Thermoregulatory Control in Cattle Exposed to the Natural Climate
Abstract
In the diurnally cyclic natural climate thermoregulative changes in vasomotor functions were related to a cycle in skin and tympanic membrane temperature. In summer vasomotor responses were correlated with tympanic membrane temperature whereas in winter responses were correlated with skin temperature. No seasonal change was found in the tympanic membrane temperature.
- In the Northern Hemisphere, from June to September and in the Southern Hemisphere from December to March, there are periods of reduced fertility (sub-fertility) in dairy cows that are described as summer infertility. Several factors contribute to sub-fertility during this time, such as ambient temperature, humidity and photoperiod. During the warm season there is a reduction in feed intake that may compromise the energy balance of the cow and/or induce an imbalance in the activity of the hypothalamo-hypophyseal-ovarian axis. These factors reduce the reproductive performance of the cow and compromise the quality of oocytes, embryos and corpora lutea. This paper reviews current knowledge on the metabolic and endocrine mechanisms that induce summer infertility and describe their effects on follicle, oocyte and embryo development in dairy cows.
- The objective of this review is to provide the reader with an overview of thermoregulatory mechanisms and the influence of climatic conditions in different housing systems on the development, performance, and health of calves. Thermic stress is observed in association with extreme temperatures and large temperature variations, but other variables such as relative humidity and wind speed can also contribute to thermic stress. Thermoregulation in calves is similar to that in adult cattle, but especially dystocial calves are more prone to heat loss. Heat or cold stress results in direct economic losses because of increased calf mortality and morbidity, as well as indirect costs caused by reduced weight gain, performance, and long-term survival. The climatic conditions in a variety of housing systems, associated health problems, and strategies to mitigate thermic stress are discussed in this review. The goal of housing is to alleviate the effect of climate on calves and provide a microclimate. Adequate ventilation with fresh air is essential to reduce respiratory disease. Common practices such as raising calves in individual outdoor enclosures have been challenged lately. Recent research seeks to evaluate the suitability of group housing under practical, economic, and animal welfare considerations. Limited results for reducing thermic stress can be achieved by simple measures such as shades or shelter, but additional heat or cold stress relieving strategies can be required depending on the housing system.
- Effects of season and incubation temperature on progesterone and prostaglandin F2alpha (PGF2alpha) production by bovine luteal cells were examined. Estrous cycles of Israeli-Holstein cows were Synchronized and ovaries collected following slaughter on Day 11 of the estrous cycle in winter and summer, and on Day 16 in winter. After enzymatic dispersion, cell count and viability determination. 300 000 luteal cells were incubated at 38 or 40-degrees-C for 2 h followed by 16 h at 38-degrees-C. Effects of bovine luteinizing hormone (LH) and forskolin, with or without PGF2alpha were examined. Cell number and viability were lower in summer than in winter. Incubation temperature did not affect cell viability. Winter Day 11 cells incubated at 40-degrees-C produced about 30% less progesterone than when incubated at 38-degrees-C. In summer Day 11 cells incubated at 38-degrees-C, LH or forskolin stimulated progesterone production was lower than in winter, and was not further reduced by incubation at 40-degrees-C. In winter Day 16 cells, less progesterone was produced compared with winter Day 11 cells (except for basal production after 2 h), and incubation temperature did not modify this result. Addition of PGF2alpha to the medium did not affect progesterone production. PGF2alpha production by luteal cells on Day 11 after 2 h incubation was higher in summer than in winter, being five and two times higher at 38-degrees-C and 40-degrees-C, respectively. These data indicate that the capacity of luteal cells to produce progesterone is impaired by both long-term seasonal heat stress and short-term incubation temperature effects.
