Twenty-six replicate observations of sweating rate were made on six Hereford x Shorthorn and six Brahman x Shorthorn steers in the field at different times of day and different seasons. Sweating rates were lower in Brahman cross than in Hereford x Shorthorn steers under mild conditions, but higher under stressful conditions. Brahman cross steers increased sweating rate at lower skin and rectal temperatures. Maximum sweating rates were significantly lower in winter than in summer. This is ascribed to an association between sweating function and other aspects of skin function which undergo seasonal changes. Under cool conditions, either there was no consistent difference between animals in sweating rate as in the Brahman cross steers, or the less heat-tolerant animals with higher body temperatures and thicker coat type showed the higher sweating rates. Under challenging conditions of heat, between-animal differences in sweating capacity were revealed and, in the British steers, animals with higher sweating rates then had lower body temperatures. Furthermore, animals with sleek coats showed a higher sweating capacity despite lower thermal stimulus to the skin. The environmental conditions under which sweating capacity is challenged, when differences between animals become important as a component of heat tolerance, depend upon genotype, season, and duration of heat stress.

A system of subjective scoring of cattle coats, ranging from very sleek to very woolly, is described. It has been applied to about 1600 animals, of which 500 were Hereford and Shorthorn cows and the rest their progeny from matings in four years to British breed and Zebu bulls. Consistency of scoring within and between observers is satisfactory. Highly significant differences between animals of the same breed persist through different seasons. The repeatability of coat score is 0.6 or more over intervals of up to 17 months. Effects of season, age, and sex are described. Heritability of coat score is estimated at 0.63. Coat scores are well correlated with body temperatures and respiration rates. Coat score and post-weaning growth rate of British breed calves are correlated to such a degree that coat score can be superior to a record of body weight as an estimate of growth capacity. The genetic correlation between coat score and growth rate is high in the British breeds. Evidence of the relation between coat score and growth rate in Zebu cross calves is inconclusive. The observed relationships are discussed with particular reference to their physiological causes. The results indicate the potential value of coat characters in selecting tropical beef cattle. The degree to which these results may be applicable to other populations in other environments is discussed. It is concluded that a sleek coat is important in favouring heat dissipation, but it may have even greater significance as an indicator of metabolic efficiency or of capacity to react favourably to stress.

The histology of early feeding lesions of the cattle tick B. microp/us has been studied using 32p labelled larvae to standardize the duration of attachment. Critical studies were made on 3-h lesions in six separate experiments on different groups of British breed animals. Each group consisted of three animals-one previously unexposed to ticks, one of high resistance and one of low resistance.The degree of mast cell disruption, eosinophil concentration and degranulation, and the extent of epidermal vesiculation were all significantly greater at the site of attachment on highly resistant hosts. In previously unexposed animals there was no mobilization of eosinophils nor mast cell breakdown and no epidermal vesiculation.Possible immune mechanisms producing mast cell disruption and the infiltration and concentration of eosinophils are suggested, and the effect of eosinophil degranulation on larval attachment and feeding is discussed.

In the reaction of Bos taurus cattle to infestation by the tick Boophilus microplus, mast cell histamine is translocated by the eosinophils to the attachment site. The concentration pattern of this cutaneous mediator for pain appears related to the grooming behaviour of the host.

Bos taurus cattle with high resistance to the tick Boophilus micro plus, whether free-grazing or in covered pens, had significantly more arteriovenous anastomoses (A VA) in their skin than did animals of low resistance. These differences in number of A VA associated with resistance level were most marked above the level of the sebaceous gland in the neck region, an area favoured for tick feeding. In this skin layer, the number of AVA in low-resistance animals (4�0�0�4 per 2�1 mm) was significantly lower than in animals of high resistance (12�3 � 2�2 per 2�1 mm) while the mean value for the naive animals (8�2 � 1 �9 per 2�1 mm) was intermediate. No differences in morphology of AVA were detectable between the three groups using light microscopy.