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Nutrient Elements in Ruminant AnimalsChapter 4
Chapter 4
Nutrient Elements in Ruminant
Animals
Outline of Digestive Physiology in Ruminant Animals
In contrast to most other animals, ruminants have a complex stomach, which consists of four compartments, the rumen, reticulum, omasum and abo- masum (Fig. 4.1). However, the rumen and the much smaller reticulum are not clearly separated, and are often referred to jointly as the reticulorumen or sometimes just as the rumen. This has a capacity of about 100–200 l in dairy cattle and about 8–10 l in sheep. The rumen contains a large population of microorganisms, mainly bacteria but with some protozoa and anaerobic fungi, and these various microorganisms secrete enzymes that carry out a partial digestion of the animal’s food. Digestion in ruminants thus has two important stages. The first stage is effected by the rumen microorganisms, while the second stage, in the abomasum and intestines, is similar to digestion
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Oesophagus
Rumen
Reticulum
Omasum Abomasum (^) Duodenum
Small intestine
Large intestine
Rectum
Fig. 4.1. Diagram of the digestive system of a ruminant animal.
in simple-stomached animals, and is effected by enzymes secreted by the animal itself. In the second stage, the substrates for the enzymes consist of dietary material, which has been partially digested, plus microbial biomass which has been synthesized in the rumen. As well as enabling ruminants to utilize diets that consist entirely of rather fibrous material, the two-stage process of digestion has additional advantages. One is that the micro- organisms in the rumen synthesize certain specific nutrients, such as some B vitamins and individual amino acids, which may be inadequate in the diet. A second advantage is that the microorganisms also incorporate non-protein forms of N and S into the proteins of their biomass, enabling these forms of N and S to be utilized in the second stage of digestion (Miller, 1979). The digestion process begins with the food being chewed and mixed with saliva. Ruminant animals produce large amounts of saliva, about 150 l day−^1 in cattle and about 10 l day−^1 in sheep (McDonald et al ., 1995), which facilitates swallowing and also contains ions such as bicarbonate and phos- phate, which buffer the pH of the rumen contents within the range 5.5–7.2. In addition, saliva provides nutrient elements for the rumen microorganisms in forms that are readily available (Miller, 1979). Of the soluble compounds produced in the rumen during the first stage of digestion, some are absorbed directly from the rumen into the bloodstream, while others are incorporated into the microbial biomass. After leaving the rumen, the partially digested food passes to the omasum, where much of the water is removed, and then into the abomasum or true stomach. Gastric juice containing hydrochloric acid is secreted, and this reduces the pH of the digesta to about 2.5, as required for the action of the digestive enzyme, pepsin. In the next portion of the digestive tract, the small intestine, the acidity is neutralized by secretions from the pancreas and liver, and partially digested proteins are hydrolysed to amino acids. Enzymes are secreted into the small intestine from the pancreas, gall-bladder and the walls of the intestine, and many of the soluble products of digestion are absorbed into the bloodstream. There is little further digestion in the large intestine, though water and bicarbonate are absorbed from this portion of the tract. Although, for many of the nutrient elements, absorption occurs mainly from the small intestine, for some elements, the relative importance of different absorption zones is difficult to establish, because substantial amounts are secreted at various points between the mouth and the large intestine (Miller, 1979). Material that remains undigested after passing through the large intestine is voided as faeces and, of this, about 10–20% consists of living and dead microbial cells (Marsh and Campling, 1970). The extent to which any particular type of plant material is digested by ruminant animals is influenced by the relative amounts of cell-wall fraction and cytoplasm. Plant cell walls, which are composed mainly of cellulose, hemicellulose and lignin, but with small amounts of protein and, in grasses, silica, have a relatively low digestibility. In addition to having a low digest- ibility themselves, the plant cell walls, especially the lignin and associated
Nutrient Elements in Ruminant Animals 71
Zn, Cu, Mo, Se) or as cofactors, which are required for the activation of enzymes, though I and Co are exceptions. Iodine is essential as a constituent of the metabolically important hormones triiodothyronine and thyroxine, and Co is essential as a constituent of vitamin B 12 , and these appear to be their only functions (Underwood and Suttle, 1999). The dietary requirement for a nutrient element can be regarded as having separate components for maintenance, for live-weight gain, for pregnancy and for lactation. The maintenance component reflects the amount of the nutrient needed to keep the tissues of the animal intact and functioning. It includes the needs for metabolic processes, such as digestion, circulation of the blood, respiration and the muscular movement of the animal. Although nutrient elements, released in soluble forms by digestion and other metabolic processes, are recycled to some extent, there are inevitable losses, through the intestines, kidneys and skin, which it is necessary to replace. For most nutrient elements, the main loss of this type occurs through the intestines, and represents part of the excretion in the faeces (Underwood and Suttle, 1999). The other components of the dietary requirement, for live-weight gain, pregnancy and lactation, apply only to appropriate categories of animal,
Nutrient Elements in Ruminant Animals 73
Ruminant animals (McDonaldet al.,
Cattle (Miller, 1979; Milleret al., 1991)
Cattle (Agricultural Research Council, 1980)
N (%) P (%) S (%) K (%) Na (%) Ca (%) Mg (%) Cl (%) Fe (mg kg−^1 ) Mn (mg kg−^1 ) Zn (mg kg−^1 ) Cu (mg kg−^1 ) Co (mg kg−^1 ) I (mg kg−^1 ) Mo (mg kg−^1 ) Se (mg kg−^1 )
a (^) 2.8 a
20– 0.2–0. 10– 1– 0.02–0. 0.3–0. b (^) 0.1–4.0 b c (^) 0.1–2.0 c
20–
< 0.06<
d (^) 0.2–0.5 d
a (^) Assuming 20% fat in live weight. b (^) Taking into account Mills and Davis, 1987; Pais and Benton Jones, 1997. cTaking into account Pais and Benton Jones, 1997. d (^) Confirmed by Ullrey, 1987.
