SUMMER MANAGEMENT of DAIRY CATTLE

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summer management of dairy cattle,effect of heat stress on dairy cattle

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SUMMER MANAGEMENT OF DAIRY CATTLE: 

SUMMER MANAGEMENT OF DAIRY CATTLE By…… Dr.PARTHASARATHI THOTA

WHY?: 

WHY? Heat stress occurs when the combination of environmental temperature, relative humidity, solar radiation and air movement cause the temperature of the cow to exceed its zone of thermoneutrality . The best way to deal with heat stress is to prevent it from occurring. The objective of the management tools are to prevent heat stress from occurring so that dry matter intake (DMI) is not reduced and animal productivity is not compromised.

Thermal neutral zone:: 

Thermal neutral zone: Environmental temperature at which an animal’s body is at equilibrium; i.e., neither tends to gain or lose heat. for cows is at an environmental temperature of 41 to 77° F

TEMPERATURE HUMIDITY INDEX (THI): 

TEMPERATURE HUMIDITY INDEX (THI) It accounts for the combined effects of environmental temperature and relative humidity. THI = 0.72 (C db + C wb ) + 40.6 where C db = dry bulb temperature ( 0 C) C wb = wet bulb temperature ( 0 C) THI exceeds 72, cows are likely to begin experiencing heat stress and their in calf rates will be affected. THI exceeds 78, cows milk production is seriously affected. THI rises above 82, very significant losses in milk production are likely, cows show signs of severe stress and many ultimately die.

THI: 

THI

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As the environmental temperature increases, the reliance on evaporative cooling (sweating and panting) to dissipate body heat increases. However, high relative humidity reduces the effectiveness of evaporative cooling and during hot, humid summer weather dairy animals can’t eliminate sufficient body heat and body temperatures rise. Lactating buffaloes and cows create a large quantity of metabolic heat and accumulate additional heat from radiant energy. Buffaloes tend to be more uncomfortable because they have fewer sweat glands under the skin than cows. Thermal environment is a major factor that can negatively affect production and reproduction in dairy animals, especially in animals of high genetic merit ( Dubey and Gnanasekar , 2008).

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high ambient temperature is also one of the major factors for reproductive failures and embryonic mortality, especially in buffaloes. Feed intake is reduced and milk yield is alarmingly decreased. The scarcity of fodder and milk make summer season lean for the dairy farmers and they are unable to keep pace with the demands of milk (Rattan, 2000).

DIRECT IMPACT OF HEAT STRESS AND ITS SIGNS: 

DIRECT IMPACT OF HEAT STRESS AND ITS SIGNS High heat loads in dairy animals may lead to depressed feed intake decreased milk yield milk fats and protein % elevated somatic cell counts as well as increased risk of mastitis weight loss and reduced reproduction.

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Once the heat load on a dairy animal becomes severe the animal may show signs of distress. This can include seeking shade refusing to lie down reducing feed intake crowding water holes splashing water from toughs laboured breathing excessive salivation convulsions and even death.

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Under heat stress, particularly buffaloes show the following signs: increased reddening of hide on the brisket, under the belly and between the legs protruded tongue panting salivation obvious blood shot eyes reduced feed intake reduced milk yield very hot to touch and increased rectal temperature (Anonymous, 2003).

HEAT TOLERANCE IN BUFFALO: 

HEAT TOLERANCE IN BUFFALO Buffalo are insufficiently heat tolerant (Mason, 1974). Reports indicate that milk yield, growth and fertility are all reduced during periods of high ambient temperature. Each of these variables shows a direct relationship with rectal temperature (Turner 1962 and unpublished; Goswani and Nair 1964). Buffalo are more prone to suffer heat stress when exposed to solar radiation and denied access to a wallow or cooling shower (Frisch and Vercoe , 1979). They are less tolerant to extremes of heat and cold than various breeds of cattle.

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The body temperature of a buffalo is lower than that of a cow in spite of the fact that its black skin absorbs much heat and its skin has only one-sixth the density of sweat glands that a cow skin has. This explains why buffalo like to wallow in water when the temperature and humidity are high (STID, 1981).

