Chapter 3

The effects of high ambient temperatures on rabbit production

GENERAL EFFECTS OF HEAT AND HUMIDITY

Heat is one of the most important environmental factors which may affect rabbit production in the tropics. The rabbit is very largely dependent on respiratory evaporation for the regulation of its body temperature and this confers only a limited power of adaption to hot climates. Heat is also dissipated by radiation and convection, but these are somewhat restricted by the rabbit's furry covering.

It was reported by Johnson, Ragsdale and Cheng (1957) that short hair and larger ears helped the cooling process in New Zealand White rabbits. According to these workers, growth and development were impaired at ambient temperatures of 28.3°C and above. Generally the higher the ambient temperature the greater was the disturbance of the rabbit's functions.

The temperature of a rabbit's body is best measured by recording rectal temperature. A rectal temperature of 38°C is considered to be within the normal range. Lee, Robinson and Hines (1944) found that at ambient temperatures above 29.4°C the rectal temperature begins to increase in the rabbit. When the rectal temperature of Angora rabbits reached 39.8°C the animals became disinclined to move; at a temperature of 40°C they lay down on their sides; and at 41.7°C the rabbits suffered obvious distress but were able to carry out ordinary movements. These rabbits were found to be unable to tolerate ambient temperatures of 37.8°C to 43.3°C for more than 7 hours, within which time the critical rectal temperature of 41.7°C was reached. El Sheik and Casida (1955) found that rabbits exposed to temperatures of 43.3°C, with relative humidities (RH) of 30% to 40%, for 1 hour experienced an increase in body temperature of 2.5°C. A progressive decrease of RH below 75% brought about definite improvements in the rabbit's ability to withstand high temperatures (as measured by respiratory increase and rectal temperature) at very high ambient temperatures above 37°C (Lee et al, 1944). Below this temperature, improvements caused by lowering RH levels were less marked. It would appear from the data of Lee et al. (1944) that any adaption the rabbit might be able to make to increasing environmental temperatures would depend largely upon the increased evaporation of water. An appreciable proportion of this can be accounted for by increased respiratory rate. Dribbling from the mouth would also account for a large part. In hot dry (RH 30%) conditions, cooling by increases in respiratory evaporation can be achieved by the provision of adequate drinking water. The rabbit can undoubtedly acclimatise to temperatures up to 31°C (Lee et al., 1944). There is evidence, however, that low night temperatures might reduce the degree of acclimatisation to high day-time temperatures (Mills and Ogle, 1933; Lee et al., 1944).

EFFECTS OF HIGH AMBIENT TEMPERATURES ON FEED AND WATER INTAKE

Ambient temperatures of 30°C were reported by Prudhon (1976) to decrease the amount and frequency of feed intake of 20 week-old rabbits. The average intake at each meal changed very little between ambient temperatures of 10°C and 20°C, but at 30°C the solid food intake had diminished from 5.6g to 4.4g for each meal and the liquid intake had increased. The rabbits were held at each temperature for two weeks successively. The water/solid food ratios for the 3 temperatures were: 1.70 (10°C), 1.85 (20°C), and 2.50 (30°C) respectively.

The increase of water intake by rabbits at higher temperatures (Johnson et al., 1957) is an important management consideration. Even at relatively moderate ambient temperatures (about 20°C) a nursing doe of 10 lbs live weight with 7 young (8 weeks old) can consume 1 gallon of water daily (Anon., 1975; Olsen, 1974).

EFFECTS OF HIGH AMBIENT TEMPERATURES ON REPRODUCTION

There is evidence that high ambient temperatures can impair the reproductive performance of rabbits. In New Zealand Whites a temperature of 32.8°C reduced fertility in the male rabbits, a continuous high temperature being more detrimental than intermittent heat (Oloufa, Bogart and McKenzie, 1951). The testes were affected directly by temperature, suffering weight loss, and indirectly by reduced thyroid activity (temperature induced). Other workers have also observed that exposure to high temperatures can lead to a marked seminal degradation in male domestic rabbits. Rathore (1970) found that temperatures of 36.1°C (RH of 50%) for 1 to 2 days affected New Zealand White males and resulted in a reduced fertilisation rate due to damaged sperm. In the female, smaller blastocysts and embryos, and an increase in embryo mortality rates were also observed. El Sheikh and Casida (1955) concluded that the maximum temperature to which rabbits could be exposed (for 1 hour) without impairing health or sperm motility was 43.3°C at a relative humidity of 30 to 40%. Chou, Yi-Ch'uan and Chen-Ch'ao (1974) heated the testes of live rabbits to 43°C for 20 min on three successive days. No spermatozoa or spermatids were present in the seminiferous tubules up to 30 days after the treatment. However, recovery was complete at about 10-14 weeks following the treatment. Hiroe and Tomitzuka (1965) observed a marked fall in semen quality in male rabbits subjected to ambient temperature of 30°C for 14 days. These changes in quality were associated with an increase in the pH of the semen, a fall in sperm motility, a decrease in sperm concentration, an increase in the percentage of abnormal spermatozoa and a decline in libido. These workers found that, with the exception of sperm concentration, the changes could be reversed by reducing the ambient temperature.

Rich and Alliston (1970) produced evidence on the effect of heat on female New Zealand White rabbits exposed to temperatures of 21.1°C and 32.2°C for 18 days. There was a trend for all does exposed to some degree of heat stress (constant or fluctuating) to suffer fertility failure compared to those housed at a constant 21.1 °C. Embryo survival was depressed by heat stress and this was more severe under constant heat stress than under fluctuating heat stress, and was due largely to postimplantation losses. Embryo survival at 12 days post insemination was lower at 32.2°C than 21.1°C. Shah (1955) reported that severe prenatal losses occur in pregnant does exposed continuously in the first 6 days of pregnancy to environmental temperatures sufficiently high to cause a rise in body temperature of 1.2°C to 1.7°C. Such losses occurred through resorbtion of embryos following implantation. The adverse affect of heat is on the maternal tissues rather than directly on the embryos. When laboratory rabbits were exposed to a Sahelo-Sudanean dry climate by Tacher (1970) breeding was only successful for 5 months out of 12.