The Effect of Applying Sodium Fertilizer on the Rate of Digestion of Perennial Ryegrass and White Clover Incubated in Rumen Liquor, with Implications for Ruminal Tympany in Cattle

C. J. C. PHILLIPS,* S. Y. N. TENLEP,* K. PENNELL,† H. OMED† and P. C. CHIY*

*Department of Clinical Veterinary Medicine, Madingley Road, University of Cambridge, Cambridge, UK †School of Agricultural and Forest Sciences, University of Wales, Bangor, UK

SUMMARY

A high herbage K:Na ratio increases the risk of ruminal tympany in cattle, which may relate to digestion rate.Experiment 1 examined whether in vitro digestibility of ryegrass was affected by NaCl fertilizer or by Naconcentration in artificial saliva. Fertilizer Na increased grass digestibility, but Na in artificial saliva decreasedit, probably due to the energy cost of sodium exclusion from bacteria. Increased herbage digestibility with fer-tilizer Na is therefore not due to additional Na, but may relate to increased water-soluble carbohydrates.

Experiment 2 examined whether NaCl fertilizer applied at 35 or 70 kg Na ha�1 to ryegrass and whiteclover affected in vitro gas production. Sodium fertilizer increased maximum gas output from grass and rateof production, confirming the increase in grass digestibility recorded previously, but in clover it had theopposite effect, thereby potentially reducing ruminal tympany in cows fed a high legume diet.

© 2001 Harcourt Publishers Ltd

KEYWORDS: Sodium; herbage digestibility; gas production; ruminal tympany; frothy bloat.

The Veterinary Journal 2001, 161, 63–71doi: 10.1053/tvjl.2000.0519, available online at http://www.idealibrary.com on

INTRODUCTION

Although a high K:Na ratio is implicated in factorsincreasing the risk of frothy bloat (Turner, 1981;Hall et al., 1988; Majak & Hall, 1990), the mode ofaction is unclear. Majak et al. (1995) attributed it tothe flocculation of chloroplasts, since potassium ismore effective than sodium in causing colloidal sus-pensions to aggregate (Gast, 1977). Ruminal chloro-phyll concentrations are high in bloated animals(Majak et al., 1985; 1986), and Majak and Hall(1990) suggested that the colloidal aggregation ofchloroplasts promotes rapid digestion. Alternatively,Kudo et al. (1985) suggested that an unknown

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Correspondence to: Dr C.J.C. Phillips, University of Cambridge,Department of Clinical Veterinary Medicine, MadingleyRoad, Cambridge CB3 OES, UK. Fax: +44 (0) 1223 33 0886; E-mail: cjcp2@cam.ac.uk

constituent in the bubbles’ membrane, derivedfrom chloroplasts, may be responsible. However,the high chloroplast density could be an artefact oflow ruminal liquid clearance rate accompanying alow sodium:potassium ratio in the forage and cor-responding low water intake (Chiy et al., 1993b). Insupport of this, low liquid clearance rates are char-acteristic in cattle that bloat (Majak et al., 1986).

Low rumen pH is an important risk factor forbloat (Cheng et al., 1998). Both applying sodium fer-tilizer to pasture and adding sodium compounds toconserved feeds increase rumen pH (Chiy et al.,1993b). The increased pH promotes the growth ofthe acetogenic bacteria at the expense of propi-ogenic bacteria (Wiedemeir et al., 1987) and conse-quently fibre digestion is enhanced (Chiy et al.,1994). Despite a decreased activity of propiogenicbacteria, cattle production is increased by usingsodium because of increased intake and rumen

© 2001 Harcourt Publishers Ltd

64 THE VETERINARY JOURNAL, 161, 1

Table IThe mineral content (g L−1) of artificial salivas

prepared with low, medium and high sodium concentrations in Experiment 1

Sodium concentration (mM)

79 (Low) 89 (Medium) 169 (High) NaHCO3 4.6 5.2 9.8 Na2HPO4 1.7 1.9 3.7 NaCl 0.47 0.47 0.47 KCl 0.57 0.57 0.57 MgCl2 0.06 0.06 0.06 CaCl2 0.04 0.04 0.04 NH2CONH2 0.9 0.9 0.9

turnover rates (Chiy & Phillips, 1991; Chiy et al.,1993a,b), which may increase the rate of gas produc-tion.

