Heat stress in dairy cattle and other livestock under southern African conditions. III. Monthly temperature-humidity index mean values and their significance in the performance of dairy cattle.

Temperature-humidity index (THI) values applicable to South Africa and Namibia have been established during this investigation for each month of the year by means of computerized modelling and mapping techniques. The data indicate that each year heat stress risk areas (HSRA's) expand from August to January and retract from February to July. The THI values classified according to the Livestock Weather Safety Index (LWSI) for lactating dairy cattle (LDC), suggest that, especially during November to March there is the risk of moderate to advanced heat stress in most South African dairy cows. This has important implications for their general health, udder health, production and reproduction. Careful planning of facilities and highly adaptable herd management are required to protect dairy cattle from heat stress.

Heat stress in dairy cattle under southern African conditions. II. Identification of areas of potential heat stress during summer by means of observed true and predicted temperature-humidity index values.

November-March are the hottest months of the year with the highest monthly mean "temperature-humidity index" (THI) in South Africa and Namibia. These 5 months are associated with severe heat stress in dairy cattle, are of critical importance for their performance and may have great economic implications for the owner as well as for the dairy industry. Firstly, compared with the existing Livestock Weather Safety index (LWSI), more relevant meteorological data can be generated when mapping South Africa and Namibia according to the LWSI modified for lactating dairy cattle (LDC). Secondly, compared with the observed true THI values alone, more relevant data on heat stress and its deleterious effects on dairy cattle performance, become available when mapping South Africa and Namibia according to the combined observed true and predicted THI values. Minimum precautions against heat stress in dairy cattle are recommended depending on THI values as classified by the LWSI for LDC.