Molybdate Topdressing and Animal Health

New Zealand Journal of Agriculture, Volume 90, Issue 2, 15 February 1955, Page 195

Molybdate Topdressing and Animal Health

By

I. J. CUNNINGHAM,

Superintendent,

Department of Agriculture Animal Research Station, Wallaceville-

MOLYBDENUM is unquestionably a trace element of special interest in New Zealand, because some areas are deficient in molybdenum while other areas have too much. . On the deficient areas molybdenum must be added as a fertiliser to grow good pastures or crops; on the excess areas there is so much molybdenum in the fodder that stock are poisoned. This excess may get into the grass because the soil is naturally high in molybdenum, or it may get there through careless or unnecessary use of molybdate fertilisers. Thus molybdenum has on the one hand possibilities of benefiting animal production by improving fodder production and on the other hand possibilities of doing harm by poisoning animals. THE main purpose of this article is to describe the harmful effects of molybdenum to stock and to show in what places and why molybdate fertilisers should not be used, but to make sure that the matter is kept in its proper perspective it is proposed first to discuss briefly the benefits which can result from the proper use of molybdenum. Molybdenum is an essential food for plants, though their requirements are very small. It is needed to convert nitrates into proteins and thus is necessary for plant growth. On soils that are deficient there will therefore be low production of pasture and low production of crops such as turnips and rape. The correct use of molybdate fertilisers on such deficient soils will greatly improve both pastures and crops. Molybdenum is an essential food for the bacteria which grow in the nodules on the roots of legumes and which are responsible for taking nitrogen from the air and converting it to a form suitable for use by plants. Fixation of atmospheric nitrogen by clovers is very important in . New Zealand grassland economy. Pastures depend in no small degree on the legume nodule to gather the nitrogen needed for growth. Clovers do not grow well on soil that is short of molybdenum and they res-

pond spectacularly to suitable topdressing with molybdates. . There is no doubt that proper use of molybdenum on deficient soils can improve clover growth, increase nitrogen supply for clover and for grasses growing in association, and raise production of pastures and crops. In other words, molybdenum can in some districts greatly increase the supply of food for stock and consequently enhance the stock carrying capacity of the land. W. R. Lobb, Instructor in Agriculture, Department of Agriculture, Oamaru, has calculated just what this may mean on the sandstone and clay soils of the lowlands ,of Waitaki County. He estimates an increase of 60 per cent. ..in stock carrying or a potential improvement of £500.000 annual gross farm income for that county alone. When all the deficient areas of New Zealand are added the total possible increase in primary production for the Dominion is likely to be very significant. s . Departmental Trials It is obvious that molybdenum is an important fertiliser which must be exploited to the full. Farmers -can be well assured that its 'use in New Zea-

land is being most actively studied. . The Extension Division of the De- . partment of Agriculture is carrying out comprehensive experiments to ... learn where molybdates should be used. Two methods are employed. In the first, experimental plots are laid down by agricultural instructors on known soil types and regular observations are made of growth response to molybdate, with and without lime, phos-

phate, and sometimes other fertilisers. Some hundreds of trials have already been laid down throughout the country and more are being laid down almost daily. The second method, at the Department of Agriculture’s Rukuhia Soil Research Station, is due to E. B. Davies and his colleagues, who have been prominent in molybdenum work from its beginnings. They have' devised a chemical test on soil by which they can diagnose adequacy or deficiency of molybdenum, and from the result of this test can give advice as to whether topdressing with molybdates is necessary. ? These two lines of work and studies made by other research groups are quickly building up in broad outline the picture of molybdenum response, in New Zealand. An article by E. B. Davies and J. L. Grigg in the December 1953 “Journal of Agriculture” covers existing knowledge. Responses in growth of pasture to molybdate topdressing have been observed in soils derived from greywacke and greywacke alluvium in Southland, Otago, Canterbury, Nelson, and Westland. In the . North Island responses have occurred on soils derived from greywacke on the Rimutaka, Tararua, and Ruahine Ranges and. on isolated areas as far north as the Bay of Islands. Some sandstones just south of Auckland and in Northland , are deficient in molybdenum, as is Te Kopuru sand in Northland. The interpretation is that it is desirable and economic to use'molybdate fertilisers on these soils. Effect of Lime Some other studies on molybdenum in soil are of direct interest here. Only a proportion of soil molybdenum is available to plants. Availability is greater near a neutral reaction in soil and decreases as soils become more acid. Liming brings acid soils nearer to neutrality and therefore in certain circumstances will increase available molybdenum. In Southland Davies has

