001|
|Rumen temperature, °C||39.51 ± 0.01||39.66 ± 0.01||<0.0001|
|MPS||0.68 ± 0.02||0.75 ± 0.02||0.0060|
|Treatment||Time of day category||MPS, units|
|EDN||AM1||0.39 ± 0.04
|DC||AM1||0.42 ± 0.04
|EDN||AM2||0.79 ± 0.04
|DC||AM2||0.86 ± 0.04
|EDN||PM1||1.10 ± 0.04
|DC||PM1||1.18 ± 0.04
|EDN||PM2||0.45 ± 0.04
|DC||PM2||0.54 ± 0.04
|Treatment||Hour code||TRUM, °C|
|EDN||1||39.57 ± 0.01|
|DC||39.66 ± 0.01|
|EDN||2||39.20 ± 0.01|
|DC||39.36 ± 0.01|
|EDN||3||39.63 ± 0.01|
|DC||39.84 ± 0.01|
|EDN||4||39.64 ± 0.01|
|DC||39.78 ± 0.01|
Effects of Third Heat Wave on PS, TRUM, MY, and DMI
|Treatment||Day||TRUM, °C||Difference, °C||THIMAX||THIMIN||THI hours ≥79, hh:min|
|EDN||1||39.47 ± 0.03|
|DC||39.85 ± 0.03||0.38||81.94||68.5||7:30|
|EDN||2||39.52 ± 0.03|
|DC||39.97 ± 0.03||0.45||81.86||68.1||7:00|
|EDN||3||39.58 ± 0.03|
|DC||39.86 ± 0.03||0.28||79.95||68.9||3:10|
|EDN||4||39.48 ± 0.03|
|DC||40.08 ± 0.03||0.60||81.17||68.6||6:10|
|EDN||5||39.59 ± 0.03|
|DC||40.24 ± 0.03||0.65||81.94||68.5||9:00|
|EDN||6||39.56 ± 0.03|
|DC||40.38 ± 0.03||0.82||83.94||69.6||10:50|
|EDN||7||39.67 ± 0.03|
|DC||40.24 ± 0.03||0.57||83.79||71.2||9:00|
|EDN||8||39.67 ± 0.03|
|DC||39.93 ± 0.03||0.26||83.12||62.8||7:20|
, and this strategy has been used for over 30 yr as a method to reduce heat stress (
) in both housed dairy cows and cows in outside yards. Evaporative heat loss from the skin and sensible heat losses are affected by relative humidity, the temperature gradient between the cow and the environment, and air velocity across the body surface of the cow (
). The efficiency of cooling over a range of environmental conditions (temperature and relative humidity) is improved if air velocity at the animal is 1 to 1.5 m/s (
). During the current study, the airflow exiting the tubes was 2.9 ± 0.8 m/s, which is twice the rate described by
needed to improve heat loss efficiency. During periods when natural air velocity is low and temperature and humidity are high, forced air flow may be required to ensure adequate cooling of cows. The use of fans in outdoor environments is often inefficient due to rapid dispersal of the air being moved by the fans.
). The use of ducted air systems using woven polythene fabric tubing was often used to improve direct air flow onto pigs (
). The use of ducted air to cool livestock has been exclusively used within fully or partially enclosed buildings. Ducted air systems are also used for ventilation and to draw air from outside the building to the inside. These systems do not cool the air but simply increase the air speed across the animals; they should not be confused with tunnel ventilation systems. Ducted systems typically do not draw air from outside the facility but use fans attached to one end of the duct to force air through holes on the bottom of the duct, which forces the air downward onto the target animals. The efficacy of ducted air to improve cooling of cows in an open environment may be influenced by natural airflow (
). It is possible that high natural airflow will negate the effect of ducted air, although this is not necessarily be a problem if the natural airflow is high enough to negate the ducted air flow because there may be sufficient natural air flow to enhance cooling.
