Alleviating heat stress in dairy cattle

Heat stress is a major source of economic distress to the US dairy industry with average annual losses of over $800 million associated with reduced performance and increased disease incidence (St. Pierre, 2003). In unusually warm summers these costs rapidly increase. For example, during the summer of 2006 a 2-week heat wave in California caused an estimated $1 billion loss in production and animals.

When effective environmental temperature exceeds the thermal zone of comfort, or thermo-neutral zone, cows experience heat stress (Armstrong et al., 1993). A cow’s thermo-neutral zone is dependent upon physiological status and level of production.

Since average milk yield in US dairy cows has doubled since the 1950s, the thermoneutral zone has shifted downward as cows become more heat sensitive and cold tolerant (Collier et al., 2004). Environmental factors that influence the effective environment around the animal include relative humidity, velocity of ambient air, degree of solar radiation, thermal radiation, and moisture loss (NRC, 1981; St. Pierre, 2003).

The Temperature Humidity Index (THI = tdb + 0.36t_dp + 41.5, where tdb = dry bulb temperature, ºC and t_dp = dew point temperature, ºC) originally developed by Thom (1958) and extended to cattle by Berry and colleagues (1964) is used to estimate cooling requirements of dairy cattle. THI values were categorized into mild, moderate and severe stress levels for cattle by the Livestock Conservation Institute (Armstrong, 1994; Whittier, 1993).

However, as pointed out by Berman (2005), the supporting data for these designations are not clear. For example, the index is based on a retrospective analysis of studies conducted at the University of Missouri in the 1950s and early 1960s on a total of 56 cows with milk yield averaging 15.5 kg/day (range 2.7-31.8 kg/day).

In contrast, average production per cow in the US is presently over 30 kg/day with many cows producing above 50 kg/day at peak lactation. Escalating milk yield increases sensitivity of cattle to thermal stress and reduces the ‘threshold temperature’ at which milk losses occur (Berman, 2005). This is because metabolic heat production increases with production level of the cow.

For example, heat production from cows producing 18.5 and 31.6 kg/day of milk was 27.3 and 48.5%, respectively, higher than non-lactating cows (Purwanto et al., 1990). In fact, Berman (2005) indicated that increasing milk production from 35 to 45 kg/day decreased threshold temperature for heat stress by 5º C. Thus, THI predictions of environmental effects on milk yield presently underestimate the magnitude of thermal stress on contemporary Holsteins.

Furthermore, the work by Berry et al. (1964) did not take into account radiant heat load or convection effects. The vast majority of cattle today are housed under some type of shade structure during warm summer months and although this greatly reduces solar heat load, there is still a radiant heat load on animals emanating from the metal roof. Berman (2005) estimated that the typical shade structure in Israel adds an additional 3º C to the effective ambient temperature surrounding animals. In addition, there are varying convection levels under shade structures depending whether fans are used as part of the cooling management system.

An additional factor in utilizing THI values is the management time interval. The time interval involved in the original THI predictions by Berry et al. (1964) was 2 weeks. In other words, the milk yield response to a given THI was the average yield in the second week at a given environmental heat load.

However, this time lag is fiscally unacceptable as dairy producers need to immediately know what level/extent of cooling is required in order to prevent present and future production losses. Collier and colleagues (1981) and Spiers and co-workers (2004) indicated that effects of a given temperature on milk yield were maximal between 24 and 48 hrs following a stress.

Additionally, it has been reported that ambient weather conditions two days prior to milk yield measurement had the greatest correlation to reductions in production and dry matter intake (West et al., 2003). Furthermore, Linvill and Pardue (1992) indicate that the total number of hours when THI exceeded 72 or 80 over a 4-day interval had the highest correlation with milk yield. Collectively, these results demonstrate that current THI values for lactating dairy cows underestimate the size of the thermal load as well as the impact of given thermal loads on animal productivity and have an inappropriate time interval associated with cooling management decisions.

Practically, if producers can avoid an acute (i.e., 48 hr) decline in production this will probably result in maintaining milk yield in the long run (i.e., 2 weeks later). Specifically, the time frame for utilizing THI values to reduce milk yield losses needs to be shortened. New studies are needed utilizing high producing dairy cows and including radiant energy impact on animal performance. Furthermore, impact of a given THI on milk yield within 48 hrs needs to be determined. This will provide meaningful data to producers in order to make cooling system decisions to improve cow comfort, animal well-being and to maintain current and long-term production.

A final component of the current THI index is the pattern of stress application. In the original work by Berry et al. (1964), cows were exposed to given THI conditions continuously (no daily fluctuations) for the entire 2-week period. This is obviously not what occurs under natural/practical management conditions where temperatures cycle (rise and fall) during a normal day. As a consequence, we presently do not know how to assess the true THI value.

