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The Loam Ranger - Micronutrients for dryland pastures

Dear Loam Ranger,

My neighbour said he increased his pasture growth by adding a trace element fertiliser. I’d like to do the same. What points do I need to know?

Provided the major soil properties such as pH and major nutrient levels are optimised, supplementation with trace elements can improve plant and animal nutrition. Soil testing is required to optimise the major chemical properties, but trace element imbalances are best assessed by leaf analysis. Ideally both soil and leaf analyses should be examined together, as many trace element deficiencies or excesses can be explained from the soil test results.

Trace element, or micronutrient, requirements for pasture used for grazing animals are best examined by plant tissue analysis, because soil testing is less effective in predicting exactly how much of these elements your pastures are taking up.

Simply supplying micronutrients doesn’t always mean animals will benefit as intended. Both positive and negative interactions can affect results. For instance, high levels of iron (Fe), manganese (Mn), molybdenum (Mo) or sulphur (S) in the soil will make copper (Cu) less available, so a copper fertiliser may prove to be a waste of money. Micronutrient status of grazing stock can ultimately be gauged from analysis of blood or liver samples (biopsy or slaughter), for which you need the services of a veterinary scientist.

There’s no point in paying good money for trace elements you don’t need, and excessive trace element applications can be harmful to stock. It is not a case of “if some is good, more is better”.

Collecting samples

When assessing the micronutrient requirements for pastures and stock (dietary mineral requirements), take a representative sample of the pasture species the stock most commonly graze – aim for about 200 g of leaves and whole shoots. Don’t include weeds, dry stems, roots or soil or any dead material, which stock would not graze. Wear plastic gloves so as not to add minerals from your skin, and use clean, sharp scissors. Collect the samples in a paper bag to start the process of drying.

You can mail or deliver samples fresh if you can be certain they will get to the laboratory within 24 hours. Alternatively, thoroughly dry the samples on paper at no more than 40 °C, and mail or deliver them to the laboratory. Alternatively, freeze them before sending them off in an esky with an ice brick so they don’t start to decay.

Make sure you label the samples with as much information as possible, including the sampling date, your address, your soil type, the stock you run and what analyses you want done.

If soil has not been previously tested, send a soil sample as well, since it is important to know the general chemistry. Acidity, for example, can have major implications for trace element balance and availability.

Essential trace elements

Here are some general comments on some of the important micronutrients for pastures and grazing animals.

Molybdenum (Mo)

Although there is little evidence for its essentiality in animals, one of the most common micronutrients required for pastures is molybdenum. It is often deficient in acidic soils and is essential to the nitrogen-fixing Rhizobium bacteria in legume roots. It is also essential to the gut flora of ruminants, as evidenced by improved digestion of feed where Mo is added to feed rations in deficient situations. Excessive Mo will antagonise copper uptake, particularly in sheep.

Moly-Super is the common method of application. Its availability to plants is related to soil pH and soil type. The more acidic the soil is (that is, a pH going from 5.5 downwards), the lower the availability of Mo for plant uptake. That is to say, although the Mo might be present, plants can’t get at it.

Liming will increase the availability of Mo already in the soil, but it is cheaper to apply Mo in fertiliser.

Soils with a pH below 5.5 (i.e. strongly acidic) will require a regular application of Mo – normally 50 g/ha every 5 to 6 years.

Boron (B)

Boron is not an essential nutrient for animals, but it is essential for plants; deficiency can suppress general pasture growth, although excessive B can result in toxicities. A wide range of pasture species can show B deficiency, especially on soils formed on sandstone and granite. If plant tissue analysis indicates a deficiency, apply about 1 kg/ha of B, which equates to 5.7 kg boric acid per ha.

Zinc (Zn)

Zinc is essential for both plants and animals. Subclinical (symptomless) Zn deficiency is common on light-textured soils such as sands or sandy loams, particularly those that are alkaline. The more alkaline the soil is (that is, a pH going from 7 upwards), the lower the availability of Zn for plant uptake. Grazing animals need around 30 mg/kg in feed. Where required, Zn can be applied at about 1 to 2 kg/ha every 5 to 6 years.

Iron (Fe) and manganese (Mn)

Iron and manganese are usually abundant in pasture. Indeed, excess of both is common on acidic soils. Very sandy soils can show deficiencies.

Cobalt (Co)

Cobalt is essential for animals but not plants. Pasture should contain around 0.1 mg/kg of this ultra-trace element. Cobalt-deficient areas of Australia are fairly well known. Often direct supplementation is used (Co pellet in the rumen) – consult your veterinarian.

Copper (Cu)

Although plants require copper for chlorophyll, the main reason for applying it is the health of livestock. Cu deficiency is more common on light-textured soils such as sands or sandy loams. Feed requirement is about 10 mg/kg for cattle and 5–10 mg/kg for sheep. Sheep have poor Cu regulation, and the requirement depends also on Mo, S and Fe levels in the feed. Cu toxicity is easily induced in sheep. Where required, copper sulphate is normally applied with fertiliser at 1 to 2 kg/ha every 3 to 6 years. Where high levels of other nutrients have made Cu unavailable, Cu drenches or injections are used. Ask your veterinarian for advice.

Iodine (I)

Iodine requirement is around 0.1–1.0 mg/kg of feed for sheep and cattle. Iodine is soluble and so is easily leached out of the topsoil, even close to the sea. Mineral licks are the most effective supplement.

Selenium (Se)

Areas of Se deficiency – "white muscle" country – are well known and confined to the lighter soils of the Central West and Tablelands. Requirements are minute, at around 0.1 mg/kg of feed for cattle and sheep. Direct supplementation is often used (e.g. licks), although sodium selenate is also effective spread on pastures at very small rates. Consult your veterinarian.