Page14 Heat Stress in Dairy Cows - Reproductive Problems and Control Measures Samal, L. Odisha University of Agriculture & Technology, Bhubaneswar -India Corresponding Author: lipismitasamal@gmail.com Rec. Date: May 17, 2013 01:45; Accept Date: Sep 20, 2013 20:54 Abstract All animals have a range of ambient environmental temperatures termed as thermo-neutral zone. This is the range of temperatures that are conducive to health and performance. The upper critical temperature is the point at which heat stress begins to affect the animal adversely. There are a number of environmental factors like high temperature, high humidity and radiant energy (sunlight) that contribute to heat stress. Heat stress has long been recognized as reducing both the productivity and reproductive efficiency of dairy cattle. Although heat stress has a direct effect on reproduction, it depends on the magnitude and duration of thermal stress, milk yield, lactation status, breed, composition of diet, dry mater intake and physical activity of animals. Further studies are needed to better understand the associations between climatic conditions and reproductive physiology and to evaluate the efficacy of various nutritional, environmental and reproductive strategies in a particular region for combating heat stress. Keywords: Heat stress, reproductive problems, dairy cows Introduction Heat stress can be defined as the point where the animal cannot dissipate adequate quantity of heat to maintain body thermal balance. Climatic factors that may influence the degree of heat stress include: temperature, humidity, radiation and wind. Figure 1 illustrates the challenges that heat stress poses for dairy cows. The environmental conditions that induce heat stress can be calculated using the temperature humidity index (THI). THI = (Dry bulb temperature 0 C) + (0.36 * dew point temperature 0 C) + 41.2 When the THI is >72 F (22.2 C), heat stress begins to occur in dairy cattle. Table 1 contains some of the signs that cows exhibit as the THI increases.
Page15 Table 1 - Effect of heat stress on dairy cattle Fig 1 - Depiction of heat stress in dairy cows THI Stress Effects level <72 None 72-79 Mild Dairy cows will adjust by seeking shade, increasing respiration rate and dilation of the blood vessels. The effect on milk production will be minimal. 80-89 Moderate Both saliva production and respiration rate will increase. Feed intake may be depressed and water consumption will increase. There will be an increase in body temperature. Milk production and reproduction will be decreased. 90-98 Severe Cows will become very much uncomfortable due to high body temperature, rapid respiration (panting) and excessive saliva production. Milk production and reproduction will be markedly decreased. >98 Danger Potential cow deaths can occur Effect of Heat Stress on Female Reproductive Functions The negative effects of heat stress on dairy cows are multifaceted. Summer heat stress has long been recognized as reducing the reproductive efficiency of dairy cattle. Here are a few ways by which reproductive function is impaired by summer heat. Oestrus expression: Cows in heat stress conditions show fewer, less intense heats than in cooler temperatures. Studies have shown that undetected oestrous events were between 76 and 82% from June to September compared to 44 to 65% from October to May. Heat stress also decreases
Page16 the length and intensity of oestrus. Heat stress decreased follicular estradiol which might decrease the oestrus intensity. Another reason of reduced oestrus expression might be the physical inactivity caused by heat stress. Cows are less active and therefore less likely to ride other cows during oestrus. So, in summer, dairy cows had approximately one-half the number of mounts per oestrus compared to dairy cows in winter. Oestrus activity is also lowered due to the cows' reduced motor activity, a means of trying to decrease her endogenous heat output. Endocrine status: Females raised under high temperatures have low estradiol. This decrease in estradiol synthesis could influence expression of oestrus, ovulation and corpus luteum. Thermal stress also alters goandotrophin, inhibin and PGF 2 secretion. The length of the luteal phase in heat-stressed cows is longer than in females kept in thermo-neutral environment. It seems that the uterus secretes less PGF 2 because of the reduction in estradiol synthesis and/or because high temperatures can interfere with the release of PGF 2 by endometrial cells. It s well known that the uterine endometrium must be primed by estradiol to produce enough prostaglandin and trigger luteolysis. Thermal stress also alters the concentrations of FSH and inhibin and corpus luteum function, as well as decreases the fluid content of follicles. High temperatures also reduce the number of granulosa cells, aromatase activity and secretion of androstenedione by theca cells (Wolfenson et al 1993). Follicular selection and development: The first reproductive challenge facing the heat stressed cow is altered follicular development. Heat stressed cows decrease feed intake causing less frequent pulses of the luteinizing hormone (LH) resulting in longer follicular waves. This lengthening of the follicular wave leads to the selection and ovulation of multiple, smaller dominant follicles (Sartori 2002). Follicles are responsible for producing estrogen, a hormone that causes cows to show signs of heat. Smaller follicles will produce less estrogen than larger ones; therefore, resulting in less oestrus activity. Ovarian follicles contain oocytes as well as somatic cells that synthesize estradiol. Estradiol has a variety of actions that include causing oestrus and the LH surge. Heat stress impairs follicle selection and increases the length of follicular waves, which reduces the quality of oocytes. It also allows for more than one dominant follicle to develop, explaining the increased twinning seen from cows conceiving in summer time heat. The somatic cells within the follicles (theca and granulosa cells) are also damaged by heat stress.
