Introduction
India
possesses the largest ruminant population in the world (520.6 millions), and
accounts for the largest number of cattle (16.1%), buffalo (57.9%), second
largest number of goat (16.7%) and third highest number of sheep (5.7%) in the
world. India ranks first in milk production (121.8 million tonnes) in world but
per capita availability (281 gm/day) is far lesser than many developed
countries which may be due to several factors out of which heat stress is one
of the important cause. High temperature of tropics along with high humidity,
in particular global warming to a great extent impairs the production (growth,
milk quantity and quality), reproductive performance, metabolism, health status
and immune response of farm animals. The surface air warming in the 21st
century by best estimate will range from 1.1 to 2.9°C for a “low scenario” and
of 2.4 to 6.4°C for a “high scenario” which will vary depending on the
geographic zones of the World. Climate change projections for India suggest
that temperature is expected to increase between 2.3 and 4.8°C because of
doubling of carbon dioxide concentration in the atmosphere which would directly
or indirectly affect the performance of animals. There are many directly
deteriorating effects of heat stress on animals which involves reduction of
feed intake, increase in respiration rate and water intake and changes in
hormonal signals that affect target tissue responsiveness to environmental
stimuli and would lead to illness, morbidity and mortality of animals. Indirect
impacts follow more intricate pathways and include those deriving from the
attempt of animals to adapt to thermal environment or from the influence of
climate on microbial populations, distribution of vector-borne diseases, host
resistance to infectious agents, feed and water shortages, or food-borne
diseases. If exposure to high air temperature is prolonged, lower feed intake
is followed by a decline in the secretion of calorigenic hormones (growth
hormone, catecholamines and glucocorticoids in particular) involved in
thermogenic processes of digestion and metabolism. All these events together
would tend to reduce metabolic heat production and might be responsible for
modifications of energy, lipid, protein and mineral metabolism, and liver
function in the body.
Effects of
Heat Stress:
Effect on
nutrient uptake and digestibility
Increase in
environmental temperature causes severe damage to the physiology, metabolism
and health of animals. Increased heat load decreases nutrient uptake in almost
all species and in case of cattle, the nutrient uptake decreases up to 30% of
dry matter intake. Digestibility at 25°C to 30°C did not change whereas
digestibility increased at 35°C and then decreased at 40°C could be attributed
to change in rumen environment (pH, rumen temperature, rumen motility, rumen
flora and fauna) due to higher intensity of thermal stress. Feed intake in
lactating cows begins to decline at ambient temperatures of 2526°C and reduces
more rapidly above 30°C and at 40°C; the dietary intake may decline by as much
as 40% resulting in a negative energy balance (NEB) and loses significant
amount of body weight and body score. During heat stress, DMI was reduced in
the lactating goats because of decreased metabolic rate and more heat
production. The concentrate intake during cool, comfort, hot-dry and hot-humid
exposures did not change in buffalo calves whereas, wheat straw intake
decreased significantly by 29.65% and 30.09% during hot-dry and hot-humid
exposures respectively.
Effect on
rumination and rumen motility
The rise in
environment temperature alters the basic physiological mechanism of rumen which
negatively affects the nutrient energy balance of ruminants. Heat stress
reduces the dry matter intake, decreases ruminal motility and contraction,
changes the fermentation pattern and volatile fatty acid production, affects
the digestibility and nutrient utilization, and thus impairs the productive and
reproductive efficiency of animals. Increase in environmental temperature
reduces the rumination time and also depresses the appetite by having a direct
negative effect on appetite centre of the hypothalamus. It was also reported
that rumination decreases during dehydration of animals resulting from heat
stress. Moreover, blood flow to rumen epithelium is depressed.during heat
stress and reticular motility and rumination is decreased whereas, the volume
of digesta in the rumen of beef cows, goats and revierine buffalo increased. A
very high concentration of lactic acid was observed in heat stressed cattle,
which lowers the ruminal pH and in turn inhibiting rumen motility. Heat stress
reduces the acetate whereas increases the propionate and buyate production.
Therefore the animal responds by consuming less roughage which leads to
variation in digestion patterns due to changes in microbial population in the
rumen and increased in rumen pH from 5.82 to 6.03 in lactating cows. Ruminal and
intestinal absorption of nutrients in ewes were found to be changed when
exposed to thermal exposure for different durations. Later on when the ewes
were chronically exposed to heat, there was lower diet digestibility and lower
pH and cellulolytic and amylolytic bacteria concentrations, slower digesta
passage rate and lower osmolarity of rumen content, indicating a possible
impairment of bacterial activity and high dilution of rumen fluid. There are
some of the gastrointestinal hormones that influence motility and also affect
feed intake in ruminants but
there is not
clear evidence of involvement of gastrointestinal hormones and peptidergic
neurons in mediating the effect of temperature on gastrointestinal motility.
Heat stress tends to decrease the ruminal fermentation and therefore reduction
in methane production, leading to lower ruminal activity.
Feeding
approach to overcome heat stress
Maintaining
optimum nutrient balance and providing highly palatable and digestible feeds
and ample supplies of fresh and clean water, along with shade and ventilation,
will go far toward keeping your animals comfortable and healthy. Low quality,
stemmy forages generate more heat of fermentation inside the rumen and
contributing to the animal's total heat load. So animals should be provided
with high quality forages but don't go below 18-19% ADF (acid detergent fiber)
which would digest faster and result in lesser heat production. Decreasing the
forage to concentrate ration (feeding more concentrate) could be practised whereas,
excess concentrate may cause problems like rumen acidosis and animal can go off
feed. Sodium
bicarbonate
or sesqui-carbonate at the rate of 0.25 to 0.5 lbs./cow/day can help buffers
the rumen to accommodate higher levels of concentrate. Increase buffer to 0.75%
of DMI (dry matter intake) in Total Mixed Rations or complete feeds and offer
free choice. Other feed additives like yeast (improved fiber digestion), fungal
cultures for example Aspergillus oryzae and niacin (improved energy
utilization) can be incorporated into the ration for better comfort to the
animals. however, all of these additives should not be used together. Ration of
ruminants should usually contain 18% protein or less on a dry basis but that
containing greater than 65% of the total protein as rumen degradable protein
should be avoided. Supplemental fat, such as cotton seeds, soybean, tallow,
rumen inert sources, or combinations can be added to rations to increase energy
intake. There is increased sweating and urination during hot weather conditions
resulting in more minerals being excreted. Therefore, additional mineral
supplementation should be incorporated in the ration containing potassium to
the level of at least 1.5%, sodium to 0.45%, and magnesium to 0.35% of dry
matter. Increasing the levels of vitamins such that supplementing 100,000
international units (IU) of vitamin A/day, 50,000 IU of vitamin D/day and 500
IU of vitamin E/day could help the animal to sustain its health and production
during heat stress.
Conclusion
Heat stress
is a cause of great concern that evokes a series of drastic changes in the
animal's biological functions that include depression in feed efficiency and
utilization, disturbances in metabolism of water, protein, energy, and mineral
balances, enzymatic reactions, hormonal secretions and blood metabolites. Such changes
lead to reduction in production and reproduction performance of animal
resulting in huge economical loss to global livestock husbandry. Therefore,
scientific feeding methods should be practised to overcome deteriorate affects
of heat stress in livestock.
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