heat stress and its impact on fertility. heat stress and its impact on fertility 1. what´s heat...

HEAT STRESS AND ITS IMPACT ON FERTILITY

Heat stress and its impact on fertility

1. What´s heat stress?2. Heat exchange in cows3. Indicators of heat stress4. Signs of heat stress5. Heat stress and reproduction6. Managing heat stress7. Summary8. References

1.What´s heat stress?There are several sources of heat that affect cows: The first and most important is the heat accumulated by direct radiation from the sun. The heat of digestion of forage. Conduction is also responsible especially when cattle are crowed together.


Air temperature and solar radiation

Humidity level

Metabolic heat as a result of eating

and digesting feed.

1.What´s heat stress?When the effective environmental temperature exceeds the thermal

zone of comfort or thermo-neutral zone, cows experience heat stress.Heat stress is one of the leading causes of decreased production

and fertility in dairy cattle. Heat stress can cost the dairy producer over 400 €/cow1.


HEAT STRESS

DECREASED PRODUCTION AND

FERTILITY

Around 80% of these losses are associated with drop of

productivity

ECONOMICAL LOSSES

20% are related with health issues (impaired reproduction,

mastitis, acidosis…)

During warm summer months, milk production

can decrease 10-35 %

2. Heat exchange in cowsIf the metabolic heat and the heat gained from the environment is higher than the heat lost, the cow experiences heat stress.The temperature difference between the cow and the environment influences on the heat exchange which is continuous and involves the processes of:


Radiation , conduction convention evaporation3, and

Transfer of energy to or from a body

by means of emission

or absorption of electromagnetic

radiation.

Transfer of energy between objects

that are in physical contact.

Transfer of energy between

an object and its environment,

due to fluid motion.

Heat loss by sweating

and breathing.

Being aware of all these processes is essential to assist the cows’ cooling processes.

2. Heat exchange in cows


Heat to the environment via radiation and

convection. Sweat evaporationfrom lungs via evaporation.

Heat transferred to the ground via

conduction and radiation.

Direct radiation from the sun and the sky.

Radiation and conduction from the ground or

other surfaces.

Heat is absorbed…

…when the air temperature is

higher than the cow's temperature.

Heat is lost…

…when the air temperature is lower than the

cow's temperature.

3. Indicators of heat stressTools to assess the risk of heat stress - Breathing rateIt is important to monitor how cows are facing hot temperatures. Counting cow´s breath rate and determine temperature humidity index allow to avoid future problems related with high temperatures.


40-60 breaths/min. Normal rate. 60 breaths/min. About 39 ºC of body temperature.

Take action. <70 breaths/min. Severe heat stress.

Cow´s breathing rates: it is the first sign of heat stress, often before elevated core temperature.

3. Indicators of heat stressTools to assess the risk of heat stress - THIHeat stress is related with temperature and relative humidity, and can be measured with the temperature-humidity index (THI).THI is currently used to estimate cooling requirements of dairy cattle in order to improve the efficiency of management strategies to reduce heat

stress and it is calculated using the following equation:


This formula uses dry bulb temperature (Tdb, ◦F) and the relative humidity (RH). The RH is divided by 100 to express the percentage in decimals.The heat stress evaluation scale for dairy cows was originally first established in the 1960s. But, today there is a reduction on the threshold THI at which losses of production occur.

THI = (Tdb – [0.55 – (0.55 x RH/100)] x (Tdb – 58)

This increased sensitivity to thermal stress is due, in part, to the increased milk potential of dairy herds.

Why?+

3. Indicators of heat stressWhich THI indicates the initial decreases in milk production?A recent study conducted in well-controlled environment, as close as possible

to real-life conditions updated this scale. A level of thermal stress for the dairy cow corresponds to each temperature/humidity couple2.


The heat stress threshold was previously set at a THI value of 72. But this study shows that a THI of 68 is low enough to cause adverse effects on milk production, health and reproduction.

This corresponds to 22°C ambient temperature with 45 % of humidity, a rather common condition in moderate climates.

University of Arizona revised heat stress scale.

Stress Threshold

Mild-Moderate stress

Moderate-Severe stress

Severe stress

4. Signs of heat stressNoticeable signs of an excessive heat loadRecognising the signs of heat stress in cows is essential to implement practical strategies. Cows experience an increase in body temperature in hot weather because they

cannot lose all of the body heat they produce to the environment.