noted that very high dressings of lime give unexpectedly high responses in pasture growth and he regards this as due to increases in availability of molybdenum. This effect of lime has attracted considerable interest and has led to the belief or hope that molybdenum topdressing will replace liming. The relation of molybdate topdressing to a liming programme therefore needs some further discussion. Lime is used to reduce soil acidity, to improve soil structure, to release some plant foods from the soil, to tie up certain substances harmful to plants, and to provide a plant food, calcium. Incidentally, reducing acidity of soils may increase availability of molybdenum and thus produce an effect greater than would be expected from lime alone. In some quarters this possible secondary effect of liming on molybdenum supply is being mistaken for the whole effect of liming. It is hoped that the use of ounces of molybdenum per acre will replace, the cost and difficulty of applying hundredweights or tons of lime per acre. Obviously this will not be true in the majority of cases, but in limited areas molybdenum can replace lime or reduce the amount of lime which must be used for pasture improvement. It is clear then that there are two very valuable - and economically important results from using molybdate fertilisers. The first is to increase pasture and crop growth. The second is to replace some of the lime needed for pasture improvement. These results are, of course, obtained only where a molybdenum deficiency occurs. Danger of Excess Molybdenum The whole story has a strong appeal to the imagination of farmers. . It is exciting to think that more grass can be made to grow by spreading something like a few handfuls of material over the whole of the farm, and it has an appeal to the pocket as well. Also it is not uncommon in farming circles to attribute production failures to lack of a trace mineral, and molybdenum is a trace mineral which appears to

do just the right things. The consequence is an urge to use molybdenum fertilisers at increasing rates and on any area irrespective of whether there is or is not a molybdenum deficiency. Unfortunately here comes the other side of the picture. If molybdenum is used in excess or if it is used on land that is already high in molybdenum, the harmful effect on stock may be very serious indeed. Poisonous effects from molybdenum have been observed in Britain, California, Florida, Ireland, Canada, and New Zealand. This article is confined to experience in this country. To make clear what happens here it is necessary to explain first that molybdenum and copper are antagonistic to each other in the animal body. Molybdenum is poisonous, but within limits copper neutralises this poisoning. If copper in the food is low, a comparatively small amount of molybdenum is poisonous, but if copper is normal, the amount of

molybdenum necessary to cause symptoms of poisoning is a great deal higher. The relative amounts of copper and molybdenum in the food are therefore predominantly important in determining the occurrence of molybdenum poisoning. An attempt is made to illustrate this in Fig. 1. This illustrates four sets of circumstances with the pasture copper shown as a clear column and alongside a black column representing the minimum amount of molybdenum which is poisonous in the same pasture. For a pasture with 10 parts per million (p.p.m.) of copper there must be 20 p.p.m. or more of molybdenum to cause poisoning. For lower levels of copper the minimum harmful level of molybdenum ,is lower. When pasture has 5 p.p.m. of copper molybdenum is toxic at only about 7 p.p.m., and when pasture copper is 3 p.p.m. molybdenum is toxic with only 3 p.p.m. These figures are not to be regarded as final, but as a general indication reached from field and experimental observations. Much of the knowledge of molybdenum poisoning in New Zealand relates to peat scours, which is caused by pasture the composition of which conforms to the two middle sets of symbols in Fig. 1. The copper in the grass is between 5 and 7 p.p.m. and molybdenum varies from 8 to 16 p.p.m., that is, just enough to be poisonous. The effect of this poisoning in stock is marked. Cattle scour severely and persistently in spring and sometimes in autumn. They lose condition and production falls. Young animals do not grow properly and bones are brittle and many have fractures from quite small knocks. The type of scouring is illustrated in Fig. 2. Proof that molybdenum is the cause of these symptoms follows from: — 1. The occurrence of a seasonal variation of molybdenum content of pasture with peak molybdenum content at seasons when scouring occurs. 2. The reproduction of symptoms by dosing molybdenum to cattle and