An alternative hypothesis for the beneficialeffect of high herbage Na:K ratio on bloat fre-quency is that there is a reduced production of gasduring bacterial fermentation. This may be due toindirect effects of sodium fertilizer on herbagecomposition, particularly the decrease in water-soluble carbohydrates in legumes, believed to arisefrom the energy cost of excluding sodium by natro-phobic species (Chiy & Phillips, 1999). Two experi-ments were, therefore, conducted to investigatewhether sodium fertilizer affected the digestionrate of perennial ryegrass (Lolium perenne) andwhite clover (Trifolium repens) by rumen bacteria.

MATERIALS AND METHODS

Experiment 1. Effects of sodium fertilizer on thedigestibility of herbage incubated with faecal bacteriain artificial saliva of different sodium concentrations Ten turves were dug from permanent pasture(909 g perennial ryegrass DM and 91 g white cloverDM kg�1 total DM). They were transferred to 5 Lplastic pots of surface area 0.04 m2 and kept in agrowth chamber with constant 20°C and 16 h lightand water provided daily for 69 days. After an initialharvest, five days after the start of the experiment,which was discarded, pots were allocated to eitherno sodium or sodium fertilizer treatments. Herbagein the no sodium treatment received 95 kg N/ha,applied as ammonium nitrate (Nitram, 345 g N,kg�1, ICI Fertilizers Ltd.), and 20 kg K ha�1, appliedas potassium chloride. Herbage in the sodiumtreatment received 95 kg N and 16 kg Na ha�1,applied as a compound fertilizer of ammoniumand sodium nitrates (Grazemore, 320 g N and 32 gNa kg�1, ICI Fertilizers Ltd), and 20 kg K ha�1,applied as potassium chloride.

After 17 days, the herbage was cut and dis-carded, and a second application of the same quan-tities of nitrogen and sodium fertilizers was madeto the two treatments. One further harvest wastaken and discarded after 17 days and a final har-vest was made after a further 30 days, by which timeit was assumed that the herbage had utilized thefertilisers that had been applied. The herbage fromthe final harvest was dried for 48 h at 90°C, milledthrough a 1 mm screen and the digestibility of theherbage samples was then determined by the artifi-cial saliva method (El Shaer et al., 1987; Omed etal., 1989).

Three samples of incubate were prepared by mac-erating 60 g fresh sheep faeces in 1 L of artificialsaliva with sodium concentrations of 79, 89 and169 mM (Table I), which represents the normalextremes of ruminal sodium concentrations (Mackieet al., 1984). The mixtures were filtered through adouble layer of muslin, which was then rinsed torecover as many of the micro-organisms as possible.The filtrate was flushed with CO2 gas for 20 min.

Three 180 mg replicates of each herbage samplewere placed in MacCartney bottles with 18 ml offresh incubate and sealed. The bottles were main-tained at 39°C for 48 h and then centrifuged at3000 r.p.m. for 30 min. The supernatant was dis-carded and the residues dried in an oven at 100°Cfor 48 h. The digestibility was then calculated usingthe following equation:

In vitro digestibility (g/kg DM) = [(180 – residue mass {mg}/180] � 1000.

Experiment 2. Effects of sodium fertilizer on the rateof gas production from herbage incubated in rumenliquor Preparation of feeds. An 18 m2 area of mixed peren-nial ryegrass (Lolium perenne) and white clover(Trifolium repens) sward was divided into 18 1 m2

plots. Twenty measurements of herbage heightwere made plot�1 with a grass meter (Bircham,1981), and the mean sward height at the start ofthe experiment was 5 cm. The plots were randomlyallocated to three fertilizer treatments: no fertilizer(treatment Nil), sodium fertilizer applied at35 kg Na ha�1 (treatment Low) or 70 kg Na ha�1

(treatment High). The fertilizer was applied usinga watering can on 16 May 1998, as sodium chloridedissolved in 10 L water per plot.

