enviado 09-07-2001 22:47
Le envio algunos articulos sobre el efecto del Stress calorico sobre la vaca productora de Leche .
REDUCING HEAT STRESS ON DAIRY COWS
REDUCING HEAT STRESS ON DAIRY COWS
1. Ambient temperature guide
Optimum temperature - 25-65 degrees F
Decreased feed intake - 80 degrees F+
Reduced performance - 90 degrees F+
3-20% or more
As low as 0%
Caution zone - 100 degrees F and 20% humidity
Take steps to ease stress and cool cow
Danger zone - 100 degrees F and 50% humidity
Lethal zone - 100 degrees F and 80% humidity
11. Cattle are more susceptible to heat stress:
A. Sweat only 10% as much as man
B. Need evaporative cooling
1. Body sprinkling
2. Good air movement
C. Feed intake is reduced by 8 to 12% or more
D. Volatile fatty acid production in rumen is decreased
III. Ration adjustments
1. Use high-quality forages to reduce heat produced in digesting
and assimilating feed.
a. Whole-plant corn silage and aftermath cuttings of hay or
haylage are higher in digestibility on many farms than
b. Make certain the neutral detergent fiber (NDF) level is at
least 26-28% in the total ration dry matter (TRDM) to keep
production and fat test reasonably normal. If NDF level
cannot be estimated, make certain that acid detergent
fiber level is at least 18-20% of the total ration dry
2. Imcrease levels of certain minerals to compensate for higher
losses from the body during hot humid weather.
% in TRDMa Milk Responseb
Potassium 1.30 +4.0%
Magnesium .30 +7.0%
Added sodium, example .50 +4.0%
Sodium bicarbonate .70
a Total ration dry matter
b In some research
3. Provide most of the ration during the cooler periods of the day
to minimize heat production when temperatures are higher.
a. Early morning hours -- Ex - 4 to 6 AM
b. Evening hours -- Ex - 9 to 11 PM
c. Keep smaller amounts of feed available during daytime
4. Add extra water to the TMR, silage or haylage if dry matter
intake (DMI) drops seriously. This sometimes will increase
5. Feed ensiled items more frequently to compensate for shorter
bunk life during hot weather, prevent heating and improve feed
a. Remove from silos at feeding time, not ahead
b. Feed more frequently if heating or appreciable drop in
6. Feed a higher fat ration (up to 4 to 5% of the total ration dry
matter from all sources) if a reasonable dry matter intake
(90%+ of usual) cannot be maintained.
Use either added fat or oilseeds such as whole cottonseed
which is also relatively high in fiber content.
7. Keep water as cool as possible and consider chilling it in a
critical situation to about 50 degrees F.
- Chilled water 50 degrees F 56.3
Control 86 degrees F 51.9
8. Consider certain feed additives that have been effective in
some, not all, studies:.
a. Aspergillus oryzae - 4 to 8%
b. Live cell yeast - +4 to 6% milk
c. Niacin - +3 to 6% milk
IV. Other management suggestions
1. Keep cows inside during the day if it is cooler for them.
2. Provide shade, especially over feeding areas and on pasture.
3. Provide extra air movement
a. Fans in stall barns
b. Fans in holding areas - Reduce time in holding areas also
by decreasing size of groups.
c. Spray cows with water in critical situations and keep
them in air stream from fans - +22% milk
d. Run water over shade or roof for evaporative cooling -
4. Imcrease sanitation during hot, humid weather
a. Imcreased risk of mastitis and other infections from
higher numbers of environmental microorganisms
b. Need more bedding, cieaning and disease control
5. Make certain ration is balanced for dry cows and springing
heifers to minimize infectious and metabolic diseases.
STRESS AND REPRODUCTION
VOLUME: NORTHEAST IRM MANUAL
The reproductive efficiency of dairy cows under stress is less than
optimal. Some cows don't show standing estrus (heat). Other cows
have a reduced chance of becoming pregnant if they are bred while
under stress. The exact way stress influences reproduction is not
fully understood. Therefore, precise recommendations are not
available to help dairy producers prevent the effects of stress.
