Review/Oorsig Volume 23, Issue 01 - Page 7

Volume 23 • Issue 01 • 2019 at concentrations less than 100 parts per million (ppm, also known as mg per kg) of the diet. The National Research Council (NRC) has identified 10 trace minerals as essential for beef cattle. However, only about 4 are generally recognized as the most problematic in grazing beef cattle: • • • • Copper (Cu) Zinc (Zn) Manganese (Mn) Selenium (Se) - which is a non-metallic element that exists in several allotropic forms. Cattle gain most of their daily trace minerals from their diets (pastures, water, licks) which is influenced by various factors such as soil type and pH, geographical region and rainfall, composition of diet (stem:leaf ratio, fibre digestibility and fertilization, sulphur containing amino acids). Trace mineral nutrition of cattle is further complicated by antagonistic reactions of which Calcium (Ca), Iron (Fe), Molybdenum (Mo) and Sulphur (S) are the main antagonists in grazed forage and water sources. These antagonistic reactions mainly take place in the rumen and negatively affect the absorption and bioavailability of trace minerals. • • • • Dietary Ca concentrations have been demonstrated to differentially affect Zn and Mn bioavailability. (Green line) Iron is the second most common trace metal on earth and is found in nearly all sources of feed, including water. A considerable amount of Fe may also be ingested through the intake of soil during grazing as well as soil contamination of harvested forage. In most soils iron is extremely high but with low solubility. Research indicated that acid conditions, like fermentation of silage, greatly increases the bioavailability of iron from soil contamination. Iron is a potent copper, zinc and manganese antagonist. (Red line) Sulphur (S) is found naturally in nearly all feedstuffs and decrease Cu and Se availability. (Yellow line) Molybdenum (Mo) is an essential trace element required by all animals and deficiencies are readily recorded. High levels of molybdenum can impact Cu nutrition in ruminants by two means: (Yellow line) º º Irreversibly bind Cu in the gut and prevent absorption. º º Post-absorption systemic depletion of Cu from tissue sites. To minimize the effect of antagonism with oral supplementation of trace minerals the minerals can be chelated when fed to the animals. Research shows that fresh semen sperm motility was increased from 55.2% to 69.1% in bulls fed chelated trace mineral supplements. Oxidative stress. Bulls, as are all animals, are subjected to various forms of stress. Such as: • • • • • Nutritional stress Environmental stress (heat stress, cold, transportation) Social stress (comingling and dominance determination) Breeding Diseases and parasite infestation All of these stress factors contribute to another type of metabolic stress, called oxidative stress, which is characterised by an increase in the amount of free oxygen radicals, and that can lead to point mutation, a catastrophic snow-ball reaction of cell dysfunction. The study of oxidative stress is a relatively young field of research in ruminant medicine. Oxidative stress results from increased exposure or production of oxidants, (which can be seen as a by-product of most metabolic processes in the body) or from decreased dietary intake or increased turnover of antioxidants. Thus, oxidative stress is a condition that results from an imbalance between oxidants and antioxidants in the body. A limited number of conditions have been investigated with regard to the effect of oxidative stress in ruminants. Studies in cattle have been sporadic and mainly focused on mastitis, pneumonia, retained placenta, metabolic diseases and more recently bull fertility. Essential trace minerals such as zinc (Zn), selenium (Se), manganese (Mn) and copper (Cu) not only act as antioxidants but also play vital roles in multiple metabolic processes. Oxidative stress (OS) results when the production of reactive oxygen species (ROS) exceeds the antioxidant defence mechanisms present in the body and can lead to DNA damage which is the root of physiologic and pathologic diseases. At high concentrations ROS produce adverse modifications to all cellular components, altering 7