SODIUM IN HERBAGE AND RUMINAL TYMPANY IN CATTLE 65

Thirty-eight and 45 days after treatments wereapplied, samples of grass and clover were obtainedby hand-plucking from the middle of each plot.They were separated manually into leaves andstems, cut into 2–4 mm segments, and crushed witha pestle and mortar until individual leaves werefragmented, to simulate mastication by cattle.Fertilizer treatment replicates were amalgamatedfor a sub-sample of the clover and grass for analysisof sodium and potassium concentrations by atomicemission spectrophotometry. Preparation of the fermentation medium. Rumen liquorwas collected from three rumen-fistulated Suffolkrams, using a 50 mL syringe. Care was taken not tointroduce air into the syringe and the collectionstook place before morning feeding to ensure a con-sistent product (Menke & Steingass, 1988). Thesheep were fed a daily ration of 600 g of hay and 400g of a concentrate (CWG). The concentrate con-tained (g.kg�1) 300 barley, 180 wheatfeed, 150maize gluten feed, 75 soyabean meal, 75 rapeseedmeal, 50 sugar beet pulp, 50 molasses, 50 Megalac,30 fishmeal, 30 mineral/vitamin mix and 10 lard;mineral concentrations were 4 g Na/kg DM, 4 gCa/kg DM, 1 g Mg/kg DM and 2 g P/kg DM. Theliquor was immediately strained through a cheese-cloth to remove coarse particles, placed in aThermos flask that had been flushed with CO2 gasand immersed in a waterbath at 39°C.

Artificial saliva was prepared from distilledwater (474 mL), 237 mL of buffer solution (35 gNaHCO3, 4 g (NH4)HCO3 in 1 L distilled water),237 mL of macroelement solution (5.7 gNa2HPO4, 6.2 g KH2O4, 0.6 g MgSO4 7H2O in 1Ldistilled water), 0.12 mL of trace element solution(13.2 g CaCl2 2H2O, 10 g MnCl2 4H2O, 1 g CoCl26H2O, 0.8 g FeCl2 6H2O in 100 mL distilled water)and 1.22 mL resazurin solution (100 mg resazurinin 100 mL distilled water) (Menke & Steingass,1988), mixed with a magnetic agitator and sub-merged in a Woulff flask in a water bath at 39°C.CO2 gas was passed through the solution in thewater bath and a reduction solution (2 mL 1 MNaOH, 285 mg Na2S 7H2O in 47.5 mL distilledwater) added. When the mixture had changedfrom its initial blue to red and then clear, therumen fluid was added in the ratio 1:2, rumenfluid to artificial saliva (v/v). The addition of CO2continued for 15 min after the addition of therumen fluid.

Gas production measurements. Forty 100 mL gradu-ated syringes, each with a capillary attachment to a

50 mm silicon tube, were used for the determina-tion of gas production, which has been demon-strated to be an effective measure of the rate ofmicrobial digestion of a feed (Blümmel & Ørskov,1993; Khazaal et al., 1993). After placing a 1 gherbage sample in each syringe, the pistons weregreased with Vaseline, re-inserted and the tubeclamped with a clip on the silicon tube. Thesyringes were warmed to 39°C in an incubator, fol-lowing which 25 mL of the solution containingrumen liquor and artificial saliva was poured intoeach inverted syringe via the silicon tube. Air wasthen expressed from the syringe by inserting thepiston into the inverted syringe as far as it wouldgo, shaking to remove air bubbles. After readingthe piston height in each tube, the tubes wereinserted into a water bath at 39°C.

For the first sampling of grass and clover, read-ings of piston height were taken at the followingintervals: 0, 2, 4, 6, 8, 10, 12, 24, 35 and 45 h. Thefrequency of recording was increased for the sec-ond sampling by including recordings at 20 and22 h. The number of syringe replicates for each ofthe 12 treatments (three fertilizer treatments � twoherbages, grass and clover, � two fractions, stemand leaf) was three, giving 36 syringes/experiment.There were in addition for each gas productiondetermination four syringes with no herbage sam-ple to enable a baseline level of gas production tobe determined.