HEAT STRESS IN CALVES: 

HEAT STRESS IN CALVES Different experiments on the effects of heat stress on calves show that blood glucose, total lipids and phospholipids contents were found to constantly increase whereas, the cholesterol levels decreased in all the groups in an experiments on buffalo calves ( Younas et al., 1979). The calves were randomly divided into four groups and were assigned different treatment viz ; open air under tree shade; treatment, inside a shed with ceiling-fan alone, inside a shed with showers alone and inside a shed with showers plus ceiling-fans both.

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The average environmental temperature during the period of study ranged from 32°C to 47°C while the mean relative humidity varied from 33 to 75 per cent. The total protein contents of the serum decreased in all the groups with the exception of group I in which a slight increase was observed. Analysis of variance revealed that body weight, body temperature and pulse rate were significantly influenced by the various treatments. ( Younas et al., 1982).

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In a study on the effect of heat stress in the male Murrah buffalo calves it was concluded that the surface temperature at all sites increased in relation to the length of radiation exposure. During summer when ambient temperature and solar radiation was maximum, young buffaloes were not able to maintain their normal rectal temperature and increased pulmonary frequency by 5–6 times, protruded tongue, and increased salivary activity. Therefore, young buffaloes should be protected from extreme hot conditions and direct sun exposure be avoided (Das et al., 1999).

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It was investigated that the mechanism of adaptation to long-term heat stress and found an adaptive response of acidbase regulation to long term (21 days) exposure of buffalo calves to hot-dry and hot-humid conditions. Decreasing rectal temperature and respiratory rate from day 1 to 21 was associated with concurrent decrease in blood pH and pCO2. Increased plasma chloride concentration with low base excess in blood and in extracellular fluid suggested compensatory response to respiratory alkalosis. Reduced fractional excretion of sodium with increased fractional excretion of potassium and urine flow rate indicated renal adaptive response to heat stress ( Korde et al., 2007).

RECTAL TEMPERATURE, RESPIRATORY RATE AND PULSE RATE: 

RECTAL TEMPERATURE, RESPIRATORY RATE AND PULSE RATE Furthermore it has been established that water excretion rate through urine in buffaloes is higher than in cattle and that buffaloes have a grater dependence on external water because of the evolutionary adaptation of buffaloes to wet environments (Koga et al.,2004). The rectal temperature and respiratory rates were significantly higher during direct exposure to solar radiation in the noon time than the values obtained when the animals were kept under shade in the barn ( Gudev et al., 2007). Therefore, when kept in the barn the buffaloes maintained their RT within the thermo neutral zone at the expense of higher respiratory rate.

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The exposure to direct solar radiation resulted in significant enhancement of both RT and RR. Plasma cortisol level at 2 p.m. tended to be lower than that at 8 a.m. inspite of the increased RT and RR at 2 p.m. showing that heat load on the buffaloes was high enough to cause stress.

HORMONAL CHANGES DURING HEAT: 

HORMONAL CHANGES DURING HEAT Meghad et al., (2008) examined the effect of heat stress in Egyptian buffaloes in summer and winter seasons and evaluated the effect of anti-oxidant treatment as a thermo-protective. the levels of oestradiol (E2) and super oxide (SOD) activities have significantly reduced in the summer when compared with the winter season. While lipid peroxide (LPO), nitric oxide (NO) and cortisol ( Cort .) Levels elevated significantly in summer in comparison with winter climate.

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They also observed a positive significant correlation between E2 and SOD activities while there is a negative significant correlation between E2 and oxidative stress (LPO and NO) particularly in summer which can be interpreted as when the level of oestradiol increases superoxide dimutase also increases, whereas, with the increase in oestradiol level a decreasing trend of lipid peroxide and nitric oxide level and vice versa was observed. It was also observed that, pregnancy rate was decreased from 90% in winter to 62.5% in summer in control group and into 75% in treated animals (injected with antioxidant).

EFFECT OF HEAT STRESS ON FEED INTAKE, DIGESTION AND FEED UTILIZATION: 

EFFECT OF HEAT STRESS ON FEED INTAKE, DIGESTION AND FEED UTILIZATION Feed intake is reduced due increased ambient temperature. High environmental temperature may affect the rumen micro organisms that synthesize Vitamin B complex, amino acids and fatty acids on which the nutrition of ruminants largely depends. In addition, reduction of blood flow to rumen epithelium (Hales et al., 1984) and reduction of rumination is noticed during dehydration and heat stress ( Aganga et al., 1990). The fractional passage rate of digesta in the GI Tract of heat stressed animal is slower than that of animals in thermo neutral zone ( Dubey and Gnanasekar , 2008).

