Molybdenum

Molybdenum is required both by plants and animals. In Ireland , shortage of the element is not a significant problem but on sphagnum peat soils some horticultural crops including swedes, cabbages and onions have responded to molybdenum applications (MacNaeidhe, private communication). Molybdenum deficiency would certainly not be expected on the majority of Irish midland cut-away peats as the high pH condition resulting from underlying calcareous parent materials would ensure that sufficient Mo was present in an available form. Under such conditions the problem is very often one of excess and many of our midland cut-away peats support molybdenum-toxic pastures. These give rise to problems particularly in young cattle when excess molybdenum in the herbage acts as an antagonist, which militates against efficient copper absorption by the animal.

From the soil/plant aspect it is essential to realise that this induced copper deficiency in animals can occur when values for both available soil copper and herbage copper are relatively high. Soil application of copper is therefore of no value in counteracting the animal problem.

The extent of molybdeniferous soils as presently known is shown in Fig. 6 (Brogan, Fleming and Byrne, 1973). Soils analysing greater than 0.3 mg/kg Mo, extractable by 0.275 ammonium oxalate buffered to pH 3.3, are regarded as molybdeniferous. Whether or not they give rise to Mo-toxic pastures depends on soil factors such as pH, organic matter content and drainage status. Because of the general complexity of Irish soils, not all farms within the shaded areas are molybdeniferous or potentially so whereas, outside the areas mapped, some soils with elevated levels of Mo undoubtedly occur. The figure should be interpreted as indicating high risk areas and analysis of herbage samples is strongly advised to establish both the molybdenum status and the seasonal variation in Mo content.

Fig. 1 Areas with high Molybdenum soils

Soil parent material

As in the case of selenium, the sources of high-molybdenum soils are Carboniferous black shales and limestones. The largest areas with such soil parent materials are found in the midlands but other areas occur in west Co. Limerick and sporadically eastward from there to Clonmel and past. An interesting enclave occurs around Kiltimagh in Co. Mayo where soil and herbage levels can be quite high. Poorly drained soils influenced by local black shale are responsible. Another molybdeniferous area occurs around Ballinamore, Co. Leitrim. Here soils occur which would normally require liming and this presents a special problem. Another area occurs in the Killorglin region of Co. Kerry where many of the soils are poorly drained. Relatively small areas of molybdeniferous soils also occur in soils reclaimed from the sea. The best known are the Wexford and Kilmore "sloblands” but there are similar areas in the Shannon estuary and in the Inishowen peninsula, Co. Donegal.

Soil analyses for molybdenum

Soil analyses for molybdenum are usually carried out using an ammonium oxalate extractant as indicated above. Data need to be interpreted with great care as the effects of organic matter, soil pH and drainage status are very important. Soil analyses can therefore be misleading unless these factors are taken into account. Soil analyses properly interpreted can indicate the potential of a particular soil for producing molybdeniferous swards but in practice herbage samples taken twice a year in spring and autumn will often yield more valuable information. As in the case of many other soil analyses, values at the extremes of the scale are readily interpreted but for analyses between the extremes due attention must be given to various soil factors as mentioned.

Fig. 13 Areas found to have elevated Molybdenum counts previously

Molybdenum in Irish Soils

Molybdenum (Mo) is essential for both plants and animals. The primary source of molybdenum in the weathering zone of the soil is the ferromagnesian minerals, with the sulphide molybdenum (MoS₂) common in certain granitoids (Chesworth, 1991). Molybdenum is soluble under a wide range of conditions, and is probably the most mobile of all the metalloid elements. Organic matter seems to act as a concentrating agent since Mo is enriched in organic sediments. Almost all the excess of Mo in terrestrial rocks is associated with Fe and Mn hydroxides and hydrated oxides (Chesworth, 1991).

Molybdenum in grassland is important from both deficiency and toxicity aspects. Deficiency affects clover establishment by influencing nitrogen fixation, whereas excess molybdenum in pastures can give rise to animal health problems. The importance of molybdenum in influencing nitrogen fixation was recognised many years ago (Bortels, 1930). Practical expression was given to the finding when the use of molybdenized superphosphate permitted the successful establishment of clovers and other pasture legumes in large areas of Australia .

