Plasma retinol concentration in grazing heifers: First data obtained from a dairy herd in the south of Chile

Abstract

Vitamins are organic molecules with a complex structure, which are essential in very small quantities for the health, growth and reproduction of ruminants. The generic term vitamin A is used when referring to retinoids that exhibit the biological activity of retinol (IUNS 1990)and includes both natural forms and synthetic analogues. Vitarnin A is physically a pale yellow crystalline solid substance, which is water insoluble. Chemically, it is a hydrophobic â-ionone ring with a conjugated isoprenoid lateral chain containing a polar group at its end (Debier and Larondelle 2005). The ability to synthesizecompoundswith vitamin A activity is limited to microorganisms and plants (Harrison 2005). Thus, animals must obtain vitamin A from the diet, where it can be found as a preformed vitamin in animal foods or as a provitarnin in plant foods. Provitamins are members of the carotenoid family, and the term is used as a generic description of all the compounds that exhibit qualitatively the biological activity of â-carotene (IUNS 1990). Â-carotene is one of the most widely available carotenoids and the main precursor of vitamin A (During and Harrison 2004). It is known that in animals, vitamin A intervenes in a variety of biological mechanisms associated to the maintenance of epithelial cells, gene regulation, immune cell function, reproduction and vision (Chew 1987, Debier and Larondelle 2005). Due to the above mentioned, the most frequent signs of deficiency are: impaired dark adaptation, xerophthalmia with the cornea becoming thick and hazy, decreased cellular and humoral immunity. In male animals it can cause degenerative damage of epithelial cells in seminiferous tubules while in the female it leads to abortion, delivering stillborn or weak foetuses and placental retention (Radostits et al 2002). According to Herdt and Stowe (1991) the evaluation of vitamin A metabolic balance is not a routine practice in animals but it should be considered as part of the general health diagnosis of the herd, especially if there is evidence of a decrease in reproductive efficiency and immunosuppression of unknown origino Hammell et al (2000) have mentioned that the hepatic concentration of vitamin A is the most reliable indicator used to evaluate the metabolic balance of vitamin A in animals. However, even though the determination of plasma concentration of retinol is a less sensitive indicator, it is the most practical and useful method to evaluate the status of vitamin A in the herd (Herdt and Stowe 1991), with plasma concentrations between 25 and 60 µg/dL indicating an acceptable level ofthe vitamin adult cattle (Frye et al 1991). Plasma concentration of vitamin A is usually expressed as the addition of retinol and retinyl palmitate concentrations; however, retinol gives more clinical information and can be measured through chromatographic techniques (Herdt and Stowe 1991). High Performance Liquid Chromatography (HPLC) is used to separate molecules with a high analytical sensitivity (Paulo et al 1999), making it a useful tool to determine vitamin concentration in bovines. Retinol requirements have been the subject of many studies. According to ARC (1980) the daily requirements of retinol are 20 µg/kg of body weight for maintenance and growth, 30 µg/kg of body weight during breeding and pregnancy and 25 to 65 µg/kg of body weight for lactating cows. NRC (2001) mentions that vitamin A, expressed as retinol equivalents, should be supplemented in amounts of 24 µg/kg of body weight during growth and 33 µg/kg of body weight during lactation, pointing out that these values do not represent the total requirement of the vitamin and consider animals being fed conserved forage. In the south of Chile, the management system for dairy cows is based on grazing pasture and the supplementation with concentrate and conserved forage is used in critical periods (Pulido et al 1999). Considering that green fresh forage is the main source of vitamin A precursors, it is possible to estimate that the contribution should be adequate for grazing cattle; however, the quality and quantity of the forage provided by the pasture changes throughout the year, under which circumstances the nutrient contribution could vary. Furthermore, most grains are void of carotenoids and the stored forages have lower concentrations of them (NRC 2001). In Chile there are no records about vitamin A status in dairy neither beef cattle; it could limit the nutritional management and the efficiency of the herd. The aim of this study was to measure the plasma retinol concentration in grazing heifers using a standardized analytic methodology and to record its variation during winter and spring in a herd in southern Chile.