Παρασκευή, 14 Μαΐου 2010
The effect of temperature stress on dairy cows
When environmental temperatures move out of the thermoneutral zone (or comfort zone) dairy cattle begin to experience either heat stress or cold stress. Either stress requires the cow to increase the amount of energy used to maintain the body temperature and there is less energy available to produce milk. Thermoneutral zone is the range of environmental temperatures where normal body temperature is maintained and heat production is at the basal level. The ranges of thermoneutral zone are from lower critical temperature (LCT) to upper critical temperature (UCT). LCT is the environmental temperature at which an animal needs to increase metabolic heat production to maintain body temperature. UCT is the environmental temperature at which the animal increases heat production as a consequence of a rise in body temperature resulting for inadequate evaporative heat loss (Yousef, 1985). Thermoneutral zone depends on the age, breed, feed intake, diet composition, previous state of temperature acclimatization, production, housing and stall conditions, tissue (fat, skin) insulation and external (coat) insulation, and the behaviour of the animal. UCT is given as 25-26 ºC , LCT as a range from -16 to -37 ºC for dairy cows (Berman et al., 1985; Hamada, 1971). LCT for newborn calves is 10 ºC in dry and draught-free environment. LCT decreases to 0 ºC by the time the calf is 1 month old
Temperature-humidity index (THI) could be used as an indicator of thermal climatic conditions. THI is determined by equation from the relative humidity and the air temperature and is calculated for a particular day according to the following formula (Kadzere et al., 2002):
THI=0.72 (W+D) +40.6
Where W – wet bulb temperature ºC
D – dry bulb temperature ºC
The principle of THI is that as the relative humidity at any temperature increases, it becomes progressively more difficult for the animal to cool itself. (RCI Technical Information. The influence of temperature humidity index on cow performance).
THI values of 70 or less are considered comfortable, 75 – 78 stressful, values greater than 78 cause extreme stress
Heat stress for the dairy cow can be understood to indicate all high temperature-related forces that induce adjustments occurring from the sub-cellular to the whole animal level to help the cow avoid physiological dysfunction and for it to better fit its environment (Kadzere et al., 2002).
Signs of heat stress
• Crowding under shade or at water tanks
• Panting (open-mouthed breathing)
• Increased salivation
• Increased respiration rate (gasping):
80 to 120 breaths per minute moderate heat stress
120 to 160 breaths per minute strong heat stress
Over 160 breaths per minute severe heat stress
• Rates of gut and ruminal motility are reduced
• Decreased activity
• Reduced feed intake
Under continuous heat stress lactating cows begin to show a decline in the intake at 25-27 ºC with a marked decline of 40% above 30 ºC. (RCI Heat stress in dairy cattle).
• Increased sweating
In dairy cows two types of sweating can be distinguished: both are involved in heat dissipation. The first type is insensible sweating or perspiration that leaves the body at all times, unless the relative humidity is 100%. The other type, thermal sweating, occurs as the principle evaporative cooling mechanism of the cow when the ambient temperature rises.
• Rise of rectal temperature
Rectal temperature is an indicator of thermal balance and may be used to assess the adversity of the thermal environment. In severe cases of heat stress the rectal temperature rise. The effect is increased when the relative humidity is greater than 50%. A rise of 1 ºC or less is enough to reduce performance in most livestock species (McDowell et al., 1976).
• Reduced heart rate
Initial increase in heart rates slows down when the heat stress persists. Reduced heart rate is more typical in heat-stressed cows as it is associated with the reduced rate of heat production as a response to high environmental temperatures
• Increased water intake
Heat stress increases water consumption by at least five times the normal level in temperate zones. Water and macro-mineral needs, influenced heavily by demands to maintain homeostasis and homeothermy, are altered for lactating dairy cows during heat stress. Milk is about 87% water, and contains large concentrations of the electrolytes Na, K, and CI. Therefore, lactating dairy cows have large turnover of water and these electrolytes (Shalit et al., 1991).
• Drop in daily milk production
It is accepted that heat stress is the major cause of lost production in dairy cattle in hostile regions. Some authors reported declines in the productions of milk and fat as a direct result of high environmental temperatures. This may be explaned by the negative effects the heat stress has on the secretory function of the udder (Silanikove, 1992). Some authors suggested that milk production is reduced 15%, accompanied by a 35% decrease in the efficiency of energy utilization for productive purposes, when a lactating Holstein cow is transferred from an air temperature of 18 to 30 ºC. Milk fat, solids-not-fat, and milk protein percentage decreased 39.7, 18.9 and 16.9% respectively (quoted by Kadzere et al., 2002).
Under heat stress metabolism is reduced, which is associated with reduced thyroid hormone secretion and gut motility, resulting in increased gut fill. Plasma growth hormone concentration and secretion rate declines with hot temperature (35 ºC). Ruminal pH is typically lower in heat stressed cattle (quoted by Kadzere et al., 2002).