EFFECT OF HEAT STRESS ON PRODUCTION: 

EFFECT OF HEAT STRESS ON PRODUCTION The high temperature causes stress due to increased body heat leading to low heat dissipation from the body surface. High heat load in lactating buffaloes reduces their milk production and shorten duration of lactation periods. The negative impact of sudden temperature change i.e. cold wave or heat wave on milk yield of buffaloes were not only observed on following day but also on the next or subsequent day(s) after extreme event, thereby indicating that extreme events cause a cumulative effect change on milk production of buffaloes ( Upadhyay et al, 2007).

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The incidence of silent heat or poor expression will be more common at high temperatures during summer particularly in buffaloes that have limited access to water for either drinking and/ or wallowing. These buffaloes at high temperatures may also fail to conceive due to silent heat or poor expression of heat, loss of conception, causing long dry periods and inter calving intervals (Roy, 1969) ultimately affecting milk production.

EFFECT OF HEAT STRESS ON REPRODUCTION: 

EFFECT OF HEAT STRESS ON REPRODUCTION Heat stress causes infertility in farm animals and represents a major source of economic loss Heat stress affects reproduction in all major farm species. Dairy buffalo and cattle are particularly sensitive to heat stress because of the metabolic heat produced( Silanikove , 2000). Zoheir et al., (2007) investigated the role of change in the temperature during the year on the Egyptian buffalo and consequently affecting the quality of their oocytes .

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In spring and winter, the percentage of good quality oocytes was (71 and 74.6%), denuded oocytes was (12.8 and 9.1%) and the fair type of oocytes was (16.3 and 16.3%) respectively. Consequently the maturation rate was 85.5 and 92.5% respectively. While in summer and autumn, the percentage of good quality oocytes was ( 50 and 56.9%) instead of denuded oocytes was ( 25.5 and 18.5%) and the fair type of oocytes was ( 24.4 and 21.5%) respectively. Consequently the maturation rate was 59.6 and 74.5% respectively. An investigation revealed that showering of buffaloes twice at 11 a.m. and 3 p.m. daily during months with temperature of 39°C and relative humidity of 47.3 per cent can give an increased conception rate from 20 to 31 per cent (Anonymous, 2003).

INCREASED INCIDENCE OF CERTAIN HEALTH PROBLEMS: 

INCREASED INCIDENCE OF CERTAIN HEALTH PROBLEMS Cause of some important health problems which not only bring great health hazards to herds but economical losses to the owners. During the summer months may be manifested as increased occurrences of mastitis, retained placentas, metritis , and ketosis (Collier et al., 1982). Heat stress can elevate cortisol levels and may partially explain the negative effects of heat stress on animal health.

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De Rensis and Scaramuzzi (2003) reviewed heat stress and seasonal effects on reproduction in the dairy cow and pointed out that There is a widely observed decrease in the fertility of postpartum dairy cows inseminated in the summer compared to cows inseminated in winter. The precise mechanism of this effect has not been conclusively identified. However, the plasma levels of LH and estradiol are decreased in heat stressed cows and this is one of the main factors contributing to low fertility during the hot months of the year. With regard to FSH , there is also general agreement that its secretion in summer is increased probably due to decreased inhibin secretion from small follicles..

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Negative energy balance produces lower blood concentrations of insulin and IGF-I , and higher blood concentrations of GH and NEFA , and this altered metabolic profile acting via the hypothalamo -pituitary system reduces GnRH and LH secretion , leading to reduced estradiol secretion by the dominant follicle. The consequences of reduced estradiol secretion from the dominant follicle are poor estrus detection, compromised oocyte quality, and in extreme situations, ovulatory failure.

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There is a reduction in LH secretion leading to reduced estrogen secretion, impaired detection of estrus, reduced oocyte quality, implantation failure and infertility. Heat stress will also change the secretion of thyrotrophic ( thyroxine ) and adrenocorticotrophic ( cortisol ) and adrenomedullary hormones (adrenaline) . These may potentially impair fertility.