Page17 Corpus luteum: In addition to influencing the ovarian follicles, heat stress can affect the corpus luteum. Progesterone from the corpus luteum is required for pregnancy and there is an association between low progesterone and infertility. Estradiol from the follicle initiates luteolysis in cattle. The cells of the corpus luteum differentiate from the cells of the follicle. Therefore, if heat stress decreases blood progesterone then the decrease could arise from the effects of heat stress on the follicle which ultimately carries over to the corpus luteum. Alternatively, changes in metabolic rate associated with heat stress may alter the metabolism of progesterone. Embryo development: Embryo quality and growth is often reduced during heat stress. Thermal stress also alters the ability of embryos to develop into blastocysts. It causes early embryonic development, increased risk of early embryonic deaths and decreased foetal growth. There are effects of heat stress on the ovary and these effects may influence the ability of cows to become pregnant. The period of greatest susceptibility is immediately after the onset of oestrus and early during the post-breeding period. Putney et al (1989) demonstrated that embryonic development was impaired in heifers subjected to heat stress for 10 hours after the onset of oestrus. This is an interesting period of development because it represents a time after the LH surge but before ovulation. The period of embryonic sensitivity to heat stress begins early during the development of the follicle and continues until about 1 day after breeding. The high uterine temperature of the heat stressed cow can impair embryonic development, resulting in poor embryo implantation and increased embryo mortality. Dry matter intake: One of the first reactions cows show to heat stress is less feed intake, supplying less energy for use which may interfere with their reproductive performance. The reduction in feed intake depends on several factors, including the proportion of concentrate and forage of the diet and milk yield. Since cows have been bred to produce high volumes of milk, cows use available energy for daily maintenance and milk production first, with fewer nutrients available for reproductive health. Carryover effects: Even after the heat of summer is over, the carryover effects of heat stress can cause fertility to be delayed. This can reduce conception rates for a longer period of time and result in poor heat detection, more services per pregnancy and longer days open. For all these
Page18 reasons, pregnancy losses increase dramatically as the temperature-humidity index (THI) rises. So, there is decrease in conception and pregnancy rates in hot seasons. Nutritional Strategies to Beat the Heat Stress Feeding of high quality feedstuffs/rations: Early lactation cows exposed to heat stress may go even further into negative energy balance because they aren t consuming as much feed as needed. Consequently, they are more likely to have lower reproductive performance due to altered follicle development and lower oestrus activity. Feeding high quality forages and balanced rations will decrease some of the effects of heat stress. Potassium levels should also be increased in the diet as it is the primary sweat gland regulator in cattle. Feeding of bypass fat: Cows in heat stress conditions are prone to rumen acidosis, so fibre quality should be enhanced to maximize rumen buffering and saliva production. Feeding a highquality bypass fat provides an energy-dense diet at a time when cows are consuming little feed. The use of fat in diets could also lower the heat load because of high energy density and lower metabolic heat when compared with other ingredients such as fibre and carbohydrate. Inclusion of fat may therefore increase milk yield but it depends on the environment where cows are raised. Moreover, fats have significant effects on concentrations of cholesterol, progesterone, rate of synthesis and metabolism of prostaglandin F 2 (PGF 2 ), follicle growth and pregnancy rates in dairy herds (Oldick et al 1997; Staples et al 1998). Environmental Strategies to Beat the Heat Stress To reduce the negative reproductive effects of heat stress, following recommendations should be followed. Cooling: Infertility during heat stress is primarily caused by elevated body temperature. Cooling dairy cows during heat stress should, therefore, improve conception rates. A variety of cooling systems are available for heat-stressed dairy cows. Perhaps the most widely used system is a combination of water sprinklers, shades, soakers and fans. Sprinkling cows with water and subsequently blowing air over the cow with a fan causes evaporative cooling. The evaporative cooling decreases body temperature.
