The most important micro mineral deficiencies are generally thought to be selenium, cobalt, copper, iodine and possibly zinc. Roles, dietary source and deficiency symptoms of these are discussed below.
Selenium (Se)
The only known role of Se is in the destruction of peroxides and consequently protecting tissues against oxidation (damage). Vitamin E (see below) acts by preventing peroxides forming and both have a role in preventing white muscle disease, muscular dystrophy, ill thrift and infertility in ewes. Symptoms may include poor growth rates, decreased wool production and scouring.
Selenium is not required for plant growth, so conditions that favour increased pasture growth in selenium deficient soils will predispose animals to deficiency, since the uptake of selenium by plants is incidental to their uptake of nutrients essential for plant growth. Clover-dominated pasture and highly fertilised pastures in general tend to lead to lower concentration of selenium in plant tissue and potential deficiency in grazing animal diets.
The syndrome can occur on both basalt and granite derived soils, acid sandy soils, high fortified super¬phosphate or gypsum use and clover dominant pastures may induce a selenium deficiency. Supplementation may include rumen bolus, injections, fortified anthelmintic drenches and subcutaneous slow release formulation.
Cobalt (Co)
While cobalt is not directly utilised by ruminants, its dietary requirement relates to its role as a precursor for synthesis of vitamin B12 by rumen micro-organisms. The effect of deficiency is manifested as a vitamin B12 deficiency.
Coastal, calcium rich or sandy soils are commonly low in cobalt but ill thrift due to deficiency of cobalt/vitamin B12 has also been observed on the tablelands. Excessive lime and lush pasture growth may also lead to deficiency symptoms. Symptoms include ill thrift, weepy eyes, anaemia, scaly ears, and infertility and poor mothering in ewes.
Cobalt / vitamin B12 deficiency is also associated with phalaris staggers and it has been found that chronic forms of staggers can be prevented through treatment with cobalt or vitamin B12 directly.
Supplementation may include slow release rumen bullets, vitamin B12 vaccination or loose licks.
Copper (Cu) and Molybdenum (Mo)
Copper is an essential part of several enzymes and is required for body, bone and wool growth. Grasses tend to contain more copper than clover-based pastures and availability is higher from dry feed than green herbage. High intakes of sulfur, zinc, iron, cadmium and calcium can decrease the availability of copper, although it has been shown that excess molybdenum is the main factor linked to copper deficiency.
Care must be taken to ensure that copper toxicity does not occur from supplementation since toxic levels are only slightly above the upper end of the target dietary intake range.
Toxicity effects may be exacerbated where liver function has been compromised by intake of alkaloids from plants such as Paterson’s curse, sencio spp (fireweed / groundsel / ragwort) and blue heliotrope. This condition also interacts with supply of cobalt since high rumen concentrations of vitamin B12 can serve to detoxify these alkaloids before they can cause any damage.
Zinc (Zn)
Zinc has a role in enzymes and their involvement in carbohydrate metabolism and protein synthesis. It is constantly required in the diet as the body does not have significant mobilisable reserves.
Deficiency symptoms may include excessive salivation, loss of wool crimp, loss of hair or wool around mouth and eyes, stiff joints, scaly and dry skin, slow wound healing and inappetence leading to reduced growth rates. Poor testicular growth and infertility are also symptoms commonly found with zinc deficiencies.
Vitamins – roles and availability
Vitamin A
Vitamin A is needed for normal bone development, maintenance of epithelial cells (cells which line the body cavities) and night vision. It is produced via conversion from beta-carotene which is found in green feed and is stored in the liver. Deficiencies, although rare, may occur during a prolonged period of drought or grazing dry, matured pasture.
Young stock are more susceptible to a vitamin A deficiency as their meagre liver reserves are more quickly depleted. Deficiency signs include night blindness, eye discharges and ill thrift, and are more common during periods of high grain feeding or in shedded animals.
Green pasture, leaves, green hay and yellow maize are good sources of beta-carotene and ultimately vitamin A. Supplementation is rarely required. Be wary of vitamin A availability in pelleted feeds and premixes as availability declines by up to 10% per month after the date of manufacture.
Vitamin B12
Vitamin B12 is produced for ruminant animals by the action of rumen microbes using cobalt as a precursor. It is needed for cell growth, energy metabolism (glucose production) and wool production (metabolisation of methionine). Production of vitamin B12 in the rumen is enhanced by increased concentration of cobalt although the efficiency with which cobalt is converted to vitamin B12 declines. The rate of absorption is inhibited if the rumen or small intestines are damaged (e.g. by worms).
Colostrum contains some vitamin B12 but milk and cereal grains contain limited amounts. Supplement-ation with vitamin B12 may be useful if you have a known cobalt deficiency (e.g. light sandy or traprock style soils as found in Tablelands) or for young stock that may not have fully functional rumens (e.g. lambs <30 kg) and are not synthesising adequate B12. In adult animals a slow release cobalt bullet is likely to be more effective and economic than direct injection with B12.
Vitamin D
Vitamin D is contained in sun-cured hay and is produced in the skin of animals through irradiation. It is stored in the liver and helps with calcium absorption by acting to regulate the Ca:P balance. Deficiencies are rare in extensively grazed animals, although in more southern latitudes a vitamin D response has been recorded in animals grazing oats during winter (see the section on Calcium).
Supplements may be used when feeding stock high grain rations for long periods (over 2 months). Supplementation may be necessary if the Ca:P ratio is less than 1:1 (recommended is 2:1). Deficiency signs include ill thrift, anorexia, hunched back and rickets (swollen joints, bow legs, knock knees). Hypocalcaemia may also result from vitamin D deficiency.
Vitamin E
Vitamin E is an antioxidant needed for unsaturated fatty acid metabolism and maintaining cell membranes. Deficiencies may occur occasionally in weaners which have had no green feed for several months but it is seldom seen in mature sheep. Symptoms may include lameness, muscle weakness and ill thrift and white muscle disease (diagnostically different from that caused by selenium deficiency).
Green feed, oils and grains (unless stored long¬term) are good vitamin E sources. Deficiency may also be addressed using an injectable multi-vitamin (A, D and E).
