Heat stress in dairy cattle in Kenya.

There are three cattle based dairy production systems in the better-watered regions' of Kenya namely: the semi-intensive dairy-meat-draught-manure system which uses predominantly pure East-African Zebu mainly in the western uplands and coastal regions; the small scale intensive and semi-intensive dairy-manure system which uses mainly first generation crossbreds with exotic or high performing grade cattle mainly in Central Province, Central Rift Region and at the Coast; and the high input system based on pure temperate dairy breeds or grade cows mainly in the Central Rift Region (Ornore et a/., 1999). The East African Zebu producing 1-2 litres milk per day on a poor diet (Stotz, 1975) helps to meet the subsistence needs of the people. This indigenous herd is, however, not the foundation of the modern Kenyan dairy industry, which supplies almost all the marketed milk, particularly to the rapidly expanding urban population. The commercial dairy herd is based on temperate breeds and or grade cows and European-Zebu crossbreds. Dairy farming using such cattle genotypes has been going on for nearly a century in the highlands located above 1500 m where its cool montane climate is considered suitable for such livestock. This area has therefore been the hub of the modern Kenyan dairy industry and boasts some of the most successful dairy enterprises in the tropics (Chamberlain, 1993). Israellson and Lindberg (1996) reported Kenya to have over three million dairy cattle. The same authors report that the improvement of the genetic potential of the Zebu for increased milk production is a continuing exercise, so that the number of crossbreds and grade dairy cattle in Kenya is set to increase, thanks also to a relatively functional artificial insemination programme. Because of its inherent high milk yields, the Holstein Friesian has been the dairy breed of choice the world over where it has been the dominant breed of the 20th century (Grothe, 1993). For the same reasons this has been the breed of choice for most farmers in Kenya and most farmers who upgrade their Zebu do so to the Holstein-Friesian (Matthewman, 1993). The Friesian is also popular because marketed milk in Kenya is paid based on volume without any premium for milk butterfat per cent and solids-not-fat content (Kahi et a/., 2000). Improved incomes as a result of a larger number of Kenyans working outside the agricultural sector, competitive prices for milk when compared with other farming enterprises and better dietary knowledge is leading to a spread of intensification of dairy farming into the other 1 These are regions where the mean rainfall can exceed 750 mm in 30 out of 100 years (Ominde, 1984). better-watered regions of Kenya in the western uplands and coastal lowlands especially among progressive fanners. In the key economic regions of these zones, Zebu are increasingly being replaced by Zebu-exotic crosses either through artificial insemination or direct purchases from the highland areas (Waters and Odero, 1986; Vorlaufer, 1990). A number of farms in these zones are also keeping pure temperate breeds including Holstein-Friesian, Ayrshire, Guernsey, Simmental and Jersey. The altitude in these 'new' dairying regions varies from about 1200 m to sea level where the climate is hot and sub-humid to humid with afternoon temperatures going up to 30?C (East African Meteorological Department, 1963). Therefore as these cattle move out of the cool highland and into these hot sub-humid to humid regions it is expected that heat stress will become an increasingly important constraint to the biological and economic efficiency of such cows. Furthermore environmental heat and humidity will become an increasingly greater constraint as the genetic potential for production is increased and other constraints are removed (Johnson 1987). McDowell (1994) states that the lower altitudes in the WCZ2 ? 1000 m elevation) represent physical environments, which can have deleterious effects on