Behavourial signs

Reduced time in lying down Reduced time in ruminating Bunching Slobbering Seeking shade Reduced activity but increased respiratory rate (>80 breaths/minute). Reduced feed intake Agitation and restlessness Open mouth breathing Lack of coordination Trembling

+

-

Heat stress consequences are: Increased body temperature (>102.5°F) Reduced milk yield (>10%) and butterfat level (0.2-0.3%). Reduced reproductive performance.

And remember…

4. Signs of heat stressAnd what about what we cannot see?Heat can have disastrous effects on the productivity, health and welfare of cows.Cattle reduces its feed intake during hot weather, about 8-12 % or more 2.

The decreased forage* intake alters the composition of the rumen, leading to acidosis and a reduced fat content of milk.


Also, under high heat load, about 15 % of this heat is lost directly from the body core via the respiratory tract. The remainder must be transferred to the skin where it is then dissipated evaporatively4.

Typically, early lactation cows are most swiftly and severely affected.

Does heat stress affect more at some stage of life? +

* Forage generates more heat than a grain ration, which contributes to more reduced intake.

FERTILITY

5. Heat stress and reproductionThe impact of heat stress in lactating cows is very important, in part due to the depressed dry matter intake. But heat stress also compromises reproductive performance

and various health outcomes in addition to the lower yields2.In order to implement strategies of management against heat stress we must first understand how and why it may affect dairy cattle:


Decreased growth and development Less feed intake Altered metabolism Decreased milk production Decreased immune function Increased incidence of disease Decreased reproductive performance

The reduced milk yield is a result of increased body temperature induced-decline in feed intake, as well as alterations in endocrine profiles, energy metabolism and other unidentified factors.

5. Heat stress and reproduction


Altered endocrine status. Reduction in rumination and nutrient absorption. Increased maintenance requirements.

Decrease in energy availability for production

Lactating cows enter into negative energy balance

RETICULUM

ESOPHAGUS

ABOMASUM

OMASUMSMALL INTESTINE

LARGE INTESTINE

RUMEN

The biological mechanism by which heat stress impacts production and reproduction is partly explained by reduced feed intake, but also includes other physiological changes (glucose and insulin) which lead to a net decrease in nutrient/energy availability for production.

This decrease in energy results in a reduction in energy balance and explains why cows lose significant amounts of body weight when subjected to heat stress 3.

5. Heat stress and reproduction


The negative effects of heat stress on reproduction parameters are not only immediately obvious, but persist well into the fall months, even after cows have returned to more comfortable environmental conditions4.

The summer-depression in fertility is greater, and lasts for more months in high producing cows compared to less producing. As more efforts are made to increase milk yield, by genetic selection or improving feeding, cows will be more susceptible to heat stress. Nowadays cows are more susceptible to heat stress than they were before which can explain partially that dairy cows have generally a lower reproductive performance today than 20 years ago5.

What heat stress does to cow´s reproduction

Decreased length and intensity of estrus behaviour Decreased conception rates Increased risk of embryo death Reduced calf birth and viability

Perc

ent M

isse

d Es

trus

esSeasonal variation in the estimated proportion of missed estruses in a Jersey herd in North Florida. Data come from Thatcher and Collier (In D.A. Morrow, ed., Current Therapy in Theriogenology 2, Philadelphia, WB Saunders, 1986)

In months associated with heat stress, 75-80% of the estrus behaviour can be missed.

Conception rates of 12% or less are common during the summer in hot areas.


5. Heat stress and reproductionWhy is heat stress related to poor fertilityThere are multiple causes for the poor fertility during heat stress. These are some

of the effects caused by high temperatures6:

Disrupt the functionof the follicle

Heat stress can affect the follicle for the last 26 days of

its growth.

Limiting inseminations to the cooler part of the day is, therefore, not enough, because by the time of insemination, the follicle has already been damaged. Moreover, if conception does occur, the resultant embryo will be susceptible to heat stress for the first two to three days of gestation6.

Direct damage to the oocyte and the embryo

Once the oocyte is released from the follicle at ovulation it can be damaged by heat stress. The

early embryo is sensitive to heat stress until about day 3 of pregnancy when it becomes

resistant to maternal hyperthermia.

Reduced levels of progesterone

Progesterone is responsible for

maintaining pregnancy.


5. Heat stress and reproductionAnd remember that feed intake decreases in heat stressed cattle…… resulting in negative energy balance which triggers:

Impact on fertility

Negative EB

Reduce intake

Hormonal changes

Anovulation Change in plasma concentrations of

Insuline: Required for follicular development.

IGF1 and glucose: Follicular growth and implantation.


6. Managing heat stress Experts recommend to be proactive and start to take measures already in cool

season to assure a good managing of heat stress in the herd.