There are great changes in dietary electrolyte balance and acid/base balance associated with heat stress. The major electrolytes involved in dietary electrolyte balance are Na+, K+, CL¯ and the buffer HCO3¯. S= is not considered so critical in the equation of dairy cattle. Na+, K+, CL¯ are the main ions involved in sweat
The effects of heat stress on dietary electrolyte balance may be summarised as:
• Appetite is depressed, there is some indigestibility of feed and gut motility is slowed
• Milk yield - at 35 ºC there is up to 33 % depression and at 40 ºC, as much as 50%
• Loss in milk quality - fat and protein content declines
• Loss in body weight
• Incidence of milk fever increases
• Metritis is more widespread
• Uterine prolapse is more common
• Mammary gland infections increase
• Uterine infections increase
• Udder oedema is more severe
• Laminitis is more frequent
• Keto-acidosis is a recurring problem
• Fertility is lowered - insemination success rate falls, embryo mortality
• Calves are often premature and small
• Growing animals have markedly reduced weight gains
• Drinking water intake increases
• Nutrient deficiencies occur in marginally adequate diets
(RCI Heat stress in dairy cattle).
The total body water is estimated to range between 75 and 81% of the body weight for lactating dairy cows. Temperature is among the most important environmental factors controlling water intake in lactating dairy cows. Heat stress simultaneously influences both energy and water metabolism (Silanikove, 1992). Under thermal stress cows tend to have increased water content in the rumen as a result of an accelerated water turnover rate. Water loss from an animal is a continuous process; taking place all the time and increasing during the heat stress because of additional evaporative water loss. Water intake of a dairy cow under heat stress increases significantly
Steps to reduce heat stress
There are two main management practices that have been proposed to ameliorate the effects of heat stress: physical protection and nutritional dietary manipulation
• Natural shade
Trees are an excellent natural source of shade on the pasture. Trees are not effective blockers of solar radiation but the evaporation of moisture from leaf surface cools the surrounding air.
• Artificial shade
Solar radiation is a major factor in heat stress. Blocking its effects through the use of properly constructed shade structures alone increases milk production remarkably. Two options are available: permanent shade structures and portable shade structures (Shearer et al., 2005).
Permanent shade structures
Major design parameters for permanent shade structures (orientation, floor space, height, ventilation, roof construction, feeding and water facilities, waste management system) depend on climate conditions. In hot and humid climates the alignment of the long-axis in an east-west direction achieves the maximum amount of shade and is the preferred orientation for tied animals, its north-south orientation is better where cows are free to move. Space requirements are essentially doubled in hot climate. Natural air movement under the permanent shade structure is affected by height and width, the slope of the roof, the size of the ridge opening etc. Painting metal roofs white and adding insulation directly beneath the roof will reflect and insulate solar radiation and reduce thermal radiation on cows.
Portable or temporary shades
Portable shades offer some advantages in their ability to be moved to a new area in different pastures. Portable shade cloth, as well as light roofing material, may be used on the temporary shades
• Cooling by reducing ambient air temperature
Air temperature of micro-environment can be lowered by air conditioning or refrigeration but the expense of such types of air cooling make these impractical.
The evaporative cooling pad (corrugated cardboard or similar material) and a fan system which uses the energy of air to evaporate water is a more economically feasible method to cool the micro-environment.
Fine mist injection apparatus – recent design of micro-environment evaporative cooling systems. This apparatus injects water under high pressure into a stream of air blown downward from above. Coolers are positioned in the roof of the shade structures or cowsheds and air is pulled through the cooler at very high rates. This system is effective in arid climates.
High pressure foggers disperse a very fine droplet of water which quickly evaporates, cooling the surrounding air and raising the relative humidity. The typical design incorporates a ring of fogger nozzles attached to the exhaust side of the fan. As fog droplets are emitted they are immediately dispersed into the fan's air stream where they soon evaporate. Animals are cooled as the cooled air is blown over their body and as they inspire the cooled air.
Misters. A mist droplet is larger than a fog droplet but cools air by the same principle. These systems do not work well in windy conditions or in combination with fans in humid environments, where mist droplets are too large to fully evaporate before setting to the ground. The consequence is wet bedding and feed.
Enhancing the cow's natural mechanism of heat loss
Cooling in hot and humid climates emphasizes shade, wetting the skin, and moving air to enhance the cow's major mechanism for the dissipation of heat – evaporative cooling from the skin.
Sprinkler and fan cooling systems (Direct evaporative cooling)
Sprinkling uses large water droplet size to wet the hair coat to the skin. Cooling is accomplished as water evaporates from the hair and skin. Upper body sprinkling followed by forced-air ventilation reduces body temperature, increase feed intake and milk yield.
Sprayers in parlour exit lanes
Exit lane sprayers are designed to automatically spray water onto the cows as they pass through.
Nutritional dietary manipulation
Evaporative heat loss through sweating and panting is the primary mechanism for heat loss at high environmental temperatures. As a result of water loss from sweating at high temperatures thirst is increased, more urine is excreted and the huge waterflux resulting from increased water consumption also causes heavy loss of electrolytes. Potassium (K+ ) loss from the skin increases by 500% in unshaded cattle. In attempts to conserve K+, cows increase urinary excretion rates of Na+.