Statistical analysis

Experiment 1. The statistical significance of differ-ences between treatments was analysed by analysis ofvariance using a general linear model in Minitab,with sodium molarity, sodium fertilizer treatmentand the interaction between the two as factors.

Experiment 2. Several lines were fitted to the datafrom a sample of the syringes using the softwarepackages JMP, TableCurve and SlideWrite, includingthe conventional logistical growth curve (Ørskov &Macdonald, 1979) and a negative exponential curve.The negative exponential line was chosen as themost appropriate because 1) there was no lag phase;2) it gave a good fit for all treatments, with the high-est adjusted R2; and 3) it was the most parsimoniousexplanation of the data. This curve was then fitted tothe gas volume (y) from each syringe, taking theform logn y = a0 + a1 /t0.5, where t = the time of incu-bation, a0 = asymptote of logn (y) (i.e. the maximum

66 THE VETERINARY JOURNAL, 161, 1

gas production) and a1 = rate of change of logn (y)with respect to 1/t 0.5 (i.e. the rate of change of gasproduction over time). The mean adjusted r2 of the72 equations was 0.96 (s.e. 0.00026).

Individual values for a0 and a1 were tested fortreatment effects. Initially the distribution of thevalues for a0 and a1 was examined, using the And-erson–Darling normality test (Minitab, 1995),and the values for a0 were raised to the cubepower to achieve normality. A positive correlationbetween values for a0 and a1 was observed (r2 =0.65). A general linear model was then createdusing the statistical package Minitab (Minitab,1995) for individual a3

0 and a1 values, with har-vesting date, species, fertilizer treatment andplant fraction (leaf/stem) as factors and plots asreplicates. As there were no significant differ-ences in the treatment effects between the twoharvesting dates, the results presented are themean of the two dates.

RESULTS

Experiment 1 The cut herbage was predominantly grass, con-taining 954 g DM perennial ryegrass and 46 gDM white clover per kg herbage DM. The sodiumfertilizer increased the concentration of sodium

Table IIThe effects of the application of

sodium fertilizer on the mineral content ofherbages (n = 5/treatment) in Experiment 1

No sodium Sodium SED treatment treatment

Sodium (g kg−1 DM) 1.5 2.0 0.19*** Potassium (g kg−1 DM) 14.6 13.8 0.88Magnesium (g kg−1 DM) 4.2 4.4 0.39Calcium (g kg−1 DM) 15.0 14.8 0.68

***P<0.001.

TableThe effects of the application of sodium fertilizer on

ment), when incubated with faecal liquor of diff

No sodium treatm

Sodium concentration 0.079 0.089(mmol gL−1)

Dry matter digestibility (g kg−1 DM) 691 660 6

*P<0.05.

in the herbage and tended to reduce the potas-sium concentration (P<0.09) (Table II). Magne-sium and calcium contents were not affected bythe sodium fertilizer application. Herbage DMdigestibility increased as the sodium concentra-tion of the artificial saliva was reduced (TableIII). The herbage from the sodium treatment wasmore digestible than herbage from the nosodium treatment, but there was less effect at thelowest sodium concentration in the artificialsaliva.

Experiment 2 Sodium fertilizer increased the sodium content ofgrass and clover leaf and stem, particularly whenapplied at the High level (Table IV). The overalleffect of sodium fertilizer on the combinedherbages demonstrated that the normalised coeffi-cient of maximum gas production, a3

0, was reducedin proportion to the level of sodium applied, butsodium fertilizer did not affect the rate of gas pro-duction (a1) (Table V). However, the effect ofsodium fertilizer differed for grass and clover(Table VI). In grass the maximum gas production(coefficient a3

0) was increased by sodiumfertilizer, particularly at the Low level, and in theLow treatment the rate of gas production was

