niacin requirement. High-energy diets and the use of particular antibiotics can increase the requirement for niacin.
Young ruminants are most susceptible to niacin deficiencies, and a dietary source of niacin or tryptophan is required until the rumen is fully developed. The first signs of niacin deficiency in most species are loss of appetite, reduced growth, general muscular weakness, digestive disorders, and diarrhea. The skin may also be affected with a scaly dermatitis. Often, these signs are followed by a microcytic anemia.
Choline is essential for building and maintaining cell structure throughout the body and for the formation of acetylcholine, the compound responsible for transmission of nerve impulses. Abnormal accumulation of fat is prevented by the lipotropic actions of choline, and labile methyl groups are furnished by choline for formation of methionine. All naturally occurring fats contain choline, but little information is available on the biological availability of choline in feeds.
Unlike most vitamins, choline can be synthesized by most animal species. Because ruminants synthesize choline, a requirement has not been determined; however, it has been recommended that milk-fed calves receive supplementation of 0.26% choline in milk replacers.
Choline from dietary sources is only of value to adult animals if it can escape rumen degradation. Rumsey (1985) determined that for choline-supplemented steers fed an all-concentrate diet, supplementation did not affect feedlot performance, carcass measurements, acidosis, or products of rumen fermentation. However, increasing dietary rumen protected choline (0.24 percent) produced a linear increase in milk production for lactating dairy cows (Erdman and Sharma, 1991).
Calves fed a synthetic milk diet containing 15 percent casein exhibited apparent signs of choline deficiency. Within a week, calves developed extreme weakness, labored breathing, and were unable to stand. Supplementation with 260 mg choline/L milk replacer alleviated the signs of choline deficiency.
B vitamins are abundant in milk and many other feeds, and synthesis of B vitamins by ruminal microorganisms is extensive (McElroy and Goss, 1940a,b; 1941a,b; Wegner et al., 1940, 1941; Hunt et al., 1943) and begins very soon after the introduction of dry feed into the diet (Conrad and Hibbs, 1954). As the concentration in the diet increases, thiamin results in a net loss; whereas niacin increases substantially in the rumen, while the duodenal concentration of thiamin, niacin, riboflavin, and biotin does not change (Miller et al., 1986a,b). Niacin decreases in the duodenum and ileum when monensin is added (22 mg/kg diet), while thiamin, riboflavin, and biotin are not affected.
Signs of insufficient intake of B complex vitamins have been clearly demonstrated for thiamin (Johnson et al., 1948), riboflavin (Wiese et al., 1947), pyridoxine (Johnson et al., 1950), pantothenic acid (Sheppard and Johnson, 1957), biotin (Wiese et al., 1946), nicotinic acid (Hopper and Johnson, 1955), vitamin B12 (Draper et al., 1952; Lassiter et al., 1953), and choline (Johnson et al., 1951) in young calves. The established metabolic functions of B vitamins are important and consequently, a physiological need for most B vitamins can be assumed for cattle of all ages.
Attempts to obtain responses to other B vitamins are numerous, but the overall results are considered inconclusive. Although B vitamin synthesis is altered by diet, considerable change is possible without producing signs of deficiency (Hayes et al., 1966; Clifford et al., 1967).
Supplemental riboflavin, niacin, folic acid, B12, and ascorbic acid are degraded and/or absorbed anterior to the small intestine, while biotin and pantothenic acid primarily escape the rumen (Zinn et al., 1987). As a result, practical vitamin-B deficiency is limited to young animals with immature rumen development and situations in which an antagonist is present or ruminal synthesis is limited by lack of precursors.
Water constitutes approximately 98 percent of all molecules in the body. Water is needed for regulation of body temperature as well as for growth, reproduction, and lactation; digestion; metabolism; excretion; hydrolysis of protein, fat, and carbohydrates; regulation of mineral homeostasis; lubrication of joints; nervous system cushioning; transporting sound; and eyesight. Water is an excellent solvent for glucose, amino acids, mineral ions, water-soluble vitamins, and metabolic waste transported in the body.
Water intake from feeds plus that consumed ad libitum as free water is approximately equivalent to the water requirements of cattle. Water requirement is influenced by several factors, including rate and composition of gain, pregnancy, lactation, activity, type of diet, feed intake, and environmental temperature.
Restriction of water intake reduces feed intake (Utley et al., 1970), which results in lower production. However,