Provide shade Increase the amount of water available to the herd Modify the diet to mantain feed intake Provide for a good air exchange in the barn

Cool season

Review existing infrastructure, such as shade and sprinklers.

Think about further infrastructure and investments.

Hot season


6. Managing heat stress How to help cows maintain cold and improve fertility? Cooling strategies Nutrition strategies Reproductive management Genetic selection


6. Managing heat stressCooling strategiesMany cooling systems for cows have changed over the

years. They must be implemented depending on the geographic location and the type of housing.

To keep cows cool, shade is the most effective way of reducing heat load because it blocks solar radiation. Using evaporative cooling, by sprinklers and fans, maximize heat loss.


6. Managing heat stressCooling strategiesA recent study7 indicated that cooling of dairy heifers for a short time before and

after artificial insemination, especially with sprinkler and fan, can increasepregnancy rate during heat stress.

When it is not feasible to provide intensive cooling (for example, in grazing dairies) it can also be advantageous to cool cows around ovulation6.


Cooling strategiesCows exhibit individual preferences for cooling strategies. This variability between

cows indicates that the behavioral response to water is an important consideration in the design of sprinkler systems used for summer cooling.

A study in 20118 used cows with free access to an overheadshower activated by floor pressure. The results indicatea great variability between individuals.

6. Managing heat stress

Average of 23 visits to the shower each day. Average of three hours total. The range of time the shower varied from 0 to 8.2 hours!


6. Managing heat stress Is it beneficial cooling dry cows? Despite the lower consume of dry matter of dry cows relative to lactating cows,

heat stress has also significant negative impacts on yield in the subsequent lactation, and those effects persist throughout lactation9.

Improves milk yield in the next lactation, and… Improves their immune status at a time of significant risk for disease

Cooling dry cows…

As early in the dry period as possible to maximize the benefits to the cow.

When should dry cows be cooled?+


6. Managing heat stressNutrition strategies During periods of heat stress, the nutrient requirements of animals are altered.

To maximize feed intake, it is necessary to reformulate rations.

Responsible for 36% of the decrease in milk production when cows are heat-stressed

HEAT STRESS

Decreased DMI Decreased rumination Decreased nutrient absorption

Net decrease in nutrient and energy availability for production

Increase in maintenance requirements


6. Managing heat stressNutrition strategiesAppropriate nutritional management during heat stress might improve the cow´s thermal

balance and reduce body temperature.

Increase the digestibility of fiber.Add fat (high energy content and low heat increment).

To increase the energy and nutrient density, we can:


6. Managing heat stressNutrition strategiesSome additives and minerals have been tested over the years to evaluate their effect on heat

stressed cows10.

Dietary Cr supplementation at level of 6 mg/head/day

Glutamine

Direct-fed microbials

Saturated fatty acids

Vitamin E

Niacin

Yeast cultures


6. Managing heat stressReproductive managementIt is well known how heat stress impacts on dairy cattle fertility. Reproductive programs can be modified through

hormonal manipulations, embryo transfer, etc. to bypass those critical periods. The aim is to improve the low fertility of lactating dairy cows during heat stress.

ATTENTIONThese programs will not overcome the negative impacts of heat stress on oocyte maturation and embryo development.

An injection of GnRH on day five of the estrous cycle may reduce pregnancy loss during heat stress by stimulating ovulation and the formation of a CL.

Embryo transfer

Injection of hormones

Increases pregnancy rate by allowing embryos to bypass the period when they are most sensitive to elevated temperature5.


6. Managing heat stressReproductive management.

The aim is to reduce losses in reproductive efficiency caused by poor detection of estrus. For example, a five-day PRID+eCG-based fixed time AI improves fertility over spontaneous estrus in heat stressed cows12.

Oestrus synchronization protocols

Some breeds are more thermotolerant than others. The use of B. t. indicus sires may result in higher conception rates in lactating Holstein cows during summer heat stress, and can affect embryonic loss, too13.

Influence of sireand breed

Manipulation of embryonic synthesis of heat shock proteins and use of antioxidants to reduce free radical damage associated with heat stress.

Other methods


6. Managing heat stressGenetic selectionThere are many studies that show that genetic selection for thermotolerance is beneficial.The aim is to develop strategies to simultaneously improve heat tolerance and increase

productivity.The heat stress response is under heat shock transcription factor (HSF) regulation. It has been

shown that the central role that HSF1 has in coordinating thermal tolerance suggest that there is opportunity to improve thermal tolerance via gene manipulation14.