For example, is it the average, the peak or the minimum that is important? Alternatively, is it the number of hours above an arbitrary THI value that is most critical? Holter et al. (1996) reported that minimum THI was more closely correlated with reduced feed intake than maximum THI. Ravagnolo et al. (2000) evaluated test day yields and found a decrease of 0.2 kg milk per unit increase in THI above 72 when THI was composed of maximum temperature and minimum humidity. A study designed to control temperature and humidity in a circadian manner, similar to natural environmental conditions, has never been conducted.

West et al. (2003) evaluated feed intake and milk yield under natural conditions and found that mean THI two days earlier had the greatest effect on both intake and yield. However, they were working under natural conditions and could not quantify the relationship between THI and milk yield.

The effects of radiant heat load can be evaluated using the Black Globe Humidity Index (BGHI = t_bg + 0.36t_dp + 41.5 where t_bg = black globe temperature ºC and t_dp = dew point temperature, ºC), developed by Buffington et al. (1981). These investigators demonstrated that BGHI had a higher correlation to rectal temperature increases and milk yield decreases than THI. They also pointed out that the correlation of BGHI to milk yield was greater (r2 = 0.36) under conditions of high solar radiation (no shade) than under a shade structure (r2= 0.23).

However, milk yields in this study were also low (average 15 kg/cow). Therefore, correlations of BGHI to milk yield under shade structures might be higher with higher producing dairy cows (they are more sensitive to increased heat loads).


Strategies to alleviate heat stress

Two primary strategies to maximize performance of cattle during warm summer months are to alter the environment around the animal and change nutritional management to maximize feed intake and substrate utilization.

Environmental modifications to alleviate heat stress include first providing shade to decrease solar radiation exposure and then to either increase convection with fans or to decrease air temperature by evaporative cooling or to directly cool the cow through use of sprinklers and soakers (Knapp and Grummer, 1991).

Evaporative cooling systems use fine mist at high pressure and large volumes of air in order to evaporate moisture and to cool the air surrounding the cow (Collier et al., 2005). Korral Kool, a company in Mesa, Arizona, manufactures a reverse chimney fixed evaporative cooling system that is mounted to a conventional corral shade (Armstrong et al., 1993).

These systems cool the environment surrounding the cow by injecting micron-sized (30-65 μm @ 300 psi) water droplets into the air moving down the cooler (Ryan et al., 1992; Armstrong, 1994). Since the systems are fixed and the shades are oriented north-south, the sun angle dramatically reduces the shade area under the roof during early morning and late afternoon. Therefore, Korral Kool also uses curtains suspended from the west edge in order to prevent exposure to solar radiation during late afternoons.

Another predominant evaporative cooling system used in the southwest US dairy industry is an oscillating evaporative cooler. One example is the system produced by Advanced Dairy Systems-Shade Tracker (Chandler, AZ). These fans are suspended from the roof structure and are equipped with variable speed water injection (5-15 μms @ 250-1250 psi) into the airstream; and the fans vary in motion 270°. This allows for cooling around and under the shade as the sun changes position. Both systems use computer driven controllers based on THI measurement to determine duration and level of cooling.

Since these systems represent a major capital investment, the dairy industry is interested in determining their relative values under semi-arid conditions.


A COMPARISON OF TWO COOLING SYSTEMS

Two independent trials were conducted by the University of Arizona from June 3rd to September 30th of 2004 and 2005 to evaluate the effectiveness of a fixed reverse chimney cooler versus an oscillating system. In each trial, 400 multiparous and 100 primiparous Holstein cows balanced for parity, stage of lactation, and milk yield were randomly assigned to one of two cooling treatments (Advanced Dairy System fan plus misters – Shade Tracker (ADS-ST) vs. Korral Kool (KK)).

Individual milk yields and pen dry matter intake (DMI) were recorded daily, respiration rates and body surface temperatures (ST) were recorded weekly, and milk components, body condition scores and body weights were obtained monthly. In 2004, production did not differ for multiparous cows housed in ADS-ST and KK pens (Table 1). However, milk yield for primiparous cows housed under KK conditions tended (P = 0.10) to be higher.

Respiration rates of multiparous cows cooled with ADS-ST were higher; however, respiration rates in primiparous cows did not differ between treatments. In 2005, milk yield for multiparous and primiparous cows housed in KK were higher (P<0.01, Table 2) compared to cows housed in ADS-ST pens. Body weight change during the 2005 trial was similar between multiparous cows housed in KK or ADS-ST pens, however primiparous cows housed under KK conditions gained more weight than heifers housed under ADS-ST. In 2005, multiparous and primiparous cows cooled with ADS-ST had higher respiration rates.

During both trials the ADS-ST cooling system used less electricity (526 vs. 723 and 517 vs. 840 kwh/day; 2004 and 2005, respectively) and water (291 vs. 305 and 290 vs. 460 L/day; 2004 and 2005, respectively) than KK coolers. The daily costs for the ADS-ST and KK systems were $27.30 vs. $36.36 in 2004 and $25.95 vs. $42.06 during the 2005 trial. During moderate to severe heat stress conditions, cows housed under KK coolers out-performed those housed under ADS-ST fans; and the increased costs of the KK system were more than compensated by increased production. These differences widened as the level of stress increased.