Excess Mo in herbage is a major factor in the incidence of scouring and ill-thrift of cattle and this was revealed by the classical researches of Ferguson and co-workers on the “teart” pastures of Somerset , U.K. (Ferguson et al., 1943; Lewis, 1943, a, b). Molybdenum is now known to interfere with copper metabolism in the ruminant. The practical effect of excess Mo in pasture is to induce a copper deficiency in the animal.

Molybdenum in Irish soils
In Ireland the molybdenum problem is one of excess rather than deficiency. Soils with elevated Mo concentrations occur in the following geochemical situations:

A. Areas where the soil parent material is influenced by or formed from black marine shales of Namurian age (mid-Carboniferous)

B. In areas where the soil parent material is influenced by the Calp limestone (shaley carboniferous limestone)

C. Areas where the soil has been reclaimed from the sea such as the sloblands of Co. Wexford, areas on the Shannon estuary and on the Inishowen peninsula, Co. Donegal

D. An area overlying a mineralised granodiorite of Mace Head, Co. Galway (Talbot and Max, 1984; Talbot and Ryan, 1988, a, b)

The distribution of the areas in Ireland where potentially molybdeniferous soils can occur are shown in Fig. 11 which is an update from 1998 of the earlier map (Fig.6). Obviously not all the pastures in the shaded areas on the figure are molybdeniferous - much will depend on local soil types, soil conditions and farm practices.

Molybdenum in Irish pastures

Pasture responses to applied Mo on mineral soils have not been recorded in Ireland. Molybdenum excess and its associated problems have therefore been the focus of attention. Hypocupraemia in cattle has been the principal syndrome associated with excess Mo, but it also has been suggested that the element is implicated in an osteodystrophic condition in horses (Walsh and O’Moore, 1953). It should be noted that a large number of stud farms are situated within the potentially molybdeniferous areas.

The identification of molybdeniferous pastures in Ireland and the occurrence of hypocupraemia in cattle were reported by Walsh et al. (1951-52) and by Neenan et al. (1956). The levels of Mo found in herbage on which the problems described above were encountered varied from approximately 2 to 25 mg/kg. Levels in clover of up to 50 mg/kg were subsequently observed. In the autumn of 1985 Mo levels of up to 100 mg/kg were found in some Irish pastures and were the result of the inordinately wet year.

Factors affecting the molybdenum content of herbage

Soil pH

A rise in soil pH increases the availability of soil molybdenum in contrast to most other trace elements. This increase can be quite significant at pH values greater than 6.3. It is generally assumed that an anion exchange mechanism involving the replacement of MoO₄^2- by OH ^- is involved in the increased Mo availability following a pH rise. Liming of pastures with high levels of available Mo results in undesirably high Mo contents in grasses and clovers and can result in molybdenosis in grazing animals. The effect of as little as 2.5 tonnes/ha of lime on Mo content of a stud farm pasture is shown in Table 1.

Soil moisture status

Although liming is generally considered to be the most potent factor in increasing the Mo content of pastures, increases in soil moisture can also have serious effects. Drainage impedance, therefore, becomes very important and its effect in relation to molybdeniferous swards must never be underestimated. Under conditions of impeded drainage reducing soil conditions prevail, and in this regime the iron oxide minerals with which molybdenum is commonly associated, break down and the element is released and becomes available for plant uptake. The effects of increased drainage impedance on Mo levels in a ryegrass sward are illustrated in Table 2 (Fleming, 1973). The data illustrate clearly what is commonly observed in practice, i.e.other factors being equal, the most molybdeniferous swards on a farm will be found in areas with the poorest drainage. Molybdenum availability will, however, also increase even in well-drained soils following an extended wet spell.

Herbage species

The molybdenum content of herbage species varies considerably. Table 3 gives the Mo values for a range of species grown on a high Mo soil (Ferguson et al., 1943) and shows that clovers are richer in Mo that are the grasses. However, on a soil with low Mo the reverse may obtain. The latter situation is probably a reflection of the ability of grass roots to scavenge more effectively than the clover roots. Amongst individual grasses, Yorkshire fog usually accumulates more Mo than the associated grasses and this seems to obtain both in high Mo soil and in a soil with normal Mo levels.