Two types of stressful conditions which affect reproduction are
discussed in this fact sheet. General recommendations to help
alleviate heat or management stress are provided for dairy
The term "heat stress" refers to the stress of hot weather and not
the estrous or heat cycle. Researchers in Florida suggest that the
body of a dairy cow begins to respond to warm temperatures in the
environment at slightly over 70 degrees F. Recent studies indicate
that the reproductive performance of cows does not suffer until
environmental temperatures are over 90 degrees F. Additionally, the
sun's rays can directly have an adverse effect on fertility. Cows
with black markings will absorb more heat from the sun's rays,
further elevating body temperature.
Blood flow may be diverted from internal circulation to peripheral
circulation in an attempt to reduce body temperature. The reduction
in blood flow to internal organs including the uterus, oviducts and
ovaries may reduce available nutrients and increase biochemical
waste products at the tissue level. Certainly some cows do become
pregnant in very hot weather, but overall fertility rates are low.
Hot weather lowers reproductive efficiency in two ways. First, cows
are harder to detect in estrus and are sometimes classified as
anestrus. Second, cows that are bred by artificial insemination
have a hard time becoming or staying pregnant. The second problem
shows up as a low conception rate or high services per conception.
Cows may not be detected in estrus due to failure of ovarian
function and absence of estrus. More commonly, the frequency of
mounting in hot weather is reduced and a producer misses seeing the
few mounts which do occur. Some dairy producers may have both
The underlying cause of improperly functioning ovaries is thought
to be only an indirect effect of high temperatures. Cows eat less
feed during hot weather. Subsequently, there are insufficient
nutrients available after milk production for the ovaries to
"switch on" and start functioning during the first 6 weeks of
Any producer who thinks this might be the problem should look at
the records from the veterinarian's palpation of the cow's ovaries.
Cows that calve immediately before or during hot weather are most
likely to be affected. Check the records for each cow not observed
in estrus. It is important to look at results of the veterinarian,s
30 to 75 day postpartum palpation record for each cow. Research
shows that over 95% of all cows will have either a corpus luteum
(CL or Yellow Body) or a follicle on one of their two ovaries at
this stage of lactation. Any herd with fewer than 75% of cows
cycling by 60 days after calving should have the energy level of
the feed checked. Scientists in Florida recommend increasing the
energy concentration of the cow's feed to account for reduced
intake during hot weather.
Providing shade over the feeding and watering area is another way
to increase feed intake by heatstressed cows. These two changes
have been shown to improve milk production and fertility under
A different way of solving the problem is suggested by researchers
in Israel, where hot weather is a major problem. Israeli dairy
producers spray cows with a fine mist of water while the cows are
in the holding pen before milking. After the spray, fans blow
across the cows and dry them. This cools the cows and improves
their appetites so they eat more.
This approach may not be practical for areas of the U.S. with high
humidity, or where the holding pen is located in the free-stall or
loafing barn. If the bedding in the resting area for cows gets wet,
the bedding can serve as an important source of bacteria to cause
Another approach to spray-cooling is to provide a fine mist of
water over the area where cows stand to eat. Cows are attracted to
the spray so they eat while they are next to the feed bunk. Water
misters are commercially available from agricultural supply
catalogs. This type of sprayer is commonly used in swine production
All dairy producers having problems catching cows in estrus during
hot weather should check their palpation records. If cows seem to
be going through normal estrous cycles, then a producer must
conclude that cows are not mounting often enough to be observed in
estrus. Dairy farmers with this problem can make three changes that
will improve their chances of catching cows in estrus. All
producers should make the first two changes, and any producer who
still has problems should consider the third change as a last
resort. The first change in management takes advantage of a cow's
natural tendency of timing during the day for showing estrus.
Research studies have shown that most of the estrous activity of
dairy cattle occurs just after dawn and just before dusk. If a herd
is milked during dawn and/or dusk, the cows should be watched for
heat for 20 minutes before A.M. and/or after P.M. milking. An
effective trick used by some producers is to close off the exercise
lot until one hour before dusk. The observer then opens the gate or
electric "hot-wire" and lets the cows out. Often times, the first
cows out are in estrus.
Using estrous detection aids is the second change that producers
can make to increase estrus detection rate. Fact Sheet IRM-7
describes the proper use of these aids.
The last resort for dairy producers who fail to observe their cows
in estrus is to utilize prostaglandins for estrous synchronization,
(Fact Sheet IRM-8). Any cow over 120 days since calving and not
observed in estrus should be considered as a candidate for "last
resort" treatment. As always, with estrous synchronization,
pregnant cows and recently bred cows may be aborted. No cow should
be treated without the direct supervision of a veterinarian.
