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If target yields or production are not being achieved, and the key macronutrients (nitrogen, phosphorus, potassium and sulphur) are adequately supplied, a micronutrient deficiency may be limiting performance.

A shortage of micronutrients (trace elements) such as copper (Cu), zinc (Zn), selenium (Se), cobalt (Co), manganese (Mn), boron (B) or molybdenum (Mo) can restrict pasture growth, animal production, or both.

Primary nutrient deficiencies occur when there is an insufficient supply of a specific nutrient and are typically corrected through fertiliser application or animal supplementation. In contrast, secondary deficiencies arise when a nutrient is present at adequate levels but its availability, uptake, or absorption is restricted by other factors.

Causes of secondary micronutrient deficiencies

A number of factors can cause secondary micronutrient deficiencies in plants, animals or both:

• Other nutrients: An abundant supply of another nutrient can affect plant uptake and animal absorption of micronutrients. Molybdenum, for example can interfere with animals’ absorption of Cu, causing molybdenosis. Zinc can reduce plant uptake and animal absorption of Cu, causing a Cu deficiency.
• Soil pH: As soil pH increases, availability of Zn, Fe, Cu, B and Mn decreases (but Mo becomes more available).
• Soil moisture: Low soil moisture promotes formation of manganese oxides which limit the availability of soil Co, and also limits availability of B.
• Soil type: Plant availability of micronutrients varies according to parent material, age of soil, soil texture and organic matter content, which binds and helps retain Cu, Zn, Fe and Mn in the soil.
• Fertiliser applications: On virgin soils or those low in phosphorus and sulphur, superphosphate can cause a large increase in plant growth, diluting the concentration of Se in herbage and creating a deficiency.

Detecting micronutrient deficiencies

As micronutrients in the soil are present in such small amounts, soil testing isn’t effective at detecting micronutrient deficiencies. Even with adequate levels in the soil, plant uptake or animal absorption could be the underlying issue.

Herbage testing is required to identify micronutrient deficiencies in plants. It can also check micronutrient levels for animal health, which can be complemented by testing micronutrient levels in animal blood and tissue samples. Two types of herbage tests are commonly used in pastoral farming:

• Clover-only: Nutrient deficiencies typically appear in clover before grasses, because clover’s shallow root system make it a weaker competitor for nutrients. In addition to assessing macronutrient levels, clover-only testing measures Mo, a key micronutrient for healthy clover growth and, in turn, improved overall pasture quality.

• Mixed pasture: This indicates the nutrient value of what stock are actually eating, as a broader selection of species in the sward are sampled. A mixed pasture sample can also be analysed to measure other feed values such as nitrate-nitrogen levels, protein content, starch levels and other feed quality tests.

Micronutrient levels in herbage can vary throughout the year due to pasture growth and nutrient uptake, so herbage testing is best carried out ahead of peak animal demand, to allow time for any required fertiliser application. The usual times are:

• late spring for Co in anticipation of weaning
• spring for Se before mating
• early April for Cu, before peak demand by deer in autumn and winter, when pasture is less able to meet animal need.

Addressing deficiency

A well planned fertiliser programme can elevate micronutrient levels in pasture to support grazing stock. Products in the NutriMax range such as NutrixMax Selenium 1%, NutriMax Copper 25% and Nutrimax Cobalt 10% make it very easy to incorporate micronutrients into fertiliser applications.

Serious deficiency requires direct-to-animal supplementation as well as correction of micronutrient levels in pasture.

If soil pH or something else you can’t fix straightaway is affecting micronutrient availability, consulting a vet or farm advisor for direct-to-animal solutions is recommended.

For more support, talk to your Ballance Nutrient Specialist or Ruralco Representative.