Nitrogen

The application of nitrogenous fertilisers tends to reduce the Mo content of herbage. The dilution produced by extra yields is obviously important but the reduction or even elimination of clover from the sward is also a factor. Different nitrogenous fertilisers are likely to effect greater or lesser reductions in Mo content. For instance calcium ammonium nitrate or urea would reduce Mo content by dilution and clover suppression effects but in the case of sulphate of ammonia other factors come into play. These are (a) the soil acidification resulting from the constant application of this fertiliser and (b) the antagonistic effects on Mo by both the ammonium and sulphate ions. The sulphate ion in particular exerts a significant depressing action on Mo uptake, an effect attributed to competition in the soil solution between the similarly sized and charged sulphate (SO₄^2-) and molybdate (MoO₄^2-) ions.

Phosphorus

The effects of phosphatic fertilisers on Mo uptake are frequently difficult to interpret as these fertilisers may contain other elements which exert their own effects. Certainly phosphatic compounds free of sulphate increase Mo uptake (Barshad, 1951). Barshad has suggested that the stimulating action of phosphate on Mo may result from the formation of a phosphomolybdate complex readily assimilable by the plant. Walsh et al. (1951-52) observed enhancement of Mo uptake by herbage from Irish soils, following fertilizing with monocalcium phosphate. Some of the most severe cases of molybdenum toxicity in animals were associated with soils which had received basic slag, which can be ascribed to the effect of the basic nature of the material with the resultant decrease in soil acidity following its application. Whereas this may have been so, the phosphate effect per se is very likely a contributory factor. The application of superphosphate will reduce Mo uptake by plants, this effect resulting from the sulphate component in this fertiliser.

Potassium

Potassium fertilisers do not seem to exert any significant effect on the uptake of Mo (Fleming, 1980a). If, however, the application of such fertiliser resulted in an increase in the clover content of a sward then levels of Mo would be increased.

Sulphur

The depressing action of sulphur in the sulphate form on Mo uptake has already been referred to as an indirect effect following basic slag application. In a field trial from which the data of Table 4 (Fleming, 1977) were taken, the application of gypsum (CaSO₄.2H₂O) reduced the Mo content of grass from 18 to 2 mg/kg. Table 4 also shows that sulphate of ammonia also reduced Mo contents in herbage. In both these cases there was an accompanying increase in sulphur content of the herbage. Feeley (1990) also showed the effectiveness of sulphate of ammonia and gypsum in reducing the uptake of Mo herbage grown on midland peat. From the animal health point of view, the use of the above methods to reduce Mo excess in herbage may be questionable because of the accompanying high sulphur intakes can exacerbate the Mo effects on their copper status. The question arises as to whether the decrease in the Mo content of the herbage following sulphate application is offset by the resultant rise in the S content.

Management of molybdeniferous pastures

Part of the management of molybdeniferous pastures involves making a decision to reduce (calculated) lime recommendations. The possible resultant loss in pasture production must be viewed against the problems likely to accrue from molybdenosis in grazing stock. Whereas copper therapy is quite effective, it must be realised that once lime has been applied, the pasture will be rendered potentially more molybdeniferous for many years to come. The position is particularly acute on acid molybdeniferous soils and normally liming above a pH of 6.2 is not advisable. In Ireland many cutaway peat soils are already at a high pH and here especially every effort must be made to discourage clover growth by suitable management practices, e.g. increased use of nitrogen. Overgrazing also must be avoided as this increases soil ingestion. Hay is less toxic than fresh grass of comparable Mo content. This probably results from the fact that the Cu content of hay is more bioavailable, as is the case with silage. Measures to prevent or reduce the severity on molybdenum-induced copper deficiency can be applied to the animal directly and these include copper injections, copper oxide needles and soluble glass boluses, which release copper at a slow rate in the rumen or by copper supplementation of the diet or of the drinking water (Price, 1989).