by grazing cattle on pastures topdressed to reproduce the levels of copper and molybdenum that occur in natural cases. However, molybdenum poisoning is not confined to peat scours. Continuing investigations have shown that in cattle and sheep too much molybdenum in the diet will wash out stores of copper and produce a copper deficiency. An example of what happens in sheep is given in Fig. 3, which summarises the results with a group of 12 ewes kept on grass with copper content of around 10 p.p.m. and dosed regularly with molybdenum so that the molybdenum intake represented about 30 p.p.m. of the food eaten. The dosing started in October 1945 and continued until 1950. The thick black line is the average amount of copper in the ewes’ livers, measured on small pieces of liver removed at different times. The steady drop in copper content of the ewes’ livers is clearly evident. The black stepped symbols indicate the copper contents in livers of newborn lambs; these are enclosed in a dotted column which shows the normal values for lambs’ livers at Wallaceville. Even in the early part of the experiment some of the newborn lambs from ewes dosed with molybdenum had low copper contents in their livers, and at the end of the experiment all such lambs had very low copper in the livers.

The same train of events occurs with cattle. Ten yearling heifers were kept on pasture containing about 10 p.p.m. of copper. Five of these were dosed with molybdenum to give about 30 p.p.m. in the food. At the end of 81 weeks the untreated cattle had on the average 96 p.p.m. of copper in the dried liver, whereas the dosed cattle had only 10 p.p.m. of copper in the liver. Dosing with molybdenum had obviously greatly reduced the amount of copper in the livers of these cattle. Investigations have also shown that diseases resulting from copper depletion and associated with excessive molybdenum in the pasture occur on other soils besides peat soils. On Omihi soil and other soils in North Canterbury ataxia in lambs occurs and Merino sheep lose wool crimp. The pastures contain about 5 p.p.m. of copper and about 9 p.p.m. of molybdenum. On pumice soils between Lake Waikaremoana and Wairoa young cattle are copper deficient, unthrifty, and show a marked loss of coat colour. The pastures have about 7 p.p.m. of copper and nearly 20 p.p.m. of molybdenum. A similar condition affects cattle in Raglan County. The appearance of the Wairoa cattle is illustrated in Fig. 4. All these circumstances show clearly that excess of molybdenum is

extremely dangerous to stock health. Great care must therefore be exercised in using this element as a fertiliser. However, a warning to be careful is not by., itself very helpful, and it is. necessary to be specific about: — 1. The areas where molybdenum content of pasture is so high that, molybdate fertilisers should on no account be used. 2. The areas where copper in pasture is low, because in these areas increases in molybdenum of pasture is especially likely to be harmful. 3. The areas where molybdate topdressing produces an abnormally high response in molybdenum content of pasture. In such areas a small dressing might convert a safepasture to a dangerous one. A certain amount of information on. the first two points had already been, accumulated, but this had to beexpanded to give a reasonable cover of the Dominion. To do this quickly and with the facilities available it was; necessary to have some basis for generalisation, and for this soil type was; selected. Samples' of pasture were collected from all the main soil types inNew Zealand in the spring of last year, since spring is the season for maximum level of molybdenum in pasture. Analyses of these pasturesfor copper and molybdenum show which soils grow pastures high in molybdenum and which grow pastures; low in copper.

The results of this work are discussed here. For measuring the response in pasture molybdenum to molybdate topdressing two methods were used. The first was to collect experience from a variety of topdressing trials with different rates of topdressing. The second was to obtain pasture samples from trial plots laid down by agricultural instructors to measure response in growth rate to molybdate dressings. From each trial a sample came from the untreated area, from the area topdressed with molybdate, and from the area topdressed with lime and molybdate. In assessing the results of surveys or of topdressing the following' criteria are used to judge the relation to animal health. Copper— Low: Below 5 p.p.m. Molybdenum— Below 3 p.p.m. Moderate: 3 to 10 p.p.m. (dangerous if copper low). High: Above 10 p.p.m. (potentially dangerous, even with normal copper). Pastures High in Molybdenum Table 1 lists soils which grow pastures high in molybdenum without any additions of molybdate fertilisers. This list is not to be regarded as complete, as further work in progress may add to it. On the North Island list Taupo silt and Taupo silt loam require some comment. Only the area of soil of these types near Wairoa is affected; the main bulk of Taupo soils produces a safe grass. The reason for this difference is probably a difference in origin of soils, despite close physical similarity. Locations of the soils included in Table 1 are indicated by the black patches shown in Fig. 5, except South Island peats, for which maps are not readily available at present. All of these soils produce pastures too high in molybdenum and on many the molybdenum is so high that stock disease

