Story: Diseases of sheep, cattle and deer
Page 13 – Mineral nutrition diseases – causes and detection
Trace elements and nutrition
To be healthy, animals need a balanced diet. Livestock need at least 13 mineral elements, or nutrients.
An animal health problem caused by lack of a nutrient is called a metabolic disease because it affects the animal’s ability to convert food into energy and growth. This is distinct from viral or bacterial diseases, which are spread from animal to animal or by other agencies.
How much is a ‘trace’?
The trace element content in pasture is measured in parts per million (ppm). There should be at least 10 ppm of copper, but only 0.25 of iodine, 0.10 of cobalt, and a mere 0.03 ppm of selenium is enough for animals to grow well.
Plant and animal needs
Grazing animals rely on pasture for their nutrition; however, animals and plants have different mineral needs. Pasture plants do not need cobalt, selenium or iodine to grow, but if the soil is low in these then the plants cannot provide enough for animals. Between 1930 and 1960, when agriculture in New Zealand expanded and intensified, trace-element deficiencies of cobalt, selenium, copper and iodine were identified in livestock. Nationally, 13% of pastures are deficient in cobalt and 30% need selenium supplementation for stock growth.
Detection and diagnosis
Detecting trace element deficiencies in animals can be difficult because poor growth and reproduction are signs associated with many diseases and nutritional problems. However, the amount of trace elements in the blood or liver is related to growth, and can be tested. A low content means the animal will have poor growth, while a high content means the animal should have a near-maximum growth rate. This is the ‘reference range’ for expected animal production and is particularly useful for managing lambs and calves, which are sensitive to trace element deficiencies.
Correction by fertilisers
Applying fertilisers with added trace elements (cobalt, selenium or copper) to pasture is one way of overcoming trace element deficiencies, and is how cobalt deficiency (called bush sickness) is remedied. The trace elements are absorbed by pasture plants, which increase their trace element content over the next three to five weeks. This drops over the 18 weeks after that, so the fertilisers should be applied annually. If animals take in more trace elements than they need, the excess is stored in their liver and other tissues and released when intake is low.
Pills and injections
Cud-chewing animals can be treated by giving them a long-acting bolus, or large pill, which remains in their reticulorumen (first and second stomachs). Another method is to inject a small quantity of an element under the skin, which is slowly released into the bloodstream for six months to a year. Oral doses with liquid supplements (drenching) lasts only two to four weeks.
Treating the mother
Sometimes the best way to supply minerals to young stock is to treat the pregnant mother. Vitamin B12, selenium, copper and iodine are readily transferred across the placenta and also, with the exception of copper, secreted into the milk. Treating the mother before mid-pregnancy will prevent deficiencies in the young from birth to weaning.