Table 1. Production variables and heat stress parameters in primiparous and multiparous cows cooled with ADS-ST or KK evaporative cooling systems from 06/03/2004 to 09/30/2004.



ADS-ST = Advanced Dairy Systems Shade Tracker Cooling System,
KK = Korral Kool Cooling System.



Table 2. Production variables and heat stress parameters in primiparous and multiparous cows cooled with ADS-ST or KK evaporative cooling systems from 06/03/2005 to 09/30/2005.



ADS-ST = Advanced Dairy Systems Shade Tracker Cooling System
KK = Korral Kool Cooling System.



NUTRITIONAL STRATEGIES: RESPONSE TO NIACIN

During periods of heat stress the nutrient requirements of animals are altered, resulting in the need to reformulate rations. For example, if DMI decreases then an increase in nutrient density is required along with recalculating mineral and water requirements due to increased potassium loss in sweat (Collier et al., 2005). Reductions in DMI are major contributors to decreased milk production (Collier et al., 2005; Collier and Beede, 1985).

When cows are heat-stressed there is also a reduction in rumination and nutrient reabsorption and an increase in maintenance requirements, which results in a net decrease in nutrient/energy availability for production (Collier and Beede, 1985; Collier et al., 2005). Recent studies by Baumgard et al. (2006) have shown the reduction in DMI may only be responsible for ~40-50% of the decrease in milk production when cows are heat-stressed and ~50-60% can be explained by other changes induced by heat stress. This raises the possibility that some of the loss in milk yield during thermal stress might be recoverable through appropriate nutritional management.

Other nutritional approaches to decrease the effect of heat stress are to reduce fiber intake to levels where the rumen can function properly, adding fat (high energy content and low heat increment), implementing higher concentrate diets with caution, and more recently niacin supplementation (Collier and Beede, 1985; Knapp and Grummer, 1991; Morrison, 1983).

Niacin (nicotinic acid) is a potentially useful supplement because it induces vasodilation, therefore transferring body heat to the periphery (Di Constanza et al., 1997). Transferring body heat to the surface through peripheral or vasomotor function can perhaps alleviate some of the decrease in DMI and thus milk production.

Researchers have reported niacin to decrease skin temperatures during periods of mild to severe heat stress when supplemented at 12, 24, or 36 g of raw niacin for three consecutive 17-day periods (Di Constanza et al., 1997). When supplementing raw niacin, the amount of niacin degraded or absorbed in the rumen is much larger than the amount that reaches the small intestine (~17-30%; NRC, 2001).

Past research observing the effects of niacin on heat stress has only looked at raw niacin, however encapsulated niacin was recently evaluated at the University of Arizona. Twelve multiparous Holstein cows producing an average of 25.4 kg/day and balanced for parity and stage of lactation were randomly assigned to either 0 g niacin (control) or 12 g encapsulated niacin daily (Niashure™, Balchem, New Hampton, NY) and were exposed to two environmental temperature patterns. Temperature patterns were thermoneutral (TN) and heat stress (HS).

The temperature humidity index (THI) range of TN pattern never exceeded 72 while HS consisted of a circadian temperature range where THI exceeded 72 for 12 hrs daily. Milk yield was measured twice daily and sampled once a day for composition analysis. Water intake was recorded daily. Cows were fed twice daily and feed refusal was measured daily.

Respiration rates, surface temperatures of both shaved and unshaved areas taken at the rump, shoulder, and tailhead, and sweating rates of the shoulder at shaved and unshaved areas were taken four times daily. Rectal temperatures were measured four times daily.

Dry matter intake was not affected by treatment (38.9 vs. 37.7 kg/day, P=0.69). Milk yields across periods (TN and HS) increased with treatment (30.9 vs. 28.5 kg/d, P<0.0001) compared to controls; however there were no treatment x period interactions (P=0.32, Table 3). Surface temperature averages were unaffected by niacin but were affected by shaving (Table 4). Cows provided cooling treatment had higher average sweating rates over the entire 24 hr period and these differences grew larger during periods of peak thermal stress.

Between 11:00AM to 4:00 PM average sweating rate for treated cows was higher than controls (81.1 vs. 68.2 g/M/hr). Treated cows had lower average rectal temperatures during heat stress compared to controls and lower body core temperatures for the 24 hr period (P<0.001) (Figure 1). Total metabolic and milk heat storage was greater during period 2 (HS) than period 1 (TN) regardless of treatment (Table 1). We concluded that cows given encapsulated niacin had higher sweating rates and lower core temperatures during acute thermal stress.




Figure 1. Body core temperatures during period 2 (heat stress) from day 4 to day 7.



Table 3. Rectal temperatures, respiration rates, heat storage, milk yield, dry matter intake, and water intake.





Table 4. Surface temperatures and sweating rates for shaved and unshaved areas.

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