TABLE I—SOILS THAT GROW PASTURE WITH MORE THAN NORMAL MOLYBDENUM (N = normal; M = moderate; H = high)

North Island Soil classifi- Pasture molybdenum Soil name cation number Class p.p.m. Rotomahana sandy loam .. 3,3 H, 3a, 3aH M 4-9 Konoti clay loam .. . . 7bH M 4-6 Taupo light silt loam I ' ... Near Wairoa Taupo sandy silt I only H 13-19 Tutira sandy loam .. .. 49a N-M 1-4 Whanganiata sandy loam .. . 53 . . N-M 2-3 Hastings clay loam .. .. 63cL . . M , 5-7 Meeanee-Farndon complex .. 11l ' M 3-5 Ahuriri Lagoon soil .. - .. 112, 112 a, 112 b M-H 6-11 Taihape silt loam ... .. . 114 a N-M 1-7 Turakina silt loam .. .. 114 b N-M • 2-6 Waitaha sandy loam .. 114 c N-M . 2-4 Mahoenui silt loam .. . . ‘ 115 N-M 1-9 Pahiatua silt loam .. .. 115 a N-M 1-4 Hangaroa sandy loam .. 115 b N-M 1&-8 Waihua stony sandy loam • and sandy silt .. . . 117 d M 5 Peat soils .. .. .. 117 M-H 4-20 South Island Motukarara saline sandy loam and silt loam . . - M 5 Omihi silt loam .. .. , M-H 3-12 Waikakahi silt loam ... ’ N-M 1-8 Harihari fine sandy loam* .. M 5 Kini peat* ’ ...... M 4 Taitapu silt loam* .... M 4 Huihui sandy loam* .... —■ M 4 Peat soils .... .. .., . . .

is now occurring. Quite obviously molybdate topdressing should not be applied on any of these soils. Pastures Low in Copper Table 2 lists the soils that produce pastures low in copper and the location of these soils is shown in Fig. 6. TABLE 2—SOILS THAT GROW PASTURES OF LOW COPPER CONTENT North Island Soil classifi- Pasture Soil name cation number copper p.p.m. Te Kopuru sand .. 47d 1-5 Marsden sand .. 23c 3-8 . Whananaki, Patea, and Poxton sands 23b and 4-10 23 South Island Lismore silts .. —• 4-9 Moutere clay .. —■ 4-4.5 Glasnevin series .. 2.5-7 Oamaru complex .. . 3.7-5 Omihi silt loam .. 3-5 ♦Fairhall silt loam .. - • • 4 * More information required before final decision. For the most part these soils grow pastures with less than 5 p.p.m. of copper. But there is some variation, as is indicated by the figures shown in Table 2, and in some of the North Island sands (types 23 and 23b) many of the pasture samples have normal copper contents. These are shown in Fig. 6 as black areas with white dots, and the soils that nearly always produce low copper pastures are shown as unmodified black. The low copper is not necessarily enough on its own to cause stock disease, but it is so low that topdressing with molybdenum could well bring on trouble in animals. Clearly molybdate fertilisers cannot be used on any of these soils without risk. If molybdenum must be used to secure good pasture growth, copper fertilisers would have to be applied at the same time to protect stock health. Pastures Showing Abnormal Responses to Molybdate Topdressing Pastures on different soils respond differently in molybdenum content to a topdressing with molybdate. This is illustrated in Table 3, which shows, for a variety of soils, the molybdenum content of pasture at different intervals after dressings of different amounts of molybdate. It is apparent from this table that pastures on different soils respond differently in molybdenum content to topdressings with molybdate. For example, 12 months after treatment with 4oz. per acre pastures on Kauru silt loam were six times as high in molybdenum as untreated pastures from the same soil; and pastures on