COOLING BY REDUCING AMBIENT AIR TEMPERATURE: 

COOLING BY REDUCING AMBIENT AIR TEMPERATURE Misters High Pressure Foggers Evaporative Cooling Pads and Fans

Misters: 

Misters The use of misters reduced water intake of the misted cows versus the unmisted controls. This was probably because the misted cows were not as hot and so transpired, sweated, less water than the unmisted controls. Misters under the shades in the loafing area should be used in conjunction with fans, at 8 to 9 feet above grade and oriented down at about 30 degrees, so that the water is blown onto the cows. The objective of misters is not to cool the air, but to put water on the cows where its evaporation provides the cooling.

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Bunk line misters should be oriented over the cows, and water volumes must be low enough to prevent slick conditions at the bunks. Bunk line misters are seldom required at night.

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Cows in freestalls should be provided with bunk line misters with fans. Misters with fans have also been used successfully over the stall area. Cooling systems for cows in freestalls are less well understood than cooling systems for cows in drylots . However be sure that there is a roof peak opening and that the sides of the building can be opened to provide a free flow of air through the covered area.

Fans: 

Fans The use of fans, particularly in areas of poor ventilation , is generally considered beneficial in preventing cows from becoming hot. one particularly critical housing area is the crowd pen prior to the milking parlor. Heat stress can occur here at relatively low environmental temperatures due to crowding. This area should have sufficient fans and misters to assure an air turnover of up to 1000 cubic feet per minute per cow. Their should also be an opening in the roof line peak to allow hot air to escape. best used in conjunction

Sprinkling: 

Sprinkling Sprinkling (not misting) the cow with water to fully wet her body and using fans to evaporate the water cools the cow and encourages greater feed intake and milk production. Research shows an 11 percent increase in milk yield when cows were cooled with fans and sprinklers compared with shading alone. Sprinklers and fans are usually placed next to the feedbunk so that the feeding area is the coolest place on the farm, helping to encourage greater feed intake.

ENHANCING THE COW'S NATURAL MECHANISM OF HEAT LOSS: 

ENHANCING THE COW'S NATURAL MECHANISM OF HEAT LOSS Sprinkler and Fan Cooling Systems (Direct Evaporative Cooling) Sprayers in Parlor Exit Lanes Cooling Ponds/ Wallowing tanks Cooled Water Shades

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Cooled Water: It should be available as fresh and clean. Drinking water three-four times a day is a need in summer months in general farm practice. Showering/splashing of water on the body thrice a day (morning, noon and afternoon) decreases heat stress.

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Wallowing : Wallowing is the cheapest and least laborious device to beat the heat in summer. Buffaloes are made to wallow in clusters in ponds, rivers, tanks or other water bodies for hours together. Wallowing is an important route of heat loss in view of the labile body temperature which enables the animal to store body temperature.

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During wallowing rectal temperature decreases gradually but falls abruptly after the buffaloes leave the water. Wallowing becomes more effective if buffaloes remain in shade after getting out of water . This device is more effective than showering, but wallowing in dirty stagnant water exposes the animal to bacterial and parasitic invasions. Wallowing is more economical due to limited recurring expenditure and increase in milk production in comparison to shower ( Agarwal and Singh 2008).

Shades: 

Shades The effect of shades on performance of dairy cows in dry lots demonstrate increased DMI and milk production from cows provided with shades in the loafing area. cows provided with shades in the loafing area are less hot (i.e., their body temperatures are lower) and that they feel less hot (i.e., their respiration rate is lower). Shaded cows also produced more milk and had fewer services per conception than unshaded cows.

Natural Shade: 

Natural Shade Trees are an excellent source of shade and if given the choice cows will generally seek the protection of trees rather than man-made structures. They are not only effective blockers of solar radiation but the evaporation of moisture from leaf surfaces cools the surrounding air without appreciably interfering with air circulation.

Artificial Shade: 

Artificial Shade Portable shades These shades should be oriented in a North/South direction, provide about 40 to 45 square feet of shade per cow, and be about 12 to 13 feet high. There should be a raised dirt mound under the shades to prevent accumulation of moisture, and it should be groomed regularly. Mesh shadecloth is lightweight, available in numerous sizes, has reinforced grommets that make installation easy, and can be used in portable installations.

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when cows were provided with feed and water under shade that they were less hot (i.e., their body temperatures were lower) and that they felt less hot (i.e., their respiration rates were lower). Most studies on shade have used solid roof structures although shade cloths, such as woven polypropylene, provide about 80 to 85% as much shading and are considerably less expensive.