Page19 Putney et al (1989) have shown that d-17 conceptuses incubated at high temperatures synthesize heat shock proteins, which may enhance their resistance to heat stress whereas in vitro endometrial tissue subjected to elevated temperatures increased release of prostaglandins into culture medium. If such an effect occurred in vivo, it might initiate premature luteal regression or compromise the function of the CL. In addition, hyperthermia on d-17 of pregnancy increased uterine production of prostaglandin F 2α in response to oxytocin (Wolfenson et al 1993). Thus, it appears that cooling is needed from at least 42 d before ovulation to over 40 d post-insemination. Reproductive Tools to Beat the Heat Stress While the changes to environment and rations will help alleviate the negative effects of heat stress on reproduction, other changes to reproductive protocols can help many folds. Improving rates of oestrus detection: Some of the effects of heat stress are caused by reduced intensity of oestrus expression. Therefore, it may be possible to improve reproduction in dairy herds by improving oestrus detection methods. Some of the oestrus detection aids like pedometers, tail chalk and pressure activated patches or electronic devices placed on the tail head can improve reproductive performance of dairy cows. Timed artificial insemination (Timed AI): If oestrus detection is a problem in heat stressed dairy cows then it may be possible to improve reproduction by using timed AI. A timed-a.i. (TAI) protocol can help improve fertility during summer months. TAI is done by administering a series of gonadotropin releasing hormone (GnRH) and PGF 2 injections. Insemination is performed at a predetermined time following the last GnRH injection. Timed AI shortened the interval to first service and increased pregnancy rates in heat stressed cows when compared to insemination at observed oestrus. The beneficial effects of the first service TAI during summer were maintained over the course of a year with fewer cows being culled (12.9 vs. 22.0%) and more cows eventually conceiving (87.0 vs. 77.9%) if TAI was used for first service compared with cows first inseminated at detected oestrus. Thus, on-farm implementation of TAI programs help to inseminate cows independent of expressed oestrus.
Page20 Embryo transfer to enhance summer fertility: Embryos are sensitive to the effects of heat stress. However, greatest sensitivity occurs during early embryonic development. During later embryonic development (morula or blastocyst stage), embryos develop some tolerance for heat stress. It should be possible, therefore to improve pregnancy rates in heat-stressed cattle by using embryo transfer of frozen embryos collected from cows that are not heat stressed. Embryo transfer nearly doubled the conception rates when compared to dairy cows inseminated artificially. So, it is possible to by-pass early embryonic stages and improves conception rates during heat stress. Although embryo transfer of good quality embryos appears to provide a methodology to enhance summer-time fertility, it is not without problems. During periods of heat stress, the number of embryos produced following superovulation may be reduced as a result of fewer oocytes released in response to superovulatory drug therapies, lower fertilization rates, or reduced embryo quality. These adverse effects appear more pronounced in dairy cattle than beef cattle, particularly when maximum air temperature exceeds 32 C (Hansen et al 2001). To mitigate the negative effects of heat stress on superovulation, the strategies are namely: reduce heat stress with cooling, alter the genetic make-up of the cattle involved, use lower producing or nonlactating donors, and enhance detection of oestrus by using synchronization schemes or other detection aids (Hansen et al 2001). The factors which have prevented widespread commercial adoption of embryo transfer (Rutledge 2001) to bypass effects of heat stress are: 1) using dairy heifers as donors may delay their first parturition, and hence productivity; 2) the negative relationship between embryo quality and ambient temperature means the fewest good to excellent quality embryos are available when the most are needed; 3) embryo recovery is highly technical and expensive; 4) for embryos that have been frozen, so far only those produced in vivo produced frozen embryos have increased the percentage of animals pregnant compared with AI; and 5) the cost of in vitro-produced embryos is reduced compared to in vivo production of embryos; however, in vitro-produced embryos currently only enhance results when transferred fresh and not frozen. Thus, the potential for commercial adoption of embryo transfer to increase reproductive success depends on enhancing the outcome while minimizing cost. By inducing heat shock proteins, the success rate of embryo transfer under heat stress can be enhanced.
