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.

Seven cows were each milked at a variety of intervals between milkings a t each of several stages of lactation, intervals being long enough for maximum attainable yields to be approximated. The udder was emptied thoroughly at the beginning and end of each interval with the aid of injections of posterior pituitary extract. Intra-mammary pressure was estimated at. each milking. Net capacity of the udder, as measured by maximum yield, declined during lactation in proportion to the concurrent decline in daily yield, so that any given long interval between milkings reduced yield to almost the same degree in late lactation as in early lactation. The decline in maximum yield or "physiological capacity" was contributed to jointly by decline in 'physical capacity' (volume contained at given intra-mammary pressure) and decline in maximum pressure or secretion pressure. In early lactation, decline in secretion pressure contributed most to decline in physiological capacity, loss of physical capacity becoming important only in late lactation. It appeared that high rate of physical involution was associated with low functional activity of secretory tissue, as measured by secretion pressure, rather than with advanced lactation per se. It is postulated that the physical and functional components of physiological capacity are paralleled by physical and functional components of daily yield, namely, amount of secretory tissue and secretion rate per unit of tissue. Accordingly, decline in secretory intensity contributes primarily to early decline in daily yield, loss of secretory tissue becoming important in late lactation or in cows in which functional activity of secretory tissue is low. Cows appear to differ in the relative contributions of mammary development and functional activity to lactational performance. The pattern and course of involution during lactation may be partly determined by these initial differences. The effect of length of interval between milkings upon secretion is discussed, with emphasis on the difference between immediate and long-term effects. It is pointed out that differences between cows in sensitivity of secretion to long intervals are morphologically rather than functionally determined.

Duplicate determinations of residual milk and fat were made at milkings following intervals of 10, 14, and 24 hr at two stages of lactation in 12 cows. The data obtained thus represented 144 milkings. The amount of residual milk was correlated with total yield whether differences in total yield arose from differences between cows, stages of lactation, milking intervals, or error variation. The effect of changes in total yield was greater in early lactation than later, but within either stage of lactation the same relationship persisted approximately over most of the variables examined. However, between different milking intervals, change in total yield produced somewhat less effect on residual milk than when total yield varied from other causes. There were also differences between cows in the degree to which residual milk was affected by variation in total yield. For early and late lactation, and for milkings at 10 and 14-hr intervals, the mean relative amount of residual milk was constant at 17.8 per cent. of total yield. For 24-hr milkings it was 13.9 per cent. Independently of differences in yield, cows differed significantly in amount of residual milk. These differences were highly correlated with differences in persistency of lactation, low persistency being associated with high proportion of residual milk. Seventy-five per cent. of variation in persistency was accounted for by differences in residual milk. Fat percentages of residual milk differed between cows, between stages of lactation, and between milking intervals. The data were used to calculate rates of secretion of milk and fat, and fat percentage of milk secreted, during intervals of 10, 14, and 24 hr. With cows normally milked at intervals of 10 and 14 hr, secretion rate was not significantly less during the longer interval, and any difference which may exist must be extremely small. Fat percentage of milk secreted increased when the milking interval was lengthened. Imposition of a single 24-hr interval on cows normally milked twice daily reduced milk secretion rate by only 15 per cent., but there was also an aftereffect of the long interval upon immediately subsequent secretion rate.

Hair follicle densities were studied in 330 cows sampled once, and 460 calves sampled twice at different ages. The calves were sampled at times varying in 2-month steps between birth and 22 months of age. Follicle density on the midside decreased with increasing body weight within animals according to a power of body weight very close to two-thirds. It is shown that data on follicle density, even from animals in the same environment sampled at the same age, are of little comparative value because of differences in body weight. Furthermore, rankings made at one age will differ from rankings at another age because of differences in growth rate. Follicle number, a figure proportional to the theoretical total number of follicles, permits valid comparisons. Male calves had higher follicle number than female calves. Africander crossbreds were no different from Herefords and Shorthorns, but Brahman crossbreds were 20% higher in follicle number. Individual animals of the same breed and sex differed significantly in follicle number, for which heritability was estimated at 29%. Between animals, follicle number was positively correlated with body weight. The significance and possible utility of follicle number as an indication of a component of body size is discussed.

Results are reported of studies to determine whether interpretations of worm egg counts per gram of faeces as an indication of the population fluctuations among gastro-intestinal helminths of cattle would benefit from adjustments to the egg counts for faecal consistency, and for changes in faecal output associated with the age and body weight of the host. Adjustments for faecal consistency proved of little value. Adjustments for age or body weight, however, were found to be important, and factors for adjustment to a standard age or weight are tabulated. Under field conditions age is more readily ascertained than weight, and estimates showed that prediction of faecal output from age varied only slightly from prediction from body weight.