Heat stress in lactating dairy cows: a review

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Heat stress in lactating dairy cows: a review

C. T. Kadzerea, M. R. Murphy, , a, N. Silanikoveb and E. Maltzb

a Department of Animal Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

b Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50 250, Israel

Received 1 September 1999; 

accepted 6 December 2001. 

Available online 9 January 2002.


Our objective was to provide a review of factors influencing heat stress in lactating dairy cows and how it affects milk production. In warmer parts of the world, during summer months in the United States, and in other temperate regions, reduced milk production resulting from heat stress counteracts tremendous genetic progress achieved in increasing milk production. Genetic progress in milk production is closely related to increased feed intake. High feed intake results in raised metabolic heat increment. High metabolic heat increment requires effective thermoregulatory mechanisms to maintain body temperature in a thermoneutral zone and in physiological homeostasis. Cows can succumb to hyperthermia if they fail to maintain thermoneutrality. Accurate measurement of when cows enter heat stress is complicated because the responses to heat stress affect not only the energy balance, but also water, sodium, potassium and chlorine metabolism. Water, sodium, potassium and chlorine are important constituents of sweat, and sweating is a major, if not the most important, thermoregulatory mechanism used to dissipate excess body heat. Due to high metabolic heat increment, and especially in the warmer months, high-producing dairy cows may enter heat stress much earlier than their lower-producing counterparts and than currently thought, or extra heat has been accommodated by physiological adaptations. Should this be the case, then strategies to reduce heat stress must be developed to enable cows to express their full genetic potential. The thermoneutral zone, heat production and heat gain, heat dissipation mechanisms, and how the lactating cow responds to heat stress are discussed.

Author Keywords: Heat stress; Responses; Lactating cows; Dairy cattle

Article Outline

1. Introduction and justification

2. Heat stress in dairy cows

3. Thermoneutral zone

3.1. Lower critical temperature

3.2. Upper critical temperature

3.3. Adaptations

4. Heat production and heat gain

4.1. Net energy of maintenance

4.2. Heat increment

4.3. Environment

5. Heat dissipation mechanisms

5.1. Radiation

5.2. Evaporation

5.3. Convection

5.4. Conduction

6. Responses to heat stress

6.1. Physical responses

6.1.1. Sweating

6.1.2. Rectal temperature

6.1.3. Blood gases

6.1.4. Respiration rate

6.1.5. Heart rate

6.2. Metabolic responses

6.2.1. Mineral metabolism

6.2.2. Water metabolism

6.2.3. Digestive tract

6.2.4. Nutrient digestibility

6.3. Production responses

6.3.1. Water intake

6.3.2. Feed intake

6.3.3. Milk production

6.3.4. Health

6.3.5. Reproduction

7. Conclusions



1. Introduction and justification

The thermal environment is a major factor that can negatively affect milk production of dairy cows, especially in animals of high genetic merit. Dairy cattle research has tended to concentrate on genetic improvements to increase milk production and on nutrient supply to the cow during early lactation. Little attention has been paid to the thermoregulatory ability of the modern cow as her capacity to produce milk has increased.

The question arises whether the temperature at which cows currently start experiencing heat stress has shifted to a lower point, considering that increased milk production is positively correlated to both feed intake and metabolic heat production. The metabolism of an animal is always in a state of dynamic equilibrium in which the influx of nutrients is balanced by the production of energy in catabolic and anabolic processes. Cows require nutrients for, among other things, maintenance of biological processes, reproduction, and lactation. The separation of metabolism into maintenance and production is somewhat artificial because energy metabolism is affected by complex interrelationships among all physiological processes. Early research to measure heat and moisture production/loss from animals had been based on the standard metabolic rate (SMR) (Gordon et al., 1968), or basal metabolic rate (BMR) ( Hayssen and Lacy, 1985). (Abbreviations used in this paper are listed in Table 1.) The concept of BMR (or fasting heat production) originates from the works of Kleiber (1932) and Brody (1945) which related it to the metabolic body size (body weight (BW) in the form of a power function in an inter-species comparison of mature animals). The concept of metabolic body size has been used as a basis for scaling other physiological/metabolic parameters, such as food intake and drug metabolism, thereby removing the effects of body size. To attain SMR or BMR, an animal must be post-absorptive, awake, at rest, and in a thermo-neutral environment ( Gordon et al., 1968); however, cows under commercial production conditions do not meet these constraints. Heat production at BMR has already been reviewed ( Brody and Hayssen) and will not be discussed further.