Permanent Shade Structures: 

Permanent Shade Structures 1) Shade structures of 40 feet or less require a minimum eave height of 12 feet . Structures widerthan 40 feet should have eave heights of 16 feet. 2) There should be at least 50 feet of clearance between adjacent buildings or other obstructions. 3) Gable roofs should have at least a 4:12 slope (6:12 is acceptable but difficult to work on) and a continuous open ridge. Ridge caps if desired should have a minimum of 1 foot of clearance between it and the roof peak. 4) Ridge openings should be a minimum of 1 foot wide plus 2 inches for each 10 feet of structure width over 20 feet. 5) Painting metal roofs white and adding insulation directly beneath the roofing will reflect and insulate from effects of solar radiation and will reduce thermal radiation on cows.

Parlor Exit: 

Parlor Exit Cows should have access to fresh water at the parlor exit even if water is available in the pens. In addition, cow activated showers as the cows walk back to their pens, have been effective at reducing heat stress. Only deliver enough water to wet the cow.

Cooling livestock buildings by integrated high pressure fogging system with air ventilation and circulation systems: Israel’s solution (Avraham Arbel) : 

Cooling livestock buildings by integrated high pressure fogging system with air ventilation and circulation systems: Israel’s solution ( Avraham Arbel ) The main evaporative cooling methods used today are sprinkling, pad-and-fan, and fog. Sprinkling systems combine fans and spraying water from sprinklers onto the animals surface (mainly used in dairy cows housing), results in an increase of the free water surface area and consequently of the evaporation rate. Also, sprinkling usually results in contagious conditions for diseases. Therefore, sprinkling is inferior in this respect to the pad-and-fan and fog systems.

The pad-and-fan system: 

The pad-and-fan system The pad-and-fan system is based on forcing outside air into the building through a wet pad , which humidifies and cools it only at the entrance, where the wet pad is situated. The disadvantages of the pad-and-fan system are: (a) the air must be forced through the pad, (b) significant temperature and humidity gradients, along the building, are created; (c) installation, operation and maintenance are expensive; (d) continuous operation and poor water quality cause progressive clogging of the pad, resulting in declining cooling performance.

Cooling Livestock Buildings by Pad and Fan Evaporative Cooling System (Pad Cooling) Vasco Fitas da Cruz, Mauricio Perissinotto and Eduardo Lucas : 

Cooling Livestock Buildings by Pad and Fan Evaporative Cooling System (Pad Cooling) Vasco Fitas da Cruz, Mauricio Perissinotto and Eduardo Lucas

The fogging system: 

The fogging system The fogging system is based on spraying the water as small droplets (in the fog range, 2-60 µ m in diameter) in order to increase the water surface in contact with the air. The free-fall velocity of the droplets is slow and they are easily carried by the air streams inside the building. These results in a high efficiency of water evaporation combined while keeping the animals and area dry. Fog droplets can be generated by several methods, but using the high pressure nozzles is the most economic. The efficiency of this system can be increased by full or partial control of the air movement and circulation through the building.

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Routine showering and keeping buffaloes in an open paddock during night are beneficial practices (Frisch and Vorcoe , 1979). Buffaloes in loose housing with a shade (may be shed) for feeding and shelter during the inclement weather and open area for night hours produce more milk than in conventional houses mismanaged and ill-equipped with cooling devices ( Aggarwal and Singh, 2008).

SOME IMPORTANT GUIDELINES IN TERMS OF FARM PRACTICES TO COMBAT HEAT STRESS: 

SOME IMPORTANT GUIDELINES IN TERMS OF FARM PRACTICES TO COMBAT HEAT STRESS The feeding, watering and milking place should always give shade and protection from direct exposure to severity of weather, either by trees or by a roof. Cool water either from a clean river or served in an earthen pit , helps the animals to maintain their temperature. Drinking bowls are used extensively for buffalo as an efficient way to provide clean, cool, fresh water at all times . Water troughs should always be placed in the shade . A paddock with trees gives very cheap and effective protection from sun.

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In hot humid climates it is better not to have walls. Walls may lead to inadequate ventilation, favoring bacteria and mould growth which makes the stable unhygienic. To protect the interior from sunshine (or heavy rain), curtains made from straw, textile or other suitable material can be used. If possible provide buffalo with a wallow . However, the wallow should be one with clean water and not far from the farm . Spending time walking in the sun to and from the wallow costs more than it saves. Showering the buffalo with cool water for three minutes twice a day has proven to be an efficient way for them to get rid of excess heat.