Lismore silt loam were only 4 times as high as untreated pastures a few months after topdressing with 6oz. per acre. Pastures on Waiareka clay loam were no higher in molybdenum than treated pastures 24 months after topdressing with 2|oz. per acre, whereas pastures o n Crownthorpe sandy loam were definitely higher 17 months after a similar treatment. Pastures on Heretaunga mottled silt loam had nearly 19 p.p.m. of molybdenum 3 months after a dressing of Boz. of molybdate per acre, and pastures on Ware-pa-like silt loam at Invermay contained only 3.8 p.p.m. from the same topdressing after the same interval. Some pastures reach a high level of molybdenum from comparatively small dressings of molybdate. Two ounces of molybdate per acre raised pastures on Heretaunga mottled silt loam to 6.9 p.p.m., and three dressings of 2oz. within 2 years raised Flock House sandy soils to 11 p.p.m. This dressing of three lots of 2oz. in 2

years is higher than would ever be recommended but not higher than might be applied by careless farmers. Table 3 thus shows two things. High or repeated dressings with molybdate will raise pasture molybdenum to dangerous levels, and responses on different soils are very different. Because of these different responses it was necessary to make measurements on a number of soil types to learn which are the soils on which abnormally high response in pasture molybdenum occurs after molybdate topdressing. A content of 3 p.p.m. or more of molybdenum in pasture 3 months after .topdressing was regarded as. a high response.

TABLE 3—EFFECT OF TOPDRESSING WITH MOLYBDATE ON MOLYBDENUM CONTENT OF PASTURE Soil Topdressing Molybdenum Months since p.p.m. in dry grass Rate/acre application Untreated Topdressed Peat .. J. .. .. soz. once 14 2-3 27 141 b. once 14 2-3 300 Sand at Flock House ... 2oz. three times 24 since first Less than 0.5 11 Crownthorpe sandy loam .. 2Joz. once 17 1.2 2.0 Kauru silt loam .. .. -4oz. once 12 Less than 1 6 Waiareka clay loam ... .. 2|oz. once 24 Less than 1 Less than 1 Lismore silt loam .. .. 6oz. once 3 0.5 2 Heretaunga mottled silt loam loz. once 3 1.4 4.3 (Wallaceville) 2oz. once 3 1.4 6.9 4oz. once 3 1.4 12.2 Boz. once 3 1.4 18.6 Warepa-like silt loam loz. once 3 1.0 1.4 (Invermay) 2oz. once 3 1.0 1.1 4oz. once 3 1.0 2.6 Boz. onee 3 1.0 3.8

Information on . this aspect, as already explained, was obtained by examining pasture samples from agricultural instructors’ trials. Circumstances made it impossible to get samples from more than about half of the trials. These included 85 trials in the North Island and 70 trials in the South Island. A summary of the results is shown in Table 4. TABLE 4—MOLYBDENUM IN PASTURE SAMPLES FROM INSTRUCTORS’ TRIALS North Island Trials . . .... .. .. 85 Pasture molybdenum above normal .. 10 Those low in copper .. .. .. 4 Soils which gave high response Waipunga silt loam Ohinepanea sand Marsden sand and peaty sandy loam Ohakune silt loam Moumahaki sandy loam Otane Wainuku sandy silt Tarawera gravel Esk sand Rissington sandy loam Te Kopuru sand South Island Trials .. .. . . .. .. 70 Pasture molybdenum above normal .. 1 Soil which gave high response Motukarara weakly saline sandy loam The first important point shown in this table is that North Island soils respond more in molybdenum content of pasture to molybdate dressings than do South Island soils. Ten out of 85 North Island soils gave pastures with molybdenum at 3

p.p.m. or higher 3 months after topdressing. There were 12 other soils on which pasture molybdenum was high, but on these topdressing had been done less than 3 months before sampling and judgment is reserved until these plots are resampled. In the South Island only one out of 70 trials gave an abnormally high response and this was on a soil high in molybdenum that produces high molybdenum pasture even without topdressing. The distribution of the 10 North Island soils in Table 4 is mapped in Fig. 7. The South Island soil is not mapped because it is included among the soils in Fig. 5. Of the 10 North Island trials that gave a high response in molybdenum, 4 were in pastures that are low in copper content. In these pastures a rise of molybdenum is more likely to be dangerous.