Low Conception Rates
High environmental temperatures may have an adverse effect directly
upon survival of a cow's egg, a bull's sperm, or the developing
embryo while in the cow's reproductive tract. The egg and sperm may
not form a healthy embryo, or a developing embryo may die,
resulting in an early abortion. This problem may result in
conception rates falling below 20% during summer months. Often,
fertility will not return to normal until late October or November,
even though environmental temperatures became unstressful in late
September or early October.
The cause of a low conception rate appears to be an increase in the
body temperature of cows because of hot weather. Practical methods
for lowering the body temperature of a cow include providing shade
or sprays of water as described previously.
Giving cows access to a pond that they may wade in to cool off is
not recommended. The cow's teats are exposed to a variety of
mastitis-causing organisms in the water.
Management or Behavioral Stress
Certain types of management situations are stressful to cattle. The
effects that management stress can have upon reproduction in cattle
have not been researched thoroughly. A cow's body responds to
stressful situations in many ways. The bodily response most
characteristic of stress is increased secretion of hormones called
glucocorticoids from the adrenal gland.
Many management situations or diseases can increase secretion of
glucocorticoids. For example, trucking cattle, competition for free
stalls, isolation from all herdmates, overmilking, surgery, and
severe mastitis have been shown to increase blood levels of
A few research studies have been conducted to look at the effect
that stress has upon reproduction in the open or nonbred cow.
The effect that stress may have on a pregnant cow has not been well
studied. Stress seems to have its biggest influence upon cows just
before they come into estrus. The balance of reproductive hormones
can be altered by stress at this time. Cows or heifers may have
delayed ovulation, and/or they will not come into standing estrus
There are many practical situations where management of cattle may
induce unnecessary stress near the time of ovulation. One example
is chasing or roping cattle before they are inseminated
artificially. Another is use of aggressive dogs to isolate cattle
for breeding. Also, when moving animals through a cattle chute for
estrous synchronization and/or breeding, handle them as gently as
Isolating a cow in a breeding stall away from the rest of the herd
several hours before being bred artificially may be stressful. A
second cow might be brought into the vicinity to keep the cow in
the breeding chute quiet.
While stress may have important effects upon detection of estrus in
some situations, the problem can be avoided. A recent research
study closely examined a herd of dairy cows that were moved into a
new milking and housing facility. Although adrenal stress hormone
levels were elevated and milk production was decreased for a short
time after relocation, there were no problems in detecting cows in
estrus. The important conclusion of this study was that cows can
undergo the stress of transportation, and adjustment to a new
milking parlor, a new freestall barn, new farm workers and new
herdmates without a decline in estrous detection rate. Extra stress
was avoided in this study by not chasing, beating, or roping the
cows, and handling the cows as gently as possible.
In conclusion, the time that management stress may have its biggest
effect on reproduction appears to be immediately before a cow comes
into standing estrus. Any management procedures that can decrease
the amount of stress in cattle at this time should be practiced.
Buffington, D.E., A. Collazo-Arocho, W.W. Thatcher, and T.C.
Skinner. 1978. Structures for modifying the environment;
Air-conditioning and shade. Large Dairy Herd Management, C.J.
Wilcox, et al., eds. University Presses of Florida, Gainesville, p.
Moberg, G.P. 1984. Adrenal-pituitary interactions: Effects on
reproduction. 10th International Congress on Animal Reproduction
and Artificial Insemination; Univ. of Ill., Urbana. June 10-14,
1984. Vol. IV (Invited Papers):1-29 to 1-36.
Thatcher, W.W., H. Roman-Ponce, and D.E. Buffington. 1978.
Environmental effects on animal performance. Large Dairy Herd
Management, C.J.Wilcox, et al., eds., University Presses of
Florida, Gainesville. p. 219-230.
Varner, M.A., B.H. Johnson, J.H. Britt, B.T. McDaniel, and R.D.
Mochrie. 1983. Influence of herd relocation upon production and
endocrine traits of dairy cows. J. Dairy Sci. 66:466-474.
Wiersma, F. 1978. Structures for modifying the environment;
Evaporate cooling and zone cooling for dairy cattle. Large Dairy
Herd Management, C.J. Wilcox, et al., eds., University Presses of
Florida, Gainesville, p. 898-908.
Gonzalo Carmona Solano M.V