III the dry matter digestibility of herbage (n = 5/treat-erent sodium concentrations in Experiment 1

ent Sodium treatment

0.169 0.079 0.089 0.169 Interaction SED

11 714 714 665 9.3*

Table IVThe sodium concentrations (g kg�1 DM) of grassand white clover leaves and stems (n = 18/treat-

ment) after the application of Nil, Low or High lev-els of sodium chloride fertilizer in Experiment 2

Sodium application rate

Nil Low High

Grass leaf 1.6 3.1 3.6 Grass stem 1.9 3.4 4.1 Clover leaf 1.5 2.0 2.1 Clover stem 3.7 5.1 4.7

SODIUM IN HERBAGE AND RUMINAL TYMPANY IN CATTLE 67

Table VEffect of sodium chloride application on the nor-malized coefficient of maximum gas production

(a30) and the coefficient for the rate of gas produc-

tion (a1) from herbage samples (n = 24/treatment) incubated in rumen liquor in

Experiment 2

Sodium application rate

Nil Low High SED

(a0)3 72.7 70.4 68.4 2.39**

a1 �3.12 �3.07 �3.08 0.0332

**P<0.01.

increased compared with the Nil treatment. Inclover both the maximum gas production and therate of gas production were decreased by sodiumapplication.

The maximum gas production was greatest fromgrass leaves compared with grass stems, cloverleaves or clover stems (Table VII). The rate of gasproduction was less for clover than grass and lessfor stems than leaves.

TableEffect of sodium chloride application on the

production (a03) and the coefficient for the rate of

samples (n = 12/treatment) incubated

Grass

Nil Low High

(a03) 68.4 77.5 74.2

a1 �3.10 �3.35 �3.15 �

***P<0.001.

Table The normalized coefficient of maximum gas

rate of gas production (a1) from samples of grass incubated in ru

Grass Clov

Leaves Stems Leaves

(a0)3 77.9 67.1 68.8

a1 �3.27 �3.09 �3.13

***P<0.001.

DISCUSSION

The increase in in vitro digestibility when sodiumfertilizer was applied to the mainly grass sward inExperiment 1, and in gas production of the grassleaves and stems in Experiment 2 when sodium wasapplied, agrees with increases in DM digestibility ofpredominantly perennial ryegrass pasture afterbeing fertilized with sodium compounds, meas-ured by incubating herbage with faecal organisms(Chiy & Phillips, 1991; 1993). Sodium fertilizer alsoincreases grass digestibility when measured in vivowith sheep (Chiy et al., 1994). In the experimentsreported here, there was no additional benefit indigestibility of applying more than 32 kg Na/haduring the grazing season. The increase in grassdigestibility is likely therefore, in the light ofthe current experiments, to be attributable tochanges in herbage composition other than anyincrease in sodium content per se. This is becausein Experiment 1, as the sodium content of the arti-ficial saliva increased, so herbage digestibilitydeclined, whereas the application of sodium fertil-izer increased the digestibility of the predomin-antly grass samples. Previously it has beensuggested (Chiy et al., 1993b) that the increase inrumen pH when sodium fertilizer is applied, due to

VI normalized coefficient of maximum gas

gas production (a1) from grass and white clover in rumen liquor in Experiment 2

Clover

Nil Low High Interaction SED

77.1 63.4 62.7 2.39***

3.14 �2.79 �3.00 0.0332***

VIIproduction (a0

3) and the coefficient for the and clover leaves and stems (n = 18/treatment) men liquor

