Trace mineral deficiencies in Australian livestock: a 1977 stocktake

Why trace mineral deficiencies were back on the research agenda#

The paper opens by noting that the study of trace element metabolism had intensified over the previous twenty years, helped along by more highly purified experimental diets and by instruments sensitive enough to pick up very small amounts of elements in animal tissues and feedstuffs. Over roughly the prior two decades, several additional elements including selenium, chromium, tin, vanadium, fluorine, silicon and nickel had come to be recognised as essential nutrients, drawing on the review cited from Schwartz (1974).

Recommended requirements for elements that had been recognised for longer were also shifting. The paper notes that the zinc requirement for chicks was then thought to be higher than the figure originally proposed by the Agricultural Research Council in 1963, while work cited from Preston and Willis (1974) suggested cattle might need less zinc than first proposed. For several elements, including zinc, copper, iodine, manganese and molybdenum, no settled requirement existed at all for laying poultry.

Against that backdrop, Norton set out to summarise three things: where trace element shortages were turning up across Australian livestock industries, what was being learned about interactions between different trace elements in the diet, and recent findings on cobalt deficiency in sheep and cattle grazing improved pastures in Queensland. The review draws mainly on Australian field reports, supplementation trials and overseas laboratory studies, with the cobalt trial as its one piece of original trial detail.

Mapping where deficiencies were occurring#

The paper presents an approximate map of copper, cobalt, selenium and iodine deficiency in grazing animals across Australia. A notable pattern emerges: these problems had mainly been recorded in areas of more intensive animal production, and Norton is careful to point out that it was not clear whether more extensive grazing areas would show similar issues if they received the same level of research attention.

Pigs and poultry were largely spared these problems, the paper explains, simply because trace element requirements were routinely built into formulated rations for those industries. Ruminants grazing pasture had no such safety net, which is why the deficiencies described through the rest of the paper are almost entirely sheep and cattle problems.

Copper, molybdenum and their tangled interaction#

Copper deficiency had been reported in sheep and cattle in every Australian state, the paper notes, producing a range of effects including skin and wool changes in sheep and digestive and reproductive problems in cattle, citing Bennetts and colleagues (1941). Supplementing copper alone was enough to prevent these effects in the Australian studies referenced.

A more complicated picture involves molybdenum. Dick (1956) had shown that high levels of molybdenum and sulphate could induce copper deficiency, and Norton cites Suttle (1974) showing that molybdenum interacts with both organic and inorganic forms of sulphur to limit copper uptake in sheep. Suttle's calculation, quoted in the paper, is striking: in a feed containing 0.5 mg molybdenum per kg, raising sulphur from 2 to 3 g/kg halved the availability of dietary copper. Norton flags that fertiliser use on pastures already marginal in copper could tip them into deficiency, though he adds that the scale of this interaction in Australian feeds was not yet known.

The paper also distinguishes how this interaction works differently depending on the digestive system involved. In animals with a rumen, dietary sulphur is quickly converted to a sulphide form, which favours an unavailable copper-molybdenum compound forming, with work by Bryden and Bray (1972) and Bird (1970) cited showing that high molybdenum intake lowers rumen sulphide and reduces the copper passing on from the rumen in soluble form. In simple-stomached animals, by contrast, dietary sulphate competes with molybdenum for absorption, which actually protects against molybdenum toxicity rather than causing a copper shortage.

Zinc, manganese and iodine shortfalls in grazing and intensive systems#

Zinc and manganese deficiency were not reported as major practical problems in Australian pigs and poultry, though the paper mentions occasional outbreaks of a skin condition called parakeratosis in Queensland piggeries. Zinc availability to simple-stomached animals is complicated by diet: plant proteins high in phytic acid reduce zinc absorption, and high calcium levels in pig diets were reported elsewhere to increase the zinc requirement further.

A more unexpected finding concerned grazing sheep. Egan (1972) found ewe fertility markedly improved with dietary manganese, zinc, or both together, even though pasture levels of these elements were not obviously low. Norton draws a general lesson from this, echoing what he says about copper and cobalt: pasture trace element levels can flag severe deficiency but tell you little about marginal deficiency affecting reproduction. He also notes that the zinc requirement for testicular development and sperm production in rams is higher than for ordinary growth, citing Underwood and Somers (1969), which raises the possibility that brief, transient shortages around breeding time matter more than static pasture figures suggest.