Page21 Inducing accessory corpus luteum: Wolfensen et al (2000) suggested that chronic heat stress reduces progesterone concentrations, although progesterone concentrations may be elevated after acute heat stress. Injection of a GnRH agonist or hcg on d 5 of the estrous cycle results in ovulation of the first wave dominant follicle and formation of an accessory corpus luteum (CL), with subsequent elevation of plasma progesterone levels. However, lactating dairy cows receiving injections of hcg on d 5 or 6 after insemination did not improve conception rate during heat stress (Schmitt et al 1996). The mechanism by which hcg might enhance conception rate is by minimizing estrogen during pregnancy recognition. When treatment occurred on d 5 after oestrus, hcg induced progesterone concentration as well as three-wave follicular cycles. However, there are two drawbacks of commercial application of hcg on d 5 post-insemination. First, hcg is a potential immunogen; therefore, repeated use should be avoided. The second concern is the additional handling of animals. To circumvent both problems, one potential method is incorporation of Deslorelin implants (long-acting GnRH agonist) into the TAI protocol. Crossbreeding: To introduce greater heat tolerance into the population of dairy cattle, crossbreeding is done. Heterosis for production traits ranges from 0 to 10%, while fertility ranges from 5 to 25%; so, there may be opportunities to improve fertility and production simultaneously by crossbreeding. The crossbred cattle had a slightly higher rate of survival than the purebred Holsteins, while the Guernseys had the lowest survival rate. Crossbreds had superior reproduction with some indication of even more favourable heterosis in the warm season, especially for Holstein crosses. However, Swan and Kinghorn (1992) attributed the lack of widespread crossbreeding in dairy cattle in temperate climates to the notable merit of purebred Holstein strains for milk production. In a study assessing heat tolerance of temperate Bos taurus, tropical Bos taurus, and Bos indicus beef cattle, Hammond et al (1996) reported that rectal temperatures in Senepol cattle were less than those in Brahman cattle, but the converse was true of respiration rates. Angus heifers on the other hand had the highest respiration rates and temperatures. Because the Hereford Senepol crosses had rectal temperatures similar to the purebred Senepol, there appears to be a high degree of dominance associated with the genes responsible for controlling rectal temperature in this tropical Bos taurus breed. Preliminary results provided evidence of a gene influencing hair length
Page22 and heat tolerance in the Senepol breed. If this gene can be identified as the bovine genome is mapped, perhaps it could be introduced into dairy breeds. Alternatively, a crossbreeding program starting with Senepol, and selecting for high milk production and low rectal temperature during heat stress, might develop a dairy cow with greater heat tolerance without compromising productivity. Conclusion Heat stress reduces reproductive efficiency of dairy cattle through a variety of different mechanisms. However, it depends on mainly milk production, housing conditions, nutrition, disease control, inbreeding and heat stress tolerance. The decrease in fertility is caused by elevated body temperature that influences ovarian function, oestrous expression, oocyte health and embryonic development. Also, the use of milk production systems based on grazing reduces the costs per kg of milk but also expose the animals to greater heat load. In response to these limitations, environmental and reproductive management of cows should be increased during heat stress. Methods that show promise in enhancing conception rate in summer are transfer of embryos collected from superovulated donors and induction of accessory CL. Research is needed to evaluate the potential of crossing traditional and non-traditional dairy breeds to enhance reproductive capability, while maintaining productivity at acceptable levels. References 1. Hammond AC, Olson TA, Chase Jr CC, Bowers E J, Randel RD, Murphy CN, Vogt DW and Tewolde A. 1996. Heat tolerance in two tropically adopted Bos taurus breeds, Senepol and Romosinvano, compared with Brahman, Angus, and Hereford cattle in Florida. Journal of Animal Science. 74: 295 303. 2. Hansen PJ, Drost M, Rivera RM, Paula Lopes FF, Al- Katanani YM, Krininger CE and Chase Jr CC. 2001. Adverse impact of heat stress on embryo production: Causes and strategies for mitigation. Theriogenology. 55: 91 103. 3. Oldick BS, Staples CR, Thatcher WW and Gyawu P. 1997. Abomasal infusion of glucose and fat effect on digestion, production, and ovarian and uterine function of cows. Journal of Dairy Science. 80: 1315-1328. 4. Putney DJ, Mullins S, Thatcher WW, Drost M and Gross TS. 1989. Embryonic development in superovulated dairy cattle exposed to elevated ambient temperatures between the onset of oestrus and insemination. Animal Reproduction Science. 19: 37-51. 5. Rutledge JJ. 2001. Use of embryo transfer and IVF to bypass effects of heat stress. Theriogenology. 55: 106 111.
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