Introduction

Domestication of ruminant animals and their use to produce milk, meat, wool, and hides represents one of the cornerstone achievements in the history of agriculture. The essential feature of the ruminant animal that has fostered its utility as a dairy animal is the presence of a large pregastric chamber where microbial digestion of feed (particularly fibrous feeds not directly digestible by humans) provides various fermentation products that serve as precursors for efficient and voluminous synthesis of milk. Without this symbiosis between animal and microbe, the dairy industry would not have developed, and indeed human culture would be vastly different in its food-gathering methods.

The dairy animal is a host to a wide variety of microorganisms. Most of these are microbes in the digestive tract that are essential for fermentative digestion of the animal's feed. However, a number of other bacteria, fungi, and viruses can induce a pathogenic state in various organ systems resulting in fatal or nonfatal diseases. This chapter will focus first upon the microbiology of digestion by the normal flora and its occasional alteration by opportunistic microbes. This will be followed by a brief overview of the major infectious diseases and their effects on the animal and on the quantity and quality of milk produced. Most of the information presented has been obtained from research with cows, but much of it applies to sheep and goats as well.

II. THE DAIRY ANIMAL A. Populations and Production

There are nearly three billion domestic ruminants in the world, the most numerous and economically important of which are cattle, sheep, goats, and buffalo (Table 1). Lactating dairy cattle (not including replacement heifers and dry cows) represent nearly one-fifth of the world's domestic cattle population and provide most of the world's milk supply. The numbers of sheep and goats actually used for milk production are difficult to estimate, but these species are of major importance in providing protein and energy to the human populations of developing countries and fill niche markets for specialty foods in developed countries. Both sheep and goats are regarded as superior to cattle in poor-quality grazing and browsing environments, in part because of more efficient retention of water and nitrogen (Devendra and Coop, 1982). Several other ruminant animal species (water buffalo, yak, camel, reindeer, and even the nonruminant horse) normally used in some cultures as sources of meat, hides, hair, or draft power are also milked for human consumption.

Because of their large size and abundant milk production, the Holstein is the predominant breed of dairy cow in use today. Improvements in animal breeding and genetics have yielded substantially larger animals over the years (Fig. 1) with corresponding increases in feed intake. This factor, combined with a gradual shift to diets having higher energy contents (i.e., higher proportions of grain) has resulted in a progressive increase in average milk production per cow, which in well-bred and well-managed herds may approach 13,600 kg (approximately 30,000 lb) per lactation.

Dairy cows are usually maintained on a 305-day lactation schedule, after which the cow is ''dried'' (by reducing feed and by not milking) for 2 months before calving to permit full development of the calf and to allow the buildup of body reserves necessary for the next lactation. After calving, milk production

Table 1 Worldwide Population of Domestic Ruminants and Worldwide Milk Production, 1998a

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