Effect of EDN on Physiological Responses
). Strategies to reduce heat load such as fans, sprinkler, cooled air, and various combinations of these have been used to reduce the impact of heat on milk production. Although all of these strategies will reduce the negative effect on milk yield, there is little evidence to show that milk production recovers sufficiently following a heat event.
using 4 heat abatement strategies for dairy cows, including ducted evaporatively cooled air (21.0 ± 3.7°C) air within an open-sided barn. In that study, the ducted air treatment did not lead to an improvement in milk production. In contrast, in the current study, we found greater milk production for the cows with access to a shower on exit from the dairy and the ducted air in a feedpad environment across the duration of the study (+2.05 kg/cow per day). Six days before the third heat wave, the EDN cows were producing 2.50 kg/d more than the DC cows. Although there was little difference between treatments during the third heat wave (MY 1.24 kg/d greater for EDN), over the 6 d following the heat wave, milk production was greater in the EDN cows (3.61 kg/cow per day), and EDN cows maintained the advantage for the rest of the study. It is evident that enhanced cooling during the heat wave allowed the EDN cows to recover faster that the DC cows. Although the major aim during a heat wave is to reduce negative effects on health, well-being, and performance, recovery after a heat event is important; that is, animals need to return, as close as possible, to performance before the heat wave, without any negative effects on health and well-being.
reported a mean rumen temperature of 39.3 ± 0.14°C for cows that were exposed to the UQ dairy’s standard cooling strategies (i.e., similar to DC in the current study). The TRUM in the current study were slightly greater, at +0.21°C for EDN and +0.36°C for DC, than the values of
. Higher mean TRUM (40.02°C) were reported in a study using high-production Holstein cows (milk production = 35.27 ± 8.44 kg/d;
). In that study, the higher TRUM is most likely attributed to the higher metabolic rate of the higher-producing cows used compared with those of the current study. The largest decline in TRUM occurred in the early morning (0400 to 0700 h), which was typically the coolest part of the day, and it is during this time that the temperature gradient between cow and environment allows the heat load of the cow to dissipate to the environment. This has been previously reported for beef cattle (
) and for dairy cattle (
). The cooling in the holding yard coupled with the temperature gradient between the cows and the environment suggest that additional cooling at this time needs to be further investigated. There may be benefits in using sprinklers at night for additional cooling, which will be enhanced by the temperature gradient. The greatest effect on TRUM occurred during the heat wave. The additional cooling allowed the EDN cows to maintain TRUM during the heat wave close to the average TRUM over the duration of the study (39.58 vs. 39.51°C), and they were able to spend 6 h/d with TRUM <39.5°C, which was only 1 h less than the overall average. In contrast, the DC cows had an elevated TRUM of 0.52°C above the study average (40.10 vs. 39.58°C) during the heat wave. In the 6 d following the heat wave, the TRUM of the EDN cows was greater than that of the DC cows (+0.09°C). Interestingly, the TRUM of the EDN cows was lower during the heat wave than in the 6 d following the heat wave. It is likely that this was due to a combination of factors, such as the greater DMI, more time spent eating, and the greater milk production leading to an increase in metabolic heat production in the EDN cows during the 6-d period following the heat wave.
) and dairy cows (
; Lees et al., 2018 a,
). In an earlier study at the UQ research dairy, Pearson correlation coefficients indicated strong relationships between respiration rate and PS (r = 0.89; P < 0.0001; our unpublished data). Similar results (r = 0.90; P ≤ 0.001) were reported by
. It has also been shown that PS increases as THI increases (
). In the current study, EDN cows had a lower overall MPS than DC cows. Both treatment groups experienced low to high heat stress across most days, with severe stress (MPS ≥1.2) occurring on d 7 and 8 of the third heat wave for EDN and on d 6, 7, and 8 for DC. During the 6 d following the third heat wave, the lower MPS of the EDN cows suggests that their recovery from the heat wave was quicker than for the DC cows, but whether this was due to the cooling effect during the heat wave or the enhanced cooling after the heat wave is difficult to determine. The MPS was always lower for EDN cows than for DC cows; however, the additional cooling did not lead to reductions in PS during the day. Because of the study design, it was not possible to determine nighttime PS. Collecting 24-h respiratory data is an essential requirement for future studies. The use of PS without respiration rate data should be reconsidered because it does not permit fine-scale data analysis.
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Figure 1Cows walking through the shower array.
Figure 2Pen layout. W1 = west 1, W2 = west 2, E1 = east 1, E2 = east 2; shaded area is the covered feedpad; X = location of weather station; O = approximate placement of water troughs. The black lines represent electric fencing.
Figure 3Fans at the feedpad (A), and fan-forced air ducts in the shaded loafing area (B).