Table 4.1. Typical concentrations reported for the nutrient elements in the body tissue of ruminant animals, excluding digesta (% or mg kg−^1 in live weight).
and reflect the amounts of the nutrient element incorporated into the weight gain, fetus or milk. All the nutrient elements can have harmful effects on animals if they are present in the diet in excessive concentration, but, in practice, this problem is restricted to N, K and a few of the micronutrients. Although excessive concentrations of N causing metabolic disturbances in animals are unusual, it is possible for them to occur in herbage when high rates of fertilizer N are applied. Excessive concentrations of K in herbage, which occur only when high rates of fertilizer K are applied, are liable to impair the absorption of dietary Mg. With the micronutrients Cu, Mo and Se, excessive concentra- tions in herbage are usually restricted to localized areas, and are due either to geochemical factors or to industrial pollution. For optimum nutrition, it is clearly important that the supply of each nutrient element in the diet should be adequate but not excessive. However, it is often difficult to assess whether a particular diet supplies an adequate amount, as, with many of the elements, a slight to moderate deficiency results only in a slower rate of growth or a reduced amount of milk or wool production, but no visible symptoms. Another problem is that for some elements, particularly Ca, it is important for the animal to maintain a reserve, which can be used when the physiological need is greatest, e.g. during lactation. Also, although the requirement for some nutrient elements increases during pregnancy and lactation, these physiological conditions often result in an increase in the extent to which the element is absorbed from the digestive tract, and they do not necessarily require a corresponding increase in the concentration in the diet. A number of the micronutrient elements (Fe, Mn, Zn, Cu, Co, I, Se) apparently influence processes that are involved in reproduction, and deficiencies or imbalances of these elements may result in a failure of animals to reproduce (Hidiroglou, 1979). There is also evidence that some micro- nutrient elements, particularly Cu and possibly Co and Se, may be involved in maintaining the animal’s resistance to infectious diseases (Suttle and Jones, 1989; Spears, 1991).
Homeostatic Control of Nutrient Elements in
Animal Tissues
The concentrations of the nutrient elements in animal tissues and in blood, at least in the forms which are metabolically active, are maintained relatively constant, whatever the composition of the diet. For any particular element, the constancy is maintained by homeostatic control mechanisms, which modify the inputs to and outputs from the body fluids. The various mecha- nisms enable an animal to adapt to changes in diet and to changes in its requirement for a particular nutrient due to its physiological state. In general, the nutrient elements are conserved when they are in relatively short supply,
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prevent a change in the pH of the body fluids, with possible harmful effects. Variations amongst the ions in the extent of absorption and metabolism make it difficult to calculate accurately the effective dietary cation–anion difference (DCAD), which is usually expressed in terms of mEq 100 g−^1 , but a number of formulae have been proposed (Roche et al ., 2000). These include:
DCAD = (K + Na) − Cl DCAD = (K + Na) − (Cl + SO 4 ) DCAD = (K + Na + 0.15 Ca + 0.15 Mg) − (Cl + 0.5H 2 PO 4 + 0.25SO (^) 4).
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Degree of importance
Major Moderate Minor Little or none
Change in % absorption Ca Fe Mn Zn
P Cu
N Mg Se
K Na Cl I Mo
Change in endogenous loss in faeces P Mn Zn
I S Co Se
Change in loss in urine N S K Na Mg Cl I Mo Se
Ca Co
P Mn Cu Zn
Change in tissue deposition Ca Fe Cu Mo
I Na Mg Mn Zn Co Se
N P S
Change in secretion in milk I Mo Se
Zn N S Mn Cu Co Se
P K Na Ca Cl Fe
Table 4.2. Homeostatic mechanisms involved in adaptation to varying intakes of nutrient elements (based on Miller, 1979, with other data).
