Solutions to Infertility Caused by Heat Stress


Depressed reproductive function during heat stress is one of the most difficult problems to solve on the dairy. Cows exposed to heat stress suffer from two problems that together make it very difficult to breed back cows quickly after calving. First, expression of estrus is reduced. In one study in Florida, 75% to 80% of the expected heats in the summer were not detected (Thatcher and Collier, 1986). Second, the proportion of cows inseminated that conceive and maintain pregnancy to term is reduced in summer (see Table 1).

Taken together, the herd pregnancy rate (the proportion of cows eligible to become pregnant in a 21-day period that actually become pregnant) takes a big hit in the summer.

Fortunately, strategies can be adopted to reduce the impact of heat stress on fertility. Some of these, like cooling cows during the summer, also increase milk yield, whereas others, like timed artificial insemination (AI) and embryo transfer, bypass the changes in the cow’s physiology during heat stress that cause infertility.

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Cooling Cows

Modification of housing or pastures to cool cows during heat stress is an important practice for boosting reproductive performance and milk yield. However, provision of cow cooling has a bigger positive impact on milk yield than it does on fertility and does not eliminate summer infertility.  

Data in Table 1 demonstrate the advantages and limitations of cooling cows for improving fertility. A total of 22 herds from Israel were categorized based on milk yield in the previous year’s winter and the intensity of cooling. Herds with intensive cooling were managed so that cows were exposed to 10 cooling sessions per day. In each session, cows were extensively wetted and then exposed to forced ventilation. Cows in the moderate cooling herds were cooled by wetting and forced ventilation only while in the holding pen before each milking (3x). As can be seen by examination of the table, herds where cows were cooled intensively had higher fertility in the summer than herds in which intensive cooling was not practiced. Nonetheless, fertility declined during the summer in all herd types.

In Israel, the Ministry of Agriculture calculates a term called the summer:winter ratio to assess the magnitude of heat stress effects on production and reproduction. As shown in Table 1, the summer:winter ratio varied between 0.49 and 0.62 for intensively cooled cows and 0.08 and 0.31 for moderately cooled cows. While not shown in the table, the summer:winter ratio of milk yield ranged from 0.96 to 1.03 for intensively cooled cows versus 0.84 to 0.90 for moderately cooled cows.  


Table 1. Summer:winter ratios for conception rate in Israeli herds as affected by production level and intensity of cooling.a

Conception rate (%)

High production

Low production

Intensive  cooling

Moderate cooling

Intensive  cooling

Moderate cooling











Summer:winter ratio





a Flamenbaum and Galon (2010).


It is important to assess how well cooling systems are working. This is particularly true for reproduction. Heat stress can compromise fertility by affecting the follicle or oocyte for at least 26 days before insemination and through the first two to three days of embryonic life. An increase in body temperature in that wide window of time can potentially compromise pregnancy success. Moreover, many of the effects of heat stress on fertility are caused by the high body temperatures that heat-stressed cows experience. Cows that experience high body temperatures for much of the day are not likely to get pregnant.

One way to assess effectiveness of cooling systems is to calculate the summer:winter ratio as shown in Table 1. Another is to actually measure the body temperature of cows. A general rule for eliminating heat stress is to ensure that rectal temperatures remain below 102.2°F since cows with body temperatures higher than that value experience reduced fertility and milk yield.

Rectal temperature should be measured in the afternoon (3:00 to 5:00 p.m.) when cows are most likely to be experiencing elevated body temperature. The thermometer should be placed in the rectum for a full minute to give the thermometer time to stabilize. While waiting for temperature measurements to stabilize, it is easy to also measure respiration rate by counting the number of flank movements for 30 seconds and multiplying by 2. A respiration rate of 60 breaths per minute or more indicates that a cow is experiencing heat stress.