er SED

Stems Herbage Fraction Interaction

68.3 1.95*** 1.38*** 1.95***

�2.87 0.293*** 0.185*** 0.293

68 THE VETERINARY JOURNAL, 161, 1

recycled sodium, may be partly responsible forincreasing herbage digestibility. The increase insodium bicarbonate content of the saliva simulatedthis effect of sodium fertilizer, although the pH ofthe incubate was not monitored in Experiment 1,but digestibility was not increased by sodium in thesaliva. This suggests that this mechanism is not theprimary reason for the increase in herbagedigestibility. Conversely the increase in sodiumcontent of the herbage is unlikely to have increasedthe incubate pH, since the pH of the extracellularfluid is homeostatically regulated, yet this applica-tion of sodium increased digestibility. The consid-erable increase in water soluble carbohydratecontent when sodium fertilizer is applied to grass(Chiy & Phillips, 1993; 1999) is most probablyresponsible for the increase in digestibility whensodium is applied as a fertilizer. This increase inwater-soluble carbohydrates is believed to occur bystimulating the activity of ATP’ase at the tonoplast,which increases sucrose synthesis (Willenbank,1983) or by activating starch synthase (Hawkeret al., 1974). Previous work has demonstratedthat moderate applications of sodium nitrate(32 kg Na ha�1 year�1) increase herbagedigestibility and water-soluble carbohydrate con-centration compared to no sodium application, butthere was no further increase when a higher levelof sodium fertilizer was applied (64 kg Na ha�1

year�1) (Chiy & Phillips, 1993).The increased sodium supply per se could also

have affected gas production in the second experi-ment, in addition to changes in herbage digestibil-ity. This could arise from the increase in theenergetic cost of sodium exclusion from bacteria,which reduces their growth rate (Arney et al., 1998)by increasing their maintenance cost and reducingthe energy available for growth. Sodium could alsoalter the proportion of gases produced. It is specifi-cally required for the growth of methanogens andacetogens (Müller et al., 1990), and sodium supple-mentation could therefore reduce the ratio ofmethane to carbon dioxide. The impact of this ongas production and the risk of bloat in cattle isunclear, since the total substrate use may remainthe same. However, the sodium pump involves theloss of energy from the cell, and in a dynamic sys-tem such as the rumen may result in less efficientenergy utilization (Müller et al., 1990). A final pos-sibility is the direct involvement of sodium inmetabolic pathways. In support of this, sodiumdependent pathways in methanogens respond notto the presence of the sodium ions per se, but to an

electrochemical sodium gradient (Müller et al.,1990). Thus it is likely that there is not an absoluterequirement for sodium, but a response to a gradi-ent across the cell wall. For example, the activeinvolvement of the sodium symport mechanism inamino acid transport (Martin, 1995), might explainthe increase in rumen protein digestion whensodium fertilizer is applied (Chiy et al., 1994).

In Experiment 1, the greater increase in DMdigestibility when sodium fertilizer was applied tograss incubated with artificial saliva of the twohigher sodium concentrations may have been in-directly due to the increase in water-soluble carbo-hydrate concentration that occurs with sodiumfertilizer. At high sodium concentrations the rumenbacteria could benefit more from the extra water-sol-uble carbohydrates to provide energy for theirsodium symport mechanism in the cell wall, whichexcludes sodium from the cell (Stein, 1995). Thefact that digestibility was still increased by sodiumfertilizer at high artificial saliva sodium concentra-tions suggests that the additional sodium was nottoxic, but that there was a significant energy cost ofexcluding sodium from the bacterial cells. Thesodium requirements of rumen bacteria differ, buta reduction in growth of Ruminococcus albus, and toa lesser extent Selenomonas ruminantium, has beenobserved at 100 mM Na compared with 10 mM Na(Mackie et al., 1984), which were both less than thehighest level used in Experiment 1 (169 mM). Thesodium requirements of bacteria vary dependingon the function that sodium fulfils, but most relev-ant are the extensive use of the sodium symportmechanism for amino transport across the cell wallof most rumen bacteria and the use of the sodiumsymport for soluble carbohydrate transport in Fibro-bacter succinogens (Martin, 1995).

In Experiment 2, the reduction in gas produc-tion when sodium fertilizer was applied to cloverplants may relate to the substantial reduction inwater-soluble carbohydrate content that has beenobserved previously when sodium fertilizer isapplied to white clover (Chiy & Phillips, 1999).This is believed to arise from the high energy costto the nitrogen-fixing bacteria of sodium exclusion.