- A study of the thyroid secretion-rate in 12 dairy cows (total of 48 determinations) was made in which breed (six Holsteins and six Guernseys), stage of lactation (seven to 14 days, and three, six, and nine months), and season of year (winter, spring, summer, and fall) were the variables under study. The mean thyroid secretion rate per 100 lb body weight for all animals and all determinations was 0.142 mg of thyroxine per day. The secretion rate for the Holsteins and Guernseys, respectively, was 0.127 and 0.156 mg of thyroxine per 100 lb body weight per day. The breed sample of animals was, however, too limited to consider this a true breed effect. Thyroid secretion rate per 100 lb of body weight was highest at the beginning of lactation (0.155 mg thyroxine per day) and lowest at the ninth month of lactation (0.132 mg of thyroxine per day). In contrast, the trend for blood thyroxine level (PBI) shows PBI to be lowest at the beginning of lactation (2.72 µg %) and to reach its height at six months of lactation (3.58 µg %). Thyroid secretion rate per 100 lb body weight was highest in the spring (0.165 mg thyroxine per day) and lowest in the fall (0.129 mg of thyroxine per day). In general, it may be stated that environmental influences, as represented by months and seasons of year, had a much greater influence upon thyroid secretion rate than did stage of lactation of the cows.
- The respiratory responses of four Israeli-Holstein heifers sheltered from direct solar radiation were measured every 4 hr during 4 consecutive days in early spring and again in late summer. Two of the animals were fed according to Fredricksen's standard, and two ad libitum. Average atmospheric conditions were 18.4°C and 65° R.H. In spring and 27.5° and 75° in summer. In spring no marked diurnal variation was evident in either ventilation rate or respiration rate, whereas in summer a well-defined cycle was evident. Tidal volume showed a distinct cycle in spring with the lower volumes occurring in the day-time, while in summer the variation was almost nil and absolute levels were lower. In both seasons the animals fed ad libitum had lower tidal volumes and a more restricted cycle. In both seasons, ventilation rates and respiration rates were not correlated with diurnal variation in climatic parameters. Ventilation rates increased by only 4% from spring to summer in both feeding groups, while respiration rate was doubled. The response of respiration rate to temperature was of the order of Q10 = 2, for changes from season to season only. In both seasons respiratory vaporization followed very similar and pronounced diurnal cycles, the maximum value being attained at 0830 hours. Higher rates were found in both seasons for the group fed ad libitum, owing mainly to consistently higher vaporization rates in the afternoon and evening hours. The diurnal cycle in vaporization was due mainly to changes in moisture content of the expired air. The partial effect of respiration rate on the moisture content of expired air and on moisture exhaled per minute was highly significant, while that of ventilation rate was not significant. The increase in absolute atmospheric humidity from spring to summer was associated with an increase in absolute humidity in the exhaled air, but this did not prevent a 12.6% reduction in respiratory vaporization.
- 1. The nychthemeral (24 hr) and seasonal cycles in thermoregulation at high (Fh) and standard (Fs) levels of feeding were studied in Holstein heifers during spring (mean daily air temperature 18.2°C) and late summer (mean daily air temp. 27.5°). 2. In the two seasons the percentage respiratory evaporative cooling (% RC) and rectal temperature (Tr) displayed marked nychthemeral cycles in both feeding groups, while heat production (Hp) cycles were evident in the Fh group only. 3. In both seasons the effect of the 22.4% larger Hp in the Fhgroup was to prolong the duration of higher % RC; Tr was relatively increased in the summer only. 4. The seasonal changes were: a 20% reduction in Hp in both groups; a larger amplitude of Hp cycling (19% of the mean in summer and 12.9% in spring) in the Fh group; a higher Tr (+ 0.40°C in the Fs group and + 0.63° in the Fh group), cycling with a larger amplitude (0.33° v. 0.52°); a 21° depression in feed intake in the Fh group and 9° in the Fs group. 5. Within each season Tr and the rate of change in Tr were significantly correlated with Hp in the Fh group only. On a between-season basis Hp and Tr were significantly correlated in both feeding groups. These suggest that the relationship between body temperature and heat production follows different patterns in nychthemeral cycles from those in seasonal changes. 6. The nychthemeral patterns of respiratory, %RC, Tr, and Hp responses indicated that the animals were acclimatized to the seasonal change in Tr, possible through a shift in the set-point for temperature control. The depression in heat production and feed intake did not prevent the maintenance of a normal rate of growth.