In high temperature there is panting respiration (an important reaction to cool the body by evaporative cooling). The rapid loss of C02 results in respiratory alkalosis. Cows compensate by increasing urinary output of HC03-. Constant replacement of this ion is critical to management of blood chemistry. Heat stress increases dietary requirements for the key electrolytes, Na+, K+ and HC03-.
Dietary electrolyte balance is especially important in locations where environmental temperatures exceed 24 ºC and is exacerbated if relative humidity exceeds 50%.
Management of the dietary electrolyte balance is based on adding essential body salts and electrolytes to the drinking water and feed. It stabilises the dietary electrolyte balance, promotes homeostasis, assists the osmoregulation of body fluids, stimulates appetite, ensures normal skeletal development etc.(RCI Heat Stress in Dairy Cattle;
European cattle tend to be tolerant to cold. It is the reason why the impact of cold stress on nutrient utilization and animal performance in cattle has received less research attention. Dairy cattle are housed in the cowsheds that minimize the impact of environmental temperature fluctuations on the animals.
During the last decade uninsulated loose housing cowsheds for dairy cattle have become common also in northern countries. The temperature in the animal's rooms is only a few degrees higher than the temperature outside the cowshed. This trend in dairy cattle keeping increases the importance of cold stress investigations
The effects of cold stress on metabolic and physiological adaptations
The next systemic reactions take place in animals suffering from cold stress:
• Increased dry matter intake
• Increased rumination
• Increased gastrointestinal tract motility
• Increased rate of passage of feed and liquid in the rumen and digestive tract
• Increased basal metabolic rate and maintenance energy requirements
• Increased body oxygen consumption
• Increased cardiac output
• Increased adrenalin, cortisol and growth hormone levels
• Increased lipolysis, glyconeogenesis, glycogenolysis
• Increased hepatic glycose output
• Decreased rumen volume
• Decreased dry matter digestibility
• Decreased insulin response to a glucose infusion
• Decreased temperature of skin, ears, legs
There are a number of factors that alter the effects of cold temperatures on animals: wind, hair depth, hair coat conditions etc. It is important to emphasize the value of a clean, dry hair coat and clean, dry environment with minimal wind for animals exposed to low temperatures
For the dairy cows cold should be considered as a local problem. Direct chilling of the udder depends as much on the thermal properties of the floor as on the temperature
The potential problems in cold weather
• Make sure the waterers or water tanks are not frozen
• Cold weather increases feed needs of cows. Hay provides more heat during digestion than concentrate feeds.
• Do not close eave inlets. This will restrict the ventilation rate and create wet, damp conditions.
• Prevent draught. Cows need dry, draught-free resting area.
• Use ample amount of good, dry bedding
• Having dry teats when the cow leaves the parlor is important. One way to lessen the risk is to dip the teats, allow the dip of about 30 seconds and then blot dry using a paper towel.
(University of Minnesota Extension Service, 2004.)
Berman, A., Folman, Y.M., Kaim, M., Mamen, Z., Herz, D., Wolfenson, A., Grabber, Y. Upper critical temperatures and forced ventilation effects for high-yielding dairy cows in a tropical climate. J. Dairy Sci. 1985, 68, 488-495.
Hamada, T. Estimation of lower critical temperatures for dry and lactating dairy cows. J. Dairy Sci., 1971, 54, 1704-1705.
Kadzere C.T., Murphy M.R., Silanikove N., Maltz E. Heat stress in lactating dairy cows: a review. Livestock Production Science 77 (2002) 59-91.
McDowell, R.E., Hooven, N.W., Camoens, J.K.. Effects of climate on performance of Holsteins in first lactation. J. Dairy Sci., 1976, 59, 965-973.
National Research Council,. Nutrient Requirements of Dairy Cattle. National Academy Press, Washington, 1989.
RCI Heat stress in dairy cattle. http://www.rci.com.au
Shalit, O., Maltz, E., Silanikove, N., Berman, A. Water, Na, K, and Cl metabolism of dairy cows at onset of lactation in hot weather. J. Dairy Sci., 1991, 74, 1874-1883.
Shearer J.K., Bray D.R., Bucklin R.A. The management of heat stress in dairy cattle> what we are learned in Florida.
Silanikove, N. Effects of water scarcity and hot environment on appetite and digestion in ruminants: a review. Livest. Prod. Sci., 1992, 30, 175-194.
Turnpenny, J.R., Wathes, C.M., Clark, J.A., McArthur, A.J. Thermal balance of livestock. 2. Applications of a parsimonious model. Agricultural and Forest Meteorology, 2000,m 101, 29-52.
University of Minnesota Extension Service. Managing Dairy Cows in Cold Weather, 2004.
Yousef, M.K. Basic Principles. Stress Physiology in Livestock. Vol. 1. CRC Press, Boca Raton, Fl, 1985).