Mined sodium chloride is one of the few fertiliz-ers that is allowed on organic farms (UK Register ofOrganic Food Standards, 1989), where cows arelikely to graze mixed perennial ryegrass and whiteclover swards. The results of this and our previousresearch suggest that the use of sodium fertilizercould reduce the rate of clover digestion, whileat the same time increasing grass water-soluble

SODIUM IN HERBAGE AND RUMINAL TYMPANY IN CATTLE 69

carbohydrate concentration and rumen turnoverrate to allow high herbage intakes and productivity.Where the herbage contains sufficient legumes topresent a risk of ruminal tympany, sodium chloridefertilizer may reduce the legume degradation rate,but further studies are required to identify theexact mechanisms and confirm the effects in vivo.

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Book Review

The Mineral Nutrition of LivestockUnderwood, E.J. and Suttle, N.F. Wallingford, Oxon,CABI Publishing, 3rd ed., 624pp. £75 (hard) ISBN0851991289

The authors describe the aims of this book as‘meeting the needs of undergraduate and graduatestudents of nutrition in colleges of agricultural sci-ence, animal husbandry and veterinary science, ofteachers and research workers in animal nutrition,of agricultural and veterinary extension officers indeveloped as well as developing regions of theworld and of progressive livestock producers, wher-ever situated, who wish to apply modern scientificknowledge of mineral nutrition to their own enter-prises’. To this end the book is a success and inshort, there is something for everybody.

The appeal of the book is largely due to theclever way that it has been laid out. The first threechapters include a general introduction, a reviewof the natural sources of minerals and a review ofthe detection and correction of mineral imbal-ances in animals. These introductory chaptersare followed by chapters on individual mineralelements including calcium, phosphorus, magne-sium, sodium and chlorine, potassium, sulphur,cobalt, copper, iodine, iron, manganese, seleniumand zinc. The final three chapters review occasion-ally beneficial elements, essentially toxic elementsand the design of supplementation trials for theassessing of mineral deprivation.

The first three chapters of the book can be readwith both pleasure and considerable relief.Pleasure because the text is very easy to follow, withplenty of examples across species illustratingboth the importance and complexity of the subject

matter. Relief because the seemingly endless diffi-culties of applying the science of mineral nutritionin practice (which to many of us often representsinconclusive detection and inconsistent correc-tion), are clearly explained in ways which will helpboth veterinary practitioners and nutritional advi-sors to usefully re-think and re-direct their efforts.

Concepts such as ‘depletion, deficiency, dysfunc-tion and disease’ will do much to improve the vet-erinary practitioners understanding of thediffering degrees of mineral imbalance, and theirrelative importance to the animal. The authorsdevelop this model throughout the book to empha-size that ‘the precise sequence of events during thedevelopment of clinical disease varies widely frommineral to mineral’. Explanations relating to thestructural, physiological, catalytic and regulatoryfunctions of minerals are used, with examples,to review mineral imbalances generally, andindividually.

The authors comment that ‘numerically andeconomically, mild abnormalities now exceedsevere abnormalities in importance’, but that‘measurement of the total concentration of amineral in the pasture or ration cannot alwaysdetect or predict inadequacy or toxicity of thatmineral in the animal’. The book, therefore, con-siders the general principles that govern the choiceand effectiveness of procedures for the detectionand correction of mineral imbalances in farmanimals in its introductory chapters, before goingon to enlarge on the importance of this in thechapters on individual minerals.

Finally, the clinician is reminded that ‘many ofthe most obvious manifestations of severe mineralimbalances, such as subnormal growth, inap-petance, impaired lactation, poor reproductive per-formance, etc., occur to varying degrees withdeficiencies of a wide range of mineral elements’,

SODIUM IN HERBAGE AND RUMINAL TYMPANY IN CATTLE 71

and that ‘the ultimate criterion of any mineral inad-equacy, imbalance or excess is the correction of suchmanifestations that occurs in response to appropri-ate changes in the intake or utilization of the min-eral or minerals in question’. At last, an experiencewhich for most of us is both pointedly and depress-ingly familiar, is openly conceded by the experts!

For the general veterinary practitioner who feelsintimidated by the knowledge and resources of his

‘competitor’ from the feed company/consultancyfirm, this book provides both an enlightening andthoroughly satisfying framework from which to‘fight back’. For the more specialist advisor, thisbook represents an essential update.

I. CUMMING