Figure 4Ambient temperature (TA, °C), black globe temperature (BG, °C), and relative humidity (RH, %) for the period from January 1 to April 23, 2019; the x-axis indicates dates in dd/mm/yy format.
Figure 5Calculated temperature-humidity index (THI; 10-min intervals) from January 1 to April 23, 2019. The solid horizontal line indicates the THI threshold (75) for shower array and ducted air use. The vertical rectangles indicate the 4 heat wave periods encountered during the study. Cow data from the third heat wave are reported in this article.
Figure 6Mean (±SE) milk yield (kg/d) by DIM categories (CAT) 1 (≤20 DIM), 2 (>20, ≤40 DIM), 3 (>40, ≤60) and 4 (>60 DIM), for enhanced day+night cooling (EDN; light gray bars) and day cooling (DC; dark gray bars). See Table 4 for a complete description of cooling treatments. Bars with different letter designation within category (CAT) are significantly different (P < 0.05); the error bars indicate SE.
Figure 7Percentage of cows within each panting score (PS) category over the duration of the study. EDN = enhanced day+night cooling; DAY = day cooling; see Table 4 for a complete description of cooling treatments. 0 = no panting; 1 = slight panting, mouth closed, no drool, slight chest movement; 1.5 = fast panting, mouth closed, no drool, easy to see chest movement; 2 = fast panting, drool present, no open mouth; 2.5 = as for 2, but occasional open-mouth panting, tongue not extended; 3 = open mouth and excessive drooling, neck extended, head held up; 3.5 = as for 3 but with tongue out slightly and occasionally fully extended for short periods; 4 = open mouth with tongue fully extended for prolonged periods with excessive drooling; neck extended and head up; 4.5 = as for 4 but head held down; cattle “breath” from flank; drooling may cease.
Figure 8Mean hourly rumen temperature of enhanced day+night cooling (EDN; gray line) and day cooling (DC; black line) cows over the entire study (90 d). See Table 4 for a complete description of cooling treatments. * indicates that there are differences between the treatment means (P < 0.0001). The dotted line is the mean hourly temperature-humidity index over the duration of the study. 0 = no panting; 1 = slight panting, mouth closed, no drool, slight chest movement; 1.5 = fast panting, mouth closed, no drool, easy to see chest movement; 2 = fast panting, drool present, no open mouth; 2.5 = as for 2, but occasional open-mouth panting, tongue not extended; 3 = open mouth and excessive drooling, neck extended, head held up; 3.5 = as for 3 but with tongue out slightly and occasionally fully extended for short periods; 4 = open mouth with tongue fully extended for prolonged periods with excessive drooling; neck extended and head up; 4.5 = as for 4 but head held down; cattle “breath” from flank; drooling may cease.
Figure 9Percentage of cows within each panting score (PS) category during the third heat wave. EDN = enhanced day+night cooling; DAY = day cooling; see Table 4 for a complete description of cooling treatments.
Figure 10Mean hourly rumen temperature of enhanced day+night cooling (EDN; gray line) and day cooling (DC; black line) cows during heat wave 3 (8 d). See Table 4 for a complete description of cooling treatments. Treatment means at all time points differ (P < 0.0001). Cows walked to the milking parlor at 0430 and 1500 h and returned to the feedpad at 0630 and 1630 h. The dotted line is the mean hourly temperature-humidity index.
- Table 1Ingredients and chemical composition of the TMR
- Table 2Panting score and breathing condition
- Table 3Monthly (January to April 2019) means (±SE) for relative humidity, ambient temperature, black globe temperature, solar radiation, wind speed, total rainfall, temperature-humidity index (THI), and number of days that maximum THI (THImax) exceeded 75 and 80
- Table 4Means (±SE) for milk yield, DIM, rumen temperature (°C), and mean panting score (MPS) for the duration of the study
- Table 5Mean panting scores (MPS; ±SE) for enhanced day+night cooling (EDN) and day cooling (DC) during morning (AM) and afternoon/evening (PM) observations for the duration of the study
- Table 6Treatment × time of day effects (hour code) on rumen temperature (TRUM) for the duration of the study
- Table 7Treatment differences for rumen temperature (TRUM), maximum and minimum THI units (THIMAX and THIMIN), and THI hours ≥79 units over an 8-d heat wave (heat wave 3)