A more comprehensive assessment of body temperature throughout the day can be obtained by the use of small computerized dataloggers attached to a blank CIDR device (see Figure 1). These can be placed in the vagina for up to a week and vaginal temperatures recorded at intervals as short as 1 minute. Examining the daily pattern in vaginal temperature can provide clues as to when additional cooling is needed. Keep in mind that vaginal temperature is about 0.2 to 0.4°F higher than rectal temperature.

Figure 1. Example of a computerized datalogger for measuring vaginal temperature. The example shown is a Thermocron® iButton (Maxim, Sunnyvale, CA) that costs about $25 each. Note that a groove has been cut into the center of the CIDR. The iButton is placed sideways into the groove and fixed into place with silicone sealant. After removal from the cow, data can be downloaded and the iButton reused.

Timed Artificial Insemination

Development of timed AI protocols such as the OvSynch procedure make it possible to avoid the need for estrus detection and to inseminate cows at a fixed time. Use of OvSynch in heat-stressed cows has been shown to increase the rate at which cows get pregnant after calving. In a Florida study, the percentage of cows that were pregnant by 90 days postpartum was 16.6% for cows in which first insemination was through timed AI using OvSynch versus 9.8% for cows bred based on visual estrus detection only (Table 2). This improvement was not due to improved fertility but rather to improved submission rate. Note that the pregnancy rate per AI at first service was very low and similar between groups. However, cows were inseminated earlier postpartum in the timed AI group.    


Table 2. Effectiveness of incorporation of an Ovsynch protocol for first service on reproductive function of cows during heat stress in Florida.ab




Number of cows


Calving to first AI (days)

Pregnancy rate (%)

First service

90 days postpartum

120 days postpartum

Bred at estrus












a Aréchiga et al. (1998).
b Voluntary waiting period = 70 days.
* P<0.05.    ***P<0.001.

Timed artificial insemination protocols have been improved recently so they not only increase the number of cows submitted to AI but also increase conception rate. First-service pregnancy rates over 40% can be realized under the best timed AI protocols in well-managed herds. One protocol for timed artificial insemination that allows such high fertility is shown in Table 3. The protocol includes a resynchronization step for cows that were diagnosed open at Day 32 after insemination. 


Table 3. Sample timed artificial insemination protocol for first-service cows (5-day Cosynch 72).a

Days in milk (+ 3 days)



Prostaglandin (PG) F2a, 25 mg


PGF2a, 25 mg


Gonadotropin releasing hormone (GnRH), 100 mg


PGF2a, 25 mg


PGF2a, 25 mg


GnRH, 100 mg; insemination


Pregnancy diagnosis by ultrasound b

——————–Nonpregnant cows only——————–


GnRH, 100 mg, insert CIDR


PGF2a, 25 mg, remove CIDR


PGF2a, 25 mg


GnRH, 100 mg; insemination c

a From Bisinotto et al. (2010).
b In a group of 593 cows, pregnancy rate to first service was 45.5% at day 32 after insemination and 38.6% at Day 60 after insemination.
c In an experiment using Ovsynch 56 for first service and the resynchronization protocol shown here, pregnancy rate was 51.3% at day 32 after insemination and 45.5% at Day 60 after insemination (n = 334).


It has not yet been tested whether newer timed AI protocols will improve fertility as compared to Ovsynch during summer heat stress although one would expect a modest increase in pregnancy rate. Unfortunately, damage to the oocyte and embryo that occurs during heat stress cannot be rectified by timed AI protocols. Therefore, the major advantage of using timed AI during the summer is to increase submission rate rather than fertility.  

Natural Breeding

There seems to be a small improvement in pregnancy rate when bulls are used during the summer, although the difference is not enough to warrant the increased costs of natural mating. De Vries et al. (2005) compared herd pregnancy rates for herds in Florida and Georgia that utilized bulls predominantly (> 90% of breedings naturally), in part (11% to 89% of breedings naturally), or seldom (<10% of breedings naturally). In the winter, pregnancy rates were similar for all three types of herds (pregnancy rates = 17.9%, 17.8%, and 18.0% for AI, mixed, and natural service herds). In the summer, there was a slight increase in pregnancy rate for herds using bulls (pregnancy rates = 8.1%, 9.1% and 9.3% for AI, mixed, and natural service herds). Bulls can be affected by heat stress just as cows can, so some of the advantage in using bulls in terms of estrus detection may be offset by reduced bull fertility.