- If livestock productivity is to be improved the relationships between the animals and the environment will have to be better understood.
- Past attempts to devise an index of climatic stress are briefly reviewed, and a new “Relative Strain Index (RSI)” is proposed. RSI is the ratio between the evaporative cooling required to compensate for the heat stress, and the maximum evaporative cooling that the animal can provide by physiological means. RSI takes into account the air temperature, vapor pressure, air movement,and radiant heat exchange,as well as the metabolic rate and insulation provided by the animal's coat. Thè index can be used to predict the probable tolerance of animals of types that have been sufficiently well studied. It can also be used to estimate the relative tolerances of animals having different physiological capacities or metabolic rates, but otherwise comparable; and it can be used to predict the relative effectiveness of proposed environmental controls.
- Sections of cattle heads were made to study the ear canal and its relationship to the tympanic membrane and the hypothala- nms. The ear canal extends fmavard and downward and tapers slightly. Approxi- mately 0.6 em distal to the tympanic mem- brane, it turns approximately 45 degrees. In nmture animals, the car canal--base of the outer ear to the 45-degree tunl--mea- sured 10-13 cm in length and 0.6 em in diameter. Semiflexible thermistor probes were used to sense the temperature near the tym- panic membrane and 20 cm in the rectum. Compariso~ls of the resl)onse of tympanic nlembrane and rectal temperatures were made with ambient temperatures cycling (20-45-20 (!) and constant (52 C). A com- parison of the speed of response to an internal stimulus--introduction of ice into the rumen--was made at 42 C. Tympanic temperature responded more rapidly to rising and declining ambient temperatures (rising tympanic 2-4 rain, recta 20 rain; declining tympanic 2 rain, rectal 6 rain). Following" the internal stimulus, tympanic declined in 2-4 rain, whereas rectal aver- aged 15 rain. It appears that temperature sensed at the tympanic membrane is more suitable than rectal temperature for determining the speed of response of cattle to both internal and external temperature changes. Tim rectum is the traditional site of internal body temperature measurement, hoth in clinical diagnosis and in environmental physiology re- search. Studies by Benzinger (3) and Kaufman (9) with man, however, showed that din'ins short-term temperature stress rectal tempera- ture was nmch slower in responding to internal and external temperature stinmli than intra- cranial temperature. They also concluded that intracranial temperature was more closely asso- ciated with sweating and other short-term phys- iological adjustments. Thus, Benzinger (2) pos-
- The thermal change in the hypothalamus, the thermoregulatory center, of the experimental animals has been usually measured by use of a thermocouple or a needleformed thermistor inserted through the skull and the brain parenchyma. However, these procedures may be unsuitable for a longterm, observation of the body temperature because the insertion of the thermodes causes considerably widespread damage of the brain tissues, particularly the hypothalamus. Recording the intracardiac, rectal and muscular temperatures of the rabbit by means of the thermistor equipped on the apex of the venous catheter or of the metal-needle, Yasuda (1-3) of this laboratory has reported the influence of various pyrogenic substances on the temperature. Using the same technique, Takashima (4) has compared the effects of pyrogenic substances in the intact and the liver-damaged rabbits. There exist some reports on the tympanic membrane temperature. Benzinger (5) has chosen the tympanic membrane of a human for the measurement of internal body temperature as close as feasible to the hypothalamic heat center. He has introduced 36-gauge twin wires of copper and constantan into the external auditory canal and has placed the thermoelectric junction of the wires at the tympanic membrane. But little work has been done to study the correlation between the brain temperature and the tympanic membrane temperature. The present experiments have been designed to determine whether the tympanic membrane temperature changes in parallel with the hypothalamic temperature or not. In these experiments a decline of the tympanic membrane and brain temperatures was produced by cooling the common carotid arteries or by the administration of some hypothermic agents. An elevation of the temperature was produced by the carotid warming or by the administration of pyrogenic substances. Moreover, the influences of the carotid cooling and warming on behaviors, respiration, blood pressure and heart rate were studied.