Vitamin A supplementation of ruminant diets is warranted when:
- feeding poor quality forage with little or no green color;
- feeding diets composed of 40% concentrate or more;
- feeding corn silage as the sole or primary forage (Jordan et al., 1963);
- feeding calves colostrum or milk from cows with a low vitamin A status;
- feeding a limited roughage or poor quality roughage weaned calves, lambs or kids;
- feeding purchased cattle of unknown background and in unthrifty condition where liver stores of vitamin A are likely to be low or suboptimal (Perry, 1980).
The use of concentrate and byproduct feeds in place of forages is probably the largest single factor that has increased the need for supplemental vitamin A in ruminant diets. Inefficient utilization of carotene from corn grain and the destruction of carotene and vitamin A in the rumen are the main reasons for adding supplemental vitamin A to high-concentrate diets (Rumsey, 1975; Weiss, 1998). Mold contamination is associated with a 98% reduction in carotene concentration in corn (Adams et al., 1975).
Supplemental vitamin A can be delivered via: (1) dry feeds, premixes, blocks or liquid feed supplements; (2) free-choice mineral mixtures; (3) injection, using a commercial preparation; (4) drinking water using a water-dispersible product. The most convenient and often most cost-effective means of providing supplemental vitamin A to livestock is in balanced rations that provide uniform consumption of the vitamin on a continuous and daily basis. Grazing livestock are often supplemented with free-choice minerals, liquid feeds or blocks (solidified products) that are formulated to be intake-limiting, usually by adjusting salt content or pH or through physical effects. These products can be both convenient and effective, but feeding directions must be followed and intake of the product monitored to ensure correct levels of supplementation. Furthermore, these products must be fed in appropriate feeders, such as covered mineral feeders, that afford adequate protection from moisture and sunlight. Liquid feed tanks must be located properly and kept in good working order.
Vitamin A is often included, along with vitamins D3 and E, in liquid feed supplements. Since the viscosity, pH and solids content of liquid feed supplements vary considerably, development of vitamin A product forms that blend uniformly and are stable in such an environment was a challenge to manufacturers. Products of choice are dispersible liquid concentrates, which include fat-soluble vitamins A, D3, and E in tested formulations of emulsifiers, antioxidants and carriers.
In recent years, some livestock producers have followed the practice of administering vitamin A by intramuscular injection. This route of administration of vitamin A has been used to correct, or more frequently to prevent, vitamin A deficiency when feed or water administration is either inconvenient or impossible. Feeder cattle with an unknown history often receive 1 million IU vitamin A by intramuscular injection during the receiving process in preparation for entry into the feedlot. Vitamin A status is important for cattle to produce a viable immune response to the vaccines that are often administered at this time. Adequate liver vitamin A stores are a necessity for feedlot cattle entering a high-concentrate feeding program.
Increased levels of vitamin A are important under stress conditions. Situations under which increased vitamin A supplementation may be valuable include:
- in calves, lambs or kids during weaning;
- as nutritional support of treatment for bacterial or viral enteric disease, intestinal parasites, ringworm or other parasites;
- for newly arrived feedlot cattle;
- at calving in beef or dairy cows that have received marginal nutrition during late pregnancy.
Administration of vitamin A at high levels in drinking water or by injection is often recommended to support any specific therapy in the treatment of disease in animals. This is of particular value in animals whose liver vitamin A stores may have been depleted or in animals with intestinal infections or disorders that have impaired vitamin A absorption. Also, high levels of vitamin A may be beneficial in reducing the incidence mastitis in dairy cows (Chew, 1987) or increasing milk yield (Oldham, et al., 1991).
The level of vitamin A supplementation used should be based on both the expected optimum requirements of the animal and the potential for subclinical deficiency from interfering factors, such as low dry matter intake, variable forage quality, ruminal destruction of vitamin A, feed product composition and storage time. As with most nutrients, a borderline deficiency of vitamin A is far more likely than a severe, outright deficiency. A marginal vitamin deficiency reduces animal performance by small increments and is not easily detected (Miller, 1979).
Optimal vitamin A fortification of ruminant diets is shown in Table 2. Higher levels of vitamin A are sometimes used when stress levels or disease pressure are high, when high-concentrate rations are being fed or when feed storage or formulation favors increased loss of vitamin A activity. Beta-carotene supplementation of dairy cattle is recommended when little or no green forage is being consumed, although this area of research remains unclear. The most recent study in this area (Aréchiga et al., 1998) reported a significant milk production response to beta-carotene in cows fed rations in which a significant proportion of forage was replaced with fibrous byproduct feedstuffs, despite being supplemented with high levels of vitamin A (200,000 to 250,000 IU per day) (Figure 1). This study also reported some beneficial effect of beta-carotene on reproductive performance during heat stress if cows received beta-carotene for 90 days or more.