Embryo Transfer

A major reason why fertility is low in the summer is damage to the growing follicle, oocyte, and embryo caused by exposure to maternal hyperthermia. This damage cannot be reversed by timed AI or by the use of natural mating. Similarly, no hormone treatment has been found that can increase fertility during heat stress (see Hansen, 2008). Fortunately, effects of heat stress on the oocyte, follicle, and embryo can be bypassed by embryo transfer (ET). This is possible for two reasons. First, the embryo that is transferred into a recipient has, for one reason or another, escaped effects of heat stress. Second, by the time embryos reach the stage of development where they are ready to be transferred into a recipient (the morula or blastocyst stage, typically Day 7 after AI), the embryo has acquired resistance to heat stress. Thus, it is much less likely that maternal hyperthermia will kill an embryo after Day 7 of pregnancy than an embryo in the first day or two of life.

The effectiveness of ET for increasing fertility during the summer is shown in Figure 2. Shown are data from a commercial dairy in Brazil in which cows were either inseminated or received a fresh or frozen-thawed embryo produced by superovulation. Note that in the cool months (June to November), pregnancy rates were similar for AI and ET cows. In the warm months (December to May), however, pregnancy rates were higher for cows receiving an embryo. Moreover, the decline in fertility from winter to summer seen for inseminated cows did not occur for ET cows.

Figure 2. Seasonal variation in fertility in Brazil for cows that were either inseminated (AI) or received a fresh or frozen-thawed embryo produced by superovulation (ET). Data are from Rodriques et al. (2004), and the figure is modified from Hansen (2007). Months in which the average dry bulb temperature was less than 72.5°F (22.5°C) are highlighted with the gray bar. The asterisks represent months in which pregnancy rate was higher for embryo transfer.  

It is possible to utilize hormonal protocols identical to those used for timed AI to perform timed embryo transfer and thereby also bypass the effects of heat stress on estrus detection. Nonetheless, embryo transfer can be an expensive proposition, and the economical benefits of ET depend upon keeping the cost of embryo production low.  

The best way to minimize cost is to produce embryos using in vitro fertilization with oocytes recovered from the slaughterhouse. Embryos of high genetic merit can be produced by using elite bulls because one straw of semen can produce dozens of embryos. Use of sexed semen results in additional value to the in vitro-produced embryo.  

Effectiveness of transfer of in vitro-produced embryos for improving fertility during heat stress is illustrated in data presented in Table 4. Note that cows receiving a fresh embryo had higher calving rates than cows that were artificially inseminated. Transfer of vitrified embryo resulted in suboptimal results, however. Vitrification is one of the processes used to cryopreserve embryos in liquid nitrogen. There was no significant difference in calving rates between cows receiving a vitrified embryo and cows bred by AI. In fact, in vitro-produced embryos do not survive freezing well with any of the current freezing processes. As a result, it is recommended that only fresh embryos be used for increasing fertility when in vitro-produced embryos are used. This recommendation creates a logistical problem because embryo production must be synchronized with preparation of the recipient. 

Table 4. Effectiveness of embryo transfer for improving calving rate in lactating cows bred during the summer in


Calving rate (%), all calves

Calving rate (%), live calves

All cows (n=550)

Synch cows (n=466)

All cows (n=550)

Synch cows (n=466)
















a Stewart et al. (2011).
b Cows assigned to ET only received an embryo if a corpus luteum was present on Day 7 after ovulation (i.e., the day of transfer). Data are expressed as the percent of cows assigned to treatment that had a calf (all cows) and as the percent of cows that had a corpus luteum on Day 7 after ovulation (synch cows).
c Embryos were produced by sexed semen. The percent of calves that were heifers was 50% for AI, 80% for cows receiving vitrified embryos, and 88% for cows receiving fresh embryos.
* Different from AI, P<0.05.