But attention! Rectal temperature (RT) during heat stress has moderate heritability, but genetic correlations with economically important traits mean that selection for RT could lead to lower productivity unless methods are used to identify genes affecting RT that do not adversely affect other traits of economic importance16.

A recent study15 showed that selection for heat stress is possible, and it could be particularly effective for environments with a high average THI.


7. SummaryHighlights Heat stress is one of the leading causes of decreased production and

fertility in dairy cattle, and can cost the dairy producer over 400 €/cow. The most severe consequences of heat stress include less feed intake,

altered metabolism, decreased milk production, decreased immune function and decreased reproductive performance.

The temperature–humidity index (THI) is an accurate indicator of heat stress in dairy cows. A THI of 68 (corresponding to a 22 ºC ambient temperature with 45 % of humidity) is low enough to cause adverse effects on milk production, health and reproduction.

A poorer reproductive performance is related toa decreased time of estrous behaviour in cows, decreased conception rates, increased risk of embryo death and reduced calf birth and viability.

Managing heat stress includes cooling, nutrition and reproductive strategiesand genetic selection.


7. SummaryHighlights Cooling dairy heifers before and after artificial insemination can

increase pregnancy rate during heat stress Cooling dry cow improves milk yield in the next lactation, and

improves their immune status. There are several nutritional strategies that can improve the cow´s

thermal balance during heat stress. Modifying reproductive programs might reduce losses in

reproductive efficiency. It is possible to do genetic selection for both production

and heat tolerance.


8. References1. St-Pierre NR, Cobanov B, Schnitkey G. Economic losses from heat stress by US livestock industries.

Journal of Dairy Science (2003); 86:E52-E77.

2. Burgos R. And Collier RJ. Feeding strategies for high-producing dairy cows during periods of elevated heat and humidity. Tri-State Dairy Nutrition Conference, April 19-20, 2011.

3. Dealing with heat stress in Australian dairy herds. http://www.coolcows.com.au/

4. Finch VA. Body temperature in cattle: Its control and relevance to production in the tropics. Journal of Animal Science (1986); 62: 531-542.

5. Managing reproductive performance during times of heat stress (2010). Dairy Cattle Reproduction Council.

6. Hansen PJ (2006). Managing the Heat-Stressed Cow to Improve Reproduction. Proceedings of the 7th Western Dairy Management Conference. .

7. Moghaddam A, Karimi I, Pooyanmehr M. Effects of short-term cooling on pregnancy rate of dairy heifers under summer heat stress. Veterinary Research Communications 2009;33(6):567-75.

8. Legrand, A., K.E. Schütz, and C.B. Tucker. Using water to cool cattle: behavioral and physiological changes associated with voluntary use of cow showers (2011). Journal of Dairy Science. 94:3376-3386.

http://www.coolcows.com.au/


8. References 9. Dahl GE. Impact of Dry Cow Cooling on Subsequent Performance and Health. Proceedings 48th Florida Dairy Production

Conference, Gainesville, March 21, 2012.

10. Rhoads M. Impact of heat stress in dairy cattle. 2nd ReprodAction Cattle Symposium – Nice 2014.

11. Soltan MA. Effect of dietary chromium supplementation on productive and reproductive performance of early lactating dairy cows under heat stress. Journal of Animal Physiology and Animal Nutrition (2010); 94(2):264-72.

12. Garcia-Ispierto I, Roselló MA, De Rensis F, López-Gatius F. A five-day progesterone plus eCG-based fixed-time AI protocol improves fertility over spontaneous estrus in high-producing dairy cows under heat stress. Journal of Reproduction and Development (2013); 59(6):544-548.

13. Pegorer MF, Vasconcelos JL, Trinca LA, Hansen PJ, Barros CM. Influence of sire and sire breed (Gyr versus Holstein) on establishment of pregnancy and embryonic loss in lactating Holstein cows during summer heat stress. Theriogenology (2007); 67(4):692-7.

14. Collier, R.J., J.L. Collier, R.P. Rhoads, and L.H. Baumgard. Invited review: Genes involved in the bovine heat stress response. Journal of Dairy Science (2008);91:445-454.

15. O. Ravagnolo and I. Misztal1 Genetic Component of Heat Stress in Dairy Cattle, Parameter Estimation. Journal of Dairy Science (2000);83:2126–2130.

16. Dikmen S, Cole JB, Null DJ and Hansen PJ. Heritability of rectal temperature and genetic correlations with production and reproduction traits in dairy cattle. Journal of Dairy Science (2012);95:3401–3405.

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heat stress and its impact on fertility. heat stress and its impact on fertility 1. what´s heat...

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