Appropriate vitamin and mineral nutrition can ensure optimal immune system function, growth rate and reproductive performance in commercial ruminant production systems.
When addressing supplementation requirements, the historic and current variables specific to the individual production system must be considered. A set and forget system typically fails to account for the fluctuating variables within production environments. The interactions between animal and environmental factors must also be considered due to their impacts on requirements and availability. These include soil type, rainfall, fertiliser history, pasture composition / stage of maturity, grazing management, and supplementary feed source as well as animal factors such as age, sex, breed, stage of production and previous growth path.
Guidance for supplementation
Whilst a standard supplementation program for all applications can be successful, a tailored regime is preferable as it optimises the efficiency of dosing and reduces the likelihood of overdosing certain elements which can easily be toxic. Some minerals are stored in body tissues (liver, adipose tissue, bone) thereby allowing some level of buffering when animal requirement or dietary levels change. Other minerals are unable to be stored in body tissues at high enough levels, and thus are required consistently in the diet. The aim of supplementation should be to manage tissue stores and meet consistent dietary requirements so that the needs are met consistently.
Selenium is a fat-soluble trace element stored in the liver that works in conjunction with Vitamin E as an antioxidant to prevent and repair cell damage associated with normal metabolic functions. Short and intermittent breaks are usually of little consequence however a continuous supply is preferable. Australian soils are generally deficient in selenium with those dropping below 300 parts per billion considered severe. WA’s most significantly affected areas are the high rainfall areas of the south-west especially those situated coastally. Contributing factors of selenium deficiency in soils include treating paddocks with sulphur containing or superphosphate fertilisers, acidic or sandy soil types, rapid pasture growth, and legume dominant pastures.
Any physiological stress induced by nutritional condition, stage of production or disease can elicit symptoms of selenium deficiency. Young calves and lambs are often the most susceptible due to their lack of body reserves, while adult breeding females are also at risk given the metabolic stresses associated with pregnancy and lactation.
Deficiency symptoms and diagnosis
The symptoms observed are similar in both cattle and sheep and include the following: weakened immune response (Increased risk of mastitis, scours), white muscle disease, suboptimal fertility, reduced milk and wool production, retained placenta, abortions/perinatal deaths, premature births, poor infant vigour, reduced postnatal survival and growth rate, ill-thrift, chronic diarrhoea and retention of winter coat. Syndromes typical of selenium deficiency such as ill-thrift, lameness, weakness and sudden death can have many different causes. Following the correct procedures will help to avoid misdiagnosis.
To adequately diagnose and manage a deficiency, consider the soil and geographical information, fertiliser regimes, farm history, and the presence or absence of clinical signs typical of deficiency. A pasture tissue test (conducted by wet chemistry, not near infrared spectroscopy) at targeted growth stages will give the best measurement of selenium levels available to livestock. Where soil tests are available, they can be considered with the pasture tissue test to determine the risk profile. A blood or post-mortem tissue sample is required to confirm a deficiency and to establish the degree of severity. It is important to consider the sampling strategy to ensure the results are representative.
Supplementation and solutions
Selenium supplementation products range from injectables, oral drenches and capsules, lick blocks, loose licks and intra-ruminal pellets/ bullets. When deciding on a product, it is important to consider the active form, delivery, rumen degradation, potency and duration of efficacy. Pastures spread with selenium containing fertilisers will generally provide a maximum of 1mg selenium/day for sheep however the economics of this method of supplementation are currently under scrutiny.
Risk of toxicity is at its greatest when multiple long acting selenium supplementation products (bullets, injectables) are used simultaneously. Despite a lack of research reporting on toxicity related deaths in ruminants, it is generally thought to occur in sheep when daily intake exceeds 3-5mg/kg DM or 5-20mg/day, depending on the form of selenium and size of animal. Generally, products given orally have a more immediate effect and considered short term with the exception of particular slow release drench capsules. Although they contain comparatively high rates of selenium, it is unclear if it is released quickly or gradually.
Several research projects on selenium deficiency in livestock are currently in progress across WA. Seeking combined information from your animal production professional and agronomist is recommended.