Another limitation of embryo transfer is that the supply of embryos is currently low. In the Southeast, at the time of this writing, one company produces and transfers embryos using slaughterhouse oocytes and in vitro fertilization. Ovatech LLC based in Gainesville, Florida ( produces embryos with conventional semen for $35 each and with sexed semen for $60 each (including costs of transfer).

Embryo transfer during the summer can be profitable. De Vries and colleagues are assessing economic return from use of embryo transfer during the summer in lactating cows (unpublished). Net return for transfer of an embryo produced using sexed semen in the summer only was calculated per milking cows in the herd (not per cows receiving an embryo). When the cost of embryo transfer per cow is $60, the estimated return is $11 when the fertility for ET is 150% of AI and $42 when the fertility for ET is 200% of AI. At a cost of $90, the return is -$10 when the fertility is 150% of AI and $21 when the fertility is 200% of AI. These returns do not include the increase in net merit that can be expected when using bulls with high estimated breeding values for this trait. 

Prospects for Feeding Antioxidants

One of the consequences of heat stress is that cells produce more oxygen free radicals that can damage cells. Accordingly, various experiments have been performed to evaluate whether administration of antioxidants can improve fertility in the summer. In general, provision of antioxidants has not been effective. However, feeding supplemental β-carotene for at least 90 days beginning at ~15 days after calving in the summer and fall increased the proportion of cows that were pregnant at 120 days postpartum (35% versus 21%) (Aréchiga et al., 1998). Work is currently under way at the University of Florida to further evaluate the feasibility of providing antioxidants to counteract the impact of increased production of free radicals in the summer.

Take Home Messages

  1. Keep cows as cool as possible by providing them with access to shade, forced air ventilation, and some form of evaporative cooling such as sprinklers or misters.
  2. Measure body temperatures in subsets of cows in each animal housing area to see how well you are doing at keeping cows cool and work to improve the situation when body temperatures are often above 102.2°F.
  3. Calculate the summer:winter ratio in pregnancy rate and milk yield to assess the overall effectiveness of cooling systems.
  4. Evaluate the use of a timed AI program in the summer to improve the number of cows submitted for insemination. Timed AI programs in the summer are likely to be profitable when the estrus detection rate is very low.
  5. Consider the use of embryo transfer as a reproductive management tool for getting cows pregnant during heat stress.

Author Information

Peter J. Hansen
Department of Animal Sciences, University of Florida


The author’s research reported here was supported in part by the funds from the Southeast Milk Inc. Milk Checkoff Program and Agriculture and Food Research Initiative Competitive Grant No. 2010-85122-20623 from the USDA National Institute of Food and Agriculture.


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Bisinotto, R.S., E.S. Ribeiro, L.T. Martins, R.S. Marsola, L.F. Greco, M.G. Favoreto, C.A. Risco, W.W. Thatcher, and J.E. Santos. 2010. Effect of interval between induction of ovulation and artificial insemination (AI) and supplemental progesterone for resynchronization on fertility of dairy cows subjected to a 5-d timed AI program. J. Dairy Sci. 93:5798-5808.

De Vries, A., C. Steenholdt, and C.A. Risco. 2005. Pregnancy rates and milk production in natural service and artificially inseminated dairy herds in Florida and Georgia. J. Dairy Sci. 88:948-956.

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Stewart, B.M., J. Block, P. Morelli, A.E. Navarette, M. Amstalden, L. Bonilla, and P.J. Hansen. 2011. Efficacy of embryo transfer in lactating dairy cows during summer using fresh or vitrified embryos produced in vitro with sex-sorted semen. J. Dairy Sci. 94: 3437-3445.

Thatcher, W.W., and R.J. Collier. 1986. Effects of climate on bovine reproduction. In: Current Therapy in Theriogenology 2. D. A. Morrow, ed., pp. 301-309. W.B. Saunders, Philadelphia.