A recent study with dairy cattle in Australia showed that only when the DCAD was less than +15 mEq 100 g−^1 for a period of about 3–4 weeks was there a substantial decrease in urine pH (Roche et al ., 2000).
Concentrations of Nutrient Elements in Animal Tissues
and Milk
In contrast to the usual practice with plant materials, concentrations of the nutrient elements in animal tissues are generally expressed on the basis of live weight. Concentrations reported on this basis as typical for the body tissue of ruminant animals are shown in Table 4.1. The nutrients having the highest concentrations in body tissue and therefore present in the greatest amounts in animal live-weight gain are N, P and Ca. Beef cattle, grazing at a stocking rate of 5.5 animals ha−^1 for approximately 6 months, might show a total gain of 1000 kg live weight ha−^1 , which would remove about 27 kg N, 8 kg P and 14 kg Ca from the system. Typical concentrations of the nutrient elements in cows’ milk are shown in Table 4.4. Some elements show a considerable range, due to the concentration changing in response to factors such as the stage of lactation, the nutrition of the animal and disease. Also there may be some contamina- tion of milk by Fe and Cu from dairy equipment, and by I through the use of iodophors for udder washing or for cleaning equipment (Renner et al ., 1989). The concentrations of many, but not all, of the nutrient elements in milk are maintained almost constant during periods of deficiency, though yields of milk may be reduced. The elements that show little change in concentration include P, Ca, Na and Fe, while the concentrations of Cu, I and Se in milk decline markedly with deficiency (Underwood and Suttle, 1999). The con- centrations of I and Mo can be increased above the normal range by high intakes in the diet. Nitrogen is the nutrient element present in greatest amount in milk, at a concentration four to six times greater than those of P, K, Ca and Cl, which are present in roughly similar amounts (Table 4.4).
Nutrient Elements in Ruminant Animals 77
Input Reserve Output
Dietary intake (g day−^1 )
Absorbed intake (g day−^1 )
Total reserve (g)
Available reserve (g)
In faeces and urine (g day−^1 )
In milk (g day−^1 )
Ca Mg K Na
6000 175 820 700
Table 4.3. Typical daily inputs and outputs (g day−^1 ), and reserves, of Ca, Mg, K and Na in a high-yielding dairy cow (from Payne, 1989).
comparison with other body tissues, wool is particularly high in S and is also high in Zn, but has relatively low concentrations of P, Ca and Mg.
Animal : Plant Concentration Ratios
Differences between animals and plants in their requirements for nutrient elements are reflected to some extent in the concentrations in their respective biomass, though, in both plant and animals, there is some variation with the type of tissue. For example, leaves differ from stems and muscle differs from liver. However, when whole plant material is compared with the whole-body tissue of animals, all the elements show a much wider variation in plants than in animals. In calculating animal : plant concentration ratios, it is necessary to express the animal concentrations on a DM basis, and the concentrations shown in Table 4.6 were calculated from the data in Table 4.1, assuming an average water content in the animal body of 70%. The content of water is rather more than 70% in young animals, and is less in fully mature animals (Naylor, 1991). Animal : plant concentration ratios, based on typical con- centrations in grass herbage material and typical concentrations in the whole-body tissue of ruminants, as in Table 4.6, provide some indication of whether or not a particular nutrient is preferentially retained by the animal. Amongst the nutrient elements, the greatest preferential retention is shown for Se, Ca, I and P, which have animal : plant ratios of > 5 : 1, while the greatest discrimination is shown against Mn and B, which have animal : plant ratios of 0.2 : 1 or less.
Nutrient Elements in Ruminant Animals 79
(i) (ii)
N (%) P (%) S (%) K (%) Na (%) Ca (%) Mg (%) Fe (mg kg−^1 ) Mn (mg kg−^1 ) Zn (mg kg−^1 ) Cu (mg kg−^1 ) Co (mg kg−^1 ) B (mg kg−^1 ) Mo (mg kg−^1 )
Table 4.5. Concentrations of nutrient elements (% or mg kg−^1 ) reported in sheep wool (washed, and with moisture content of about 5%): (i) 50 samples from USA and Australia (Burnset al., 1964); and (ii) three samples from New Zealand (Grace and Lee, 1992).