On iodine, the paper discusses Tasmanian studies of congenital goitre in sheep, both of which suggest that relying on palpable thyroid enlargement alone to diagnose deficiency can understate the true scale of the problem; one of these studies found that a sizeable share of lambs without any palpable thyroid enlargement had apparently still died from iodine deficiency. In northern Queensland, Hopkins and Pratt (1976) found that giving iodine supplements together with access to shade noticeably lifted fertility rates in Merino ewes.

Selenium supplementation and its practical limits#

Selenium responses had been recorded in every Australian state except Queensland by the time of writing. In sheep, selenium therapy prevented a muscle-wasting disease in lambs, increased body weight gain and wool growth, and improved reproductive performance, drawing on work cited from McDonald (1975) and Wilkins (1977). In cattle, a cardiac form of a related condition had been diagnosed in calves, but Gabbedy and colleagues (1977) found no body weight response to selenium treatment in that case.

The paper mentions the development of selenium pellets for direct placement into the rumen, citing Handreck and Godwin (1970), as a practical way of maintaining supply continuously rather than relying on repeated dosing. It also notes that uptake of selenium by plants from soil is reduced by certain competing soil nutrients, so continued cropping of low-selenium soils was expected to further reduce the selenium content of feed grains over time. Because adding selenium directly to stockfeed was against the law in most states at the time, Norton argues that monitoring selenium levels in grain was going to matter more as a long-term watching brief rather than something correctable at the feed mill.

Cobalt deficiency on improved tropical pastures in Queensland#

The most detailed original findings in the paper concern cobalt deficiency in sheep and cattle grazing improved tropical pastures, work described by Norton and Hales (1976) and Winter and colleagues (1977). Cobalt deficiency had previously been associated mainly with Western Australia, South Australia and Tasmania, but these newer reports described it in subtropical Queensland pasture systems for the first time.

The supplementation trial reported in detail used breeding ewes grazing nitrogen-fertilised Pangola grass, a tropical species, in south-eastern Queensland. Cobalt supplementation, whether applied to the pasture or given directly as a pellet, increased lamb birth weight, weight gain to weaning and lamb survival compared with unsupplemented ewes. In the unsupplemented group, 24% of lambs ended up dead at birth or did not live beyond their first day, and milk output in these ewes was markedly reduced compared with the supplemented groups, which the paper identifies as the main reason behind the poor lamb growth seen. Even ewes that received supplements lost weight after weaning, and lamb growth after weaning remained poor despite earlier supplementation, suggesting the dosing regime used was not yet sufficient on its own.

Pasture cobalt concentration in this trial measured 0.11 ppm, within the range usually regarded as adequate, yet total cobalt intake was clearly insufficient for the animals' needs. Norton attributes this gap to the generally low feed intake associated with the poorer digestibility of tropical pasture, meaning that even an acceptable pasture concentration can translate into an inadequate total intake when animals simply eat less of a lower-quality feed. This is presented as a wider caution: judging trace element adequacy purely from pasture concentration figures may be misleading wherever pasture quality itself is low enough to suppress intake.

Trace element interactions and how much weight to put on the findings#

Norton also sets out a general framework for thinking about interactions between trace elements, citing a classification from Davies (1974) that splits these into competitive interactions, where one element displaces another through similar chemistry with mutually negative effects, and non-competitive interactions, where the presence or absence of one element changes how another is handled in the body. Examples given include high dietary zinc inhibiting copper use in pigs and high dietary copper inhibiting zinc use, alongside a positive interaction in which dietary copper assists iron mobilisation for haemoglobin production.

Taken as a whole, the paper reads as a synthesis of scattered field reports and trial results rather than a single controlled study, so its conclusions carry the weight of a review rather than of one definitive experiment. The cobalt trial is the exception, offering concrete figures from a specific Queensland pasture system, but it covers one pasture type, one fertiliser rate and one class of stock. We would treat the broader claims about tropical pastures predisposing animals to several interacting trace element shortages as a reasonable hypothesis worth testing further, as Norton himself suggests, rather than as an established rule for all improved tropical pasture country.

The practical message the paper lands on is that frank, visible deficiencies of copper, cobalt and selenium were still being found under grazing conditions in ruminants, and that marginal or transient shortages, harder to detect and easy to miss with pasture testing alone, deserved more research attention, particularly in Australia's tropical grazing areas. This conference paper sits within the wider RAAN Conference Proceedings collection, gathered together with related reviews under the Recent Advances in Animal Nutrition community, both searchable from the Livestock Library research index.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Meat & Livestock Australia: red meat industry research and marketing body
• Australian Wool Innovation: wool industry research and marketing body
• Feedipedia animal feed database: an open-access database of animal feeds

Questions#

— The Livestock Library team