On only one of the soils, was the pasture growth so improved by molybdate topdressing that the instructor would recommend using molybdate fertiliser. This case is Te Kopuru sand, but this is an important case, for copper in the pasture is very low. The molybdenum content of the pasture was 3| p.p.m. 9 months after applying 2Joz. of molybdate per acre and the copper was 3.1 p.p.m. On pasture of such composition copper deficiency and molybdenum poisoning could occur in grazing stock. On all the soil types in Table 4 molybdate fertilisers must be used with care. It appears that the 2joz. per acre used in these trial plots is too high and that on these soil types and indeed on the North Island in general a lower level should be employed to guard against stock disease. For soil types like Te Kopuru sand where low pasture copper accompanies high response to molybdate

dressing the only safe procedure would be to combine a topdressing of copper with molybdate topdressings. On the basis of results discussed above it is possible to make specific comments on the use of molybdate fertilisers. .. 1. All the soil types listed in Table 1 and shown in Fig. 5 at present produce pastures above normal in molybdenum content. Topdressing with molybdate should be unnecessary and would. further raise pasture molybdenum with possibility of harm to stock health. On these soils molybdates should not be used. 2. The soil types listed in Table 2 and shown in Fig. 6 produce pastures low in copper content. Topdressing with molybdate is likely to raise molybdenum content of grass, and a raised molybdenum content of grass with low copper is especially dangerous to stock. If molybdate topdressing is necessary to improve pasture growth, copper topdressing at 51b. of bluestone per acre should be applied at the same time to protect stock. 3. The soils listed in Table 4 and shown in Fig. 7 give abnormally high responses to a topdressing of 2|oz. of molybdate per acre. Molybdate should not be used at this rate on these soils without concurrent use of copper. 4. The high response to molybdate dressings in a big proportion of the North Island trials (Table 4 and discussion of it) suggests that 2|oz. per acre is too high for the North Island. It is probable that a lower rate of topdressing would not produce sufficient increase in pasture molybdenum to cause any concern about stock health and yet would produce adequate growth responses in pasture. Responses in the South . Island are not nearly as high and on most soils of that island a rate of topdressing with molybdate higher than in the North Island could safely be employed. (The lists of soils commented on in 1,2, and 3 have been arrived at from a year’s work only. These lists may be extended by further work proceeding at present. The results so far obtained are presented, however, as even in the present form they offer some assistance in reaching a policy on the use of molybdate fertiliser.) 5. The abnormally high responses on some soils to small rates of molybdate topdressing and the marked response to higher single dressings or repeated dressings (Table 3) point very clearly to the need to adhere closely to recommended rates of molybdate. Despite the discouraging outlook that these results might engender, they should not in any way be interpreted as suggesting that molybdate is unsafe where a deficiency exists. Molybdenum is a very valuable fertiliser and its correct use can be of great value. Careless and irresponsible use on the other hand can be harmful. This article shows where molybdates should not be used. Agricultural instructors in each district can, if necessary, give farmers information on the soil types on their farms and can tell them where molybdate should be used and the amount it is safe to employ.

Dual-purpose Gate for Woolsheds THE dual-purpose gate -illustrated has been designed by Mr. P. D. Roberts, of Pleasant Point, South Canterbury, for use in woolsheds. The gate can be used as a swing gate or as a lift-up gate, for which use a counterweight is attached to rope A, which runs over an overhead, pulley. . ■ '. In place of hinges two flat pieces of iron, B, are bolted to the gate with the outside ends welded to two short pieces of pipe, C, with an outside diameter of 2in. The pieces of pipe are slipped over a length of pipe, D, with an outside diameter of l|in. D is. supported by two small blocks of wood, E, each about 2in. x 2in. x 6in. On the under side of upper E and the upper side of lower E are fixed flat pieces of iron, each with a small stud attached, which, fit inside D. The size of the gate and the length of D can be made to suit individual requirements.

—W. J. HANSEN,

Sheep and Wool Instructor,

Department of Agriculture, Dunedin

* Further data required before soils can be included as types that produce high molybdenum pasture. .. ■ ' >