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In tied up systems it is advisable to provide partitions between buffalo . This helps to reduce the number of cases of teat trampling and other udder injuries. Partitions are also useful while milking buffalo with bucket milking machines or in a pipe-line milking system.

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There are also various roof systems which give improved natural ventilation by means of roof openings, enhanced solar chimney effect, etc. Painting the roof white may increase the level of sunlight reflected, thus reducing the amount of absorbed solar energy (All heat at the same wave length must be either reflected or absorbed, so increasing the amount of reflected heat reduces the amount of heat absorbed).

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Energy is a critical nutrient because of the decline in feed intake which occurs during hot weather. Because energy is usually the nutrient which is most limiting in dairy diets, especially during high production and heat stress, the diet must be made more energy dense to provide sufficient energy to maintain milk yield. Increasing the energy in the diet can be achieved by increasing concentrates (grains) and decreasing forages in the diet. However increasing concentrates to greater than 55 to 60% of the diet dry matter is risky and can result in depressed milk fat content, acidosis, off-feed, laminitis, and reduced efficiency of nutrient use . Added dietary fat is an excellent way to increase energy content of the diet, especially during summer when feed intake is depressed.

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Fat is high in energy (about 2.25 times as much as carbohydrate), does not add starch to the diet (minimizing rumen acidosis), and may reduce heat load in summer. Added dietary fat often boosts milk fat test a point or two. Addition of rumen protected fat is a good option to add more energy to the ration. One rule of thumb when high fat addition is required is that 1/3 comes from natural feed ingredients, 1/3 comes from oilseeds, and 1/3 comes from rumen bypass fats. Minerals play a very important role in reducing heat stress. Care should be taken to supplement extra minerals as there will be a reduction of minerals available to the animals due to low DMI. Under stressor influence, secretion of the cortisol increases , acting as an insulin antagonist through increasing blood glucose concentration and reduction of glucose utilization by peripheral tissues. Increase in blood glucose levels stimulate the mobilization of the Chromium reserve, Cr being then irreversibly excreted in urine.

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Nutrient Change and dietary concentration (DM basis) Energy Increase to compensate for reduced DMI. A .80 Mcal NE L per pound is probably the maximum that can be obtained with adequate fiber levels. Fiber ADF minimum - 18 %. NDF minimum - 25%. NDF from forage or effective fiber - 21%. Fat Added amount should not exceed 4%. Strategy suggested is add 1 lb from animal, 1 lb from vegetable and a final 1 lb from a rumen inert source. Protein Meet overall CP requirement by adjusting concentration as needed. Use a combination of rumen degradable and undegradable sources achieving a rumen undegradable level of 36 to 40% of CP.

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Sodium Increase using buffers to .45 to .55% Potassium Increase to 1.2% or more. High quality alfalfa is a good source. Salt Feed 3 to 4 ounces per cow per day. Chlorine Minimum - .25%, maximum - .35%. DCAB Na + K - Cl ~ 35 to 45 meq/100g DM(Na + K) - (Cl + S) ~ 2 5 to 35 meq/100g DM Magnesium Increase to between .3 and .35%. Niacin 6 g per cow per day Aspergillus oryzae 3 g per cow per day

REFERENCES: 

REFERENCES Dairy bovine production;C.K.THOMAS,N.S.R.SASTRY http://vetconcerns.org/content/managing-dairy-cattle-indian-summer http://www.thecattlesite.com/articles/881/livestock-housing-ventilation-natural-ventilation-design-and-management-for-dairy-housing https://secure.hosting.vt.edu/www.dasc.vt.edu/extension/nutritioncc/shear99b.pdf http://www.ksre.ksu.edu/library/lvstk2/mf2319.pdf http://www.ansci.umn.edu/dairy/dairyupdates/du125.htm http://vet.unne.edu.ar/revista/21-suple-1/2%20bufalos.pdf http://www.caes.uga.edu/applications/publications/files/pdf/B%20956_1.PDF http://animalscience.tamu.edu/images/pdf/dairy/dairy-is-your-dairy-mgmt-program.pdf http:// animalscience.­ucdavis.­edu /­faculty/­ robinson /­Articles/­ FullText /­PDF/­heat1.­pdf