SummaryThe heart rates and corresponding energy expenditure of adult Brahman cattle (Bos indicus) and water buffalo (Bubalus bubalis) were measured whilst they were standing, walking at speeds of up to 1 m/sec and pulling loads of up to 50 kg for periods of 1–2 h/day. Correlations of heart rate with energy expenditure showed a positive linear relationship within species although a distinct difference was exhibited between species. However, when heart rate and energy expenditure were expressed relative to their respective resting values (RHR and REE respectively), the results for all animals fitted the same line:REE = 2·251 RHR – 0·954 (r = 0·93; n = 49).More importantly, the relative heart rate and actual energy expenditure per unit metabolic body weight (EEW) for all animals fitted a common line:EEW = 24·94 RHR – 16·25 (r = 0·91; n = 49),where EEW is in watts/kg0·75 and RHR = heart rate of the working animal/heart rate at rest. This assessment of the energy expenditure from measurement of heart rate and live weight thus allows a prediction to be made of the energy requirements of working draught cattle under field conditions.

Four lactating Friesian cows housed in climate chambers were exposed to the following sequential treatments: I, a four-week period when they were allowed constant access to an ad lib supply of a complete diet and water in a thermoneutral environment (ambient temperature (Ta) 14 to 20 degrees C and relative humidity (r.h.) 50 to 75%); II, a five-week period during which they were exposed to Ta 38.5 degrees C/r.h. 85% for 61/2 h during daytime and Ta 14 to 20 degrees C/r.h. 50 to 75% for the remainder of each day with constant access to ad lib complete diet and water; III, a four-week period under the same environment as treatment II but with access to food restricted to between 19.00 and 07.00; IV; a four-week period under the environment and feeding regime described for treatment II. The voluntary food intake (VFI) of cows subjected to II was not depressed as a result of high daytime temperature despite the fact that they exhibited daytime clinical features indicative of heat stress; this was because of a 27.8% compensatory increase in night-time feeding. Nevertheless there was an accompanying significant increase in the rate of lactation decline of 8.7% per month greater than that normally expected under the thermoneutral conditions (I). Subsequent restriction of food access to night-time (III) resulted in a significant reduction in mean VFI of 7.5%; however, the rate of lactation decline was drastically reduced to a value not dissimilar to the natural decline of 3.97% per month observed under thermoneutral conditions (I).

INTRODUCTIONAs a consequence of the rapidly expanding population and the general recognition of the need for more efficient food production in the less developed countries, there is currently much interest in the establishment of intensive animal production units therein. However, traditional animal production enterprises in these countries, particularly in those with hot humid climates, are noted for their low productivity and poor reproductive rates. Consequently, only a few of the intensive units established in these locations have been successful. The most common factors associated with this failure are, inexperience in 'farming' animals, mismanagement, inappropriate animal types, inadequate housing and planning and ignorance of the deleterious effects of tropical environments on animals and production systems and on means of alleviating them.Ever since animals were domesticated, man realized that climatic changes resulted in dramatic influences on their behaviour and performance. In an attempt to ameliorate these effects, enormous energy has since been expended in the developed world in modifying traditional housing, feeding and management practices. However, the problems faced by animals in the tropics are particularly difficult, especially in the low income countries, not only because they are exposed to periods of extremely hot/humid weather but also because financial constraints have precluded the employment of many of the available methods of ameliorating the effects of climate on production. Clearly, the fate of many of these nations will depend on man's ability to overcome these difficulties. Thus far, research and development efforts have been mainly concerned with: * the importation of high producing animals or their genes from temperate regions, crossing them with tropically adapted species and subsequently selecting those that are adaptable and productive; * designing suitable housing to counteract the rigours of climate; and * developing the most efficient ways of animal management and feeding.The object of this paper is to briefly describe the physiological and productive responses of animals maintained in hot environments and to categorise the practical methods currently available of alleviating the depressive effects. The majority of the work quoted will relate to cattle. RESPONSE OF CATTLE TO HOT/HUMID TROPICAL.CLIMATESClimate is one of many features of the tropical environment which limit animal production; others include nutrition, endo-and ecto-parasites, disease, soil fertility, etc. (McDowell, 1968). The depressive effects of tropical climates are very difficult to quantify because they reflect responses to four factors: * to the direct effect of diurnal heat loads -various combinations of solar radiation, environmental temperature, relative humidity, wind, rainfall and oxygen tension (altitude) -on the energy needs of the animal; 58 RICHARDS * to the indirect effects of heat loads on the type, amount, availability and quality of feed and water, the prevalence of disease, helminth load, ...

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