Table 1. Abbreviations

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Nutrient intake by high-producing cows is closely related to the amount of milk produced. The process of metabolizing nutrients generates heat, which contributes to maintaining body temperature in a cold environment. In a warm climate this heat has to be dissipated if thermal neutrality, a prerequisite for normal physiological function, is to be maintained. This complex interplay of physical and environmental effects influences the physiological functions of the cow and affects not only milk production but also the efficiency and profitability of dairy enterprises. We estimated the amount of heat stress encountered by dairy cows in the United States using records from the National Climatic Data Center (part of the Department of Commerce, National Oceanic and Atmospheric Administration) for the mean number of days per year with maximum temperatures of 32.2 °C or higher and the mean number of cooling degree-days (base 18.3 °C) per year for weather stations across the country. Cooling degree-days are used to estimate the amount of energy required to maintain comfortable indoor temperatures and are computed from each day’s mean temperature [(minimum+maximum)/2] by accumulating the difference between the mean temperature and 18.3 °C for days that it exceeded this temperature. A weighted average for these variables was then calculated for the 100 counties in United States with the most dairy cows in 1997. The top 100 counties had from 19,368 to 277,922 cows and accounted for 4.3 million (48%) of the 9.1 million dairy cows in the country. The average dairy cow experienced 47 days when the maximum temperature was 32.2 °C or higher and 625 cooling degree (Celsius)-days per year. Temperature data alone do not tell the whole story; however, similar information for the heat index in the US is not available.

For the 110,000 dairy cows in Israel, heat stress was quantified in a similar manner. Temperature and relative humidity data, based on a temperature humidity index (Kibler, 1964), for 17 locations were used ( Gat et al., 1999). Heat units, positive differences between the index and 22 units, were accumulated daily from May through October. Total heat units for each location were then combined with milk production data ( Israel Cattle Breeders Association, 1997) for the corresponding districts in 1996 to calculate a weighted average. The average cow in Israel was exposed to 196 heat units during the 6-month period.

We hypothesized that the thermal regulatory physiology of the cow may have changed in response to genetic selection for increased milk production. This hypothesis is justified by the fact that data from the USDA reveal a 338% increase in average milk production per cow per 300-day lactation between 1940 and 1995, from 2096 kg (2090 kg of 4% FCM) to 7462 kg (7080 kg 4% of FCM) (Table 2). In Israel the average milk production per year has increased from 3690 kg (3516 kg of 4% FCM) to 10,447 (9293.8 kg of FCM) between 1934 and 1997 (Table 3). The average milk production per cow per 300-day lactation is higher now and is projected to increase further in the future.

Table 2. Average milk production per cow per 300-day lactation from 1940 to 1995 in the US

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Table 3. Average annual milk production per Holstein cow from 1934 to 1997 in Israel

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The lactating cow uses metabolizable energy (ME) for milk production at an average efficiency (q) of 65% (Moe et al., 1970). It should be mentioned that heat increment (HI) during milk synthesis is also dependent on the quality and quantity of feed that the animal consumes. The quality of dairy cattle feed has changed enormously between 1940 and 1995. Using this information and the fact that the gross energy (GE) content of 4% FCM is 3.14 MJ/kg, we calculated the HI. The classical work on heat stress ( Brody, 1945) was done close to when dairy record keeping started in the US (1940) and is used as a point of departure. Against this background, we calculated the average HI between 1940 and 1995, in the US, at four efficiencies of milk production, q=50, 60, 65 and 70% (Table 4; and for 1934 to 1997 in Israel, Table 5), to estimate the impact that changes in efficiency would have on HI. The results indicated that increased milk production is related to elevated HI. Total milk production and HI have increased over time; however, the rate of increase of HI has been slower than of milk production (Fig. 1, US; Fig. 2, Israel).

Table 4. Average daily heat increment of dairy cows from 1940 to 1995 at four efficiencies of milk production in the US; (a) q=50%; (b) q=60%; (c) q=65%; (d) q=70%

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Table 5. Average daily heat increment of Holstein cows from 1934 to 1997 at four efficiencies of milk production in Israel; (a) q=50%; (b) q=60%; (c) q=65%; (d) q=70%

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Fig. 1. Milk energy and heat increment of lactating cows in the US from 1940 to 1995 calculated from USDA annual milk production data. Conversion of ME into milk energy at an average efficiency of 65% was assumed.

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Fig. 2. Milk energy and heat increment of lactating Holstein cows in Israel from 1934 to 1997 calculated from average annual milk production data (Israel Cattle Breeders Association, 1997). Conversion of ME into milk energy at an average efficiency of 65% was assumed.

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Furthermore, the average BW of dairy cows has increased over time as cows have been selected to produce more milk (Fig. 3). Data from Ragsdale (1934), Davis and Hathaway (1956) and Heinrichs and Hargrove (1987) suggest that, over time, the size of 24-month-old Holstein heifers has increased by 46 kg (or about 10%) from 485 to 531 kg. Larger cows have larger gastrointestinal tracts that allow them to consume and digest more feed. This in turn provides more substrates for milk synthesis.

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Fig. 3. Bodyweight of Holstein heifers at 24 months of age between 1934 and 1987 in the US (Ragsdale; Davis and Heinrichs).

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There is an apparent lack of realization that changes in the physical and genetic constitution of cows may have affected their thermoregulatory capability as well as how they cope with heat stress. The objective of our review was to focus on responses of high-producing dairy cows to heat stress. The literature on this particular topic is scant and does not provide a holistic view of factors that influence the incidence of heat stress in high-producing dairy cows, especially during early lactation. The thermoneutral zone, heat production and heat gain, heat dissipation mechanisms, as well as how the lactating cow responds to heat stress are reviewed.

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