Search

It is often desired to raise heavy or fat animals in hot climates but conflicting ideas are held as to whether heavy feeding renders them unsuitable for tropical and subtropical conditions.Experiments were carried out with White Leghorn hens, Australorp hens, Middle White sows and cross-bred Border Leicester-Merino ewes, in which some animals of each kind were reared upon a high-plane diet and some upon a low-plane diet. Once a week each animal was exposed to a hot atmosphere for seven hours. Hot-wet and hot-dry atmospheres were used on alternate weeks. The behavior of the rectal temperature, pulse rate, respiratory rate and weight loss was measured.It was found that, in all animals studied, the reactions of the high-plane animals to hot conditions were significantly higher than those of the low-plane animals. This was much more pronounced under the hot-dry conditions. When, after several weeks, the diets of hens and ewes were reversed, the relative reactions of the two groups upon exposure to heat were reversed much more rapidly than the body weights.In another series of experiments, animals were fed successively upon different diets which were of equal caloric value but contained proportions of protein varying from 5 to 28.5 percent. Hens, a dog, a ewe and a sow were used. After a week's feeding with a particular diet, the animals were exposed to a critically hot atmosphere for seven hours and the reactions studied. In no case was any evidence obtained that a high proportion of protein had any significant effect upon the reactions of the animals to heat.It is concluded that the caloric value of the current diet has a significant, and at times practically important, effect upon the reactions of animals to hot conditions, but the proportion of protein contained in the diet is of no significant moment.

A method is described for measuring the rate of water evaporation from selected areas of skin without altering markedly the humidity of the air to which it is exposed. Air is circulated alternately through a capsule attached to the skin and an absorber containing a saturated solution of a salt giving the desired vapor pressure. The gain in weight of the absorber indicates the evaporation from the skin. A supplementary “maintenance” circuit is used to keep up air flow through capsules when they are not subject to measurement.Examples are given of results obtained by the use of this apparatus on cows under hot conditions. Different areas give different rates, especially under the less hot conditions. The rate of evaporation is increased with the hotter conditions. No significant differences are found between Jersey and Sindhi-Jersey crossbred cows for areas on the forechest and paunch. The rates for these areas under severely hot conditions (105° F., and 34 mmHg vapor pressure) reach 660 gm./sq. m., hr.The same technique applied to rabbits under the same hot conditions revealed virtually no evaporation.

A method is described for visualizing the significance of series of temperature and humidity readings made at different times, in different places, or under different circumstances. Each series is plotted on a psychrometric chart; statistical treatment carried out, using the chart lines as class limits; the outlying points joined to make an envelope; and the mean and standard error clearly plotted on the chart. The ease of interpretation is illustrated with data from Louisiana.

Jersey and Sindhi-Jersey crossbred cows were subjected for six hours to a standard hot atmosphere (105° F., and vapor pressure 34 mm. Hg) at repeated intervals of two months or longer, and observations made on rectal temperature and respiratory rate. Between tests the animals were kept in the barn or on pasture under climatic conditions prevailing at Beltsville, Maryland.The rectal temperature responses are found to vary markedly with the time of year. The responses show a maximum in February, and a smaller maximum in August, with a minimum in May–June.Repeatability coefficients for the rectal temperature responses in heifers (8–22 months of age) remain fairly low until the responses are sorted into three seasonal groups: April–July (low); August–September and February–March combined (high); and October–January (intermediate). They then reach high and satisfactory levels.The results are explained in terms of two sets of response to stress—yield or slip responses, the compensatory responses—which are differentially affected by the seasonal conditions (hot, temperate, cold) under which the animals live between tests.Repeatability coefficients for respiratory rate responses to the test hot atmosphere remain low, even after seasonal sorting.

The surface area was measured on 20 adult Jersey and 20 adult Sindhi-Jersey crossbred cows with a surface integrator, the construction and use of which is described. The areas were compared with the body weights. Neither the ratio of surface area to body weight, nor the ratio of surface area to the two-thirds power of body weight, showed a significant difference between the two groups.In spite of the close similarity in proportional surface area, the heat tolerance of the two groups was markedly different. This indicates that differences in heat tolerance of animals such as these are not primarily due to differences in proportional surface area even though the presence of large dewlaps, etc., might suggest that such a difference in surface area would be found.The relationship between surface area and body weight in the animals reported here, and in animals of adult size reported elsewhere in the literature, is well enough expressed by a straight line. It is suggested that there is no advantage to be gained from the more complex exponential relationship introduced by using a fractional power of body weight when comparisons are to be made between adult animals.

Twenty purebred Jersey cows and 20 Red Sindhi × Jersey (F1) cows were examined (a) at a “normal” temperature of 57°–84°F., and (b) after 3 hours exposure to 105° F. with a vapor pressure of 34 mmHg. Observations were made on rectal temperature, respiratory rate, and respiratory volume. The method of measuring respiratory volume is described.The crosses showed a smaller rise in rectal temperature (1.87± 0.152° F.) than did the Jerseys (3.06±0.23° F.). Both the final respiratory volume and the rise in respiratory volume, however, were also slightly smaller in the crosses. Furthermore, the correlation between respiratory volume and rectal temperature in cows exposed to comparable heat stress was a positive one, often of high magnitude.It is concluded that the superior heat tolerance of the crosses cannot be attributed to a more sensitive respiratory response, but that the higher respiratory activities are compensatory responses displayed by the individuals with the lower heat tolerance. The crosses, however, did show a more marked reduction of tidal volume (−0.48±0.131 l./resp.) than the Jerseys (−0.29±0.113 l./resp.), which should further reduce the risk of an induced alkalosis.Respiratory rate proved to be a less sensitive index of heat tolerance than respiratory volume. Under these conditions there was no significant correlation between rectal temperature and respiratory rate.