Absorption of Nutrient Elements from the Diet
The nutrient elements present in herbage may be present in one of three broad categories. They may be present in organic compounds of high molecular weight that are insoluble in water, such as proteins, nucleic acids and pectin; they may occur in soluble compounds of low molecular weight, such as amino acids and metallo-organic complexes; or they may be present as inorganic ions in solution. The first category is not absorbed through the digestive tract, the second is absorbed to a variable extent and, in general, the third is absorbed readily. With most elements, there is some conversion from one category to another during digestion, and the extent of absorption is therefore influenced not only by the form in which the element occurs in the diet, but also by the extent to which it is converted, within the digestive tract, to other forms, which may be either more or less soluble. The mechanism of actual absorption through the wall of the digestive tract varies among the elements. Those present in the digesta as monovalent ions, e.g. K, Na and Cl, are absorbed readily by simple diffusion, whereas the absorption of others, especially those occurring as trivalent or divalent cations, depends on metabolically active transport, often involving special carrier proteins (Miller, 1979). The extent of absorption of those elements depending on metabolically active transport is influenced by a range of
80 Chapter 4
Typical concentration in plant DM
Typical concentration in animal body DMa
Animal : plant concentration ratio (x : 1)
N (%) P (%) S (%) K (%) Na (%) Ca (%) Mg (%) Cl (%) Fe (mg kg−^1 ) Mn (mg kg−^1 ) Zn (mg kg−^1 ) Cu (mg kg−^1 ) Co (mg kg−^1 ) B (mg kg−^1 ) Mo (mg kg−^1 ) I (mg kg−^1 ) Se (mg kg−^1 )
a (^) Derived from data in Table 4.1 assuming live weight to contain 30% DM.
Table 4.6. Typical animal : plant concentration ratios, based on grass herbage and ruminant animals.
The determination of the apparent absorption of an element is straight- forward, requiring only the dietary intake and the amount excreted in the faeces to be measured, though to be accurate, it is necessary for the measure- ments to be continued for a period that is long enough to allow for fluctua- tions in body reserves. The determination of true absorption, sometimes regarded as availability (e.g. McDonald et al ., 1995), is more complicated. This is because, with most, if not all, elements, there is some endogenous excretion into the digestive tract and, as a result, the faeces contain variable amounts of elements that have been absorbed, metabolized and excreted, as well as the proportion that has not been absorbed. An additional complica- tion is that there may be some reabsorption of an element after its excretion (or secretion) into the digestive tract (Suttle, 1987; Underwood and Suttle, 1999). Distinguishing the endogenous fraction of an element that is absorbed and then excreted in the faeces ( Fz above) from the fraction that is present in the undigested portion of the diet is difficult. One way of assessing Fz is to measure faecal excretion on a diet free of the element, so that endogenous excretion ( Fz ) then equals total excretion ( F ). For some elements, a second means of assessing endogenous excretion is to use a radioisotope technique, which can differentiate between the unabsorbed fraction in the faeces and the fraction that has been absorbed and re-excreted, for example, by comparing two sources of the element labelled with different radioisotopes. More detail is provided in a recent review (Underwood and Suttle, 1999). Many of the estimates that have been reported for availability are of apparent absorption and are therefore lower than the true absorption. Even when true absorption has been assessed, the values obtained will have been influenced by the various factors described above. For these reasons, it is doubtful whether ‘availability’, assessed by absorption, can be regarded as an attribute of the particular diet (Suttle, 1987). Certainly, for absorption to approximate to potential availability, it is important that the animals should have a substantial physiological requirement for the nutrient element being studied, due to growth or lactation or to the element being marginally deficient. Another approach that has been used in assessing the availability of nutrient elements in herbage involves placing samples of the herbage in mesh bags of a non-degradable material, such as nylon, and incubating them either in an animal’s digestive system or in conditions that simulate one or more stages of digestion. In one study of this type, samples of several grasses and lucerne were incubated first in the rumen of a cow for 24 h and then in an acid–pepsin solution, simulating the abomasum, for 1 h, after which they were inserted into the duodenum (via a cannula) and subsequently collected in the faeces. The total release of P during this procedure ranged from 84 to 98%, while the release of K was 100%, the release of Ca was (with one excep- tion) 72–91% and the release of Mg was 86–95% (Emanuele et al ., 1991). Despite some nutrient elements being associated with the cell walls or other insoluble components of plant material, all the elements except Mn appear to
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be solubilized to a large extent in the rumen, suggesting that permanent retention by plant components is normally not a major factor limiting absorption. However, after solubilization in the rumen, a nutrient element may form less soluble products with other constituents of the diet and/or with microorganisms, and these interactions may result in some curtailment of absorption (Spears, 1991).
Assessment of the Dietary Requirements of
Nutrient Elements
The minimum requirement for a nutrient element is usually defined as the intake needed to maintain a normal physiological concentration of the element in the bloodstream and/or to avoid any impairment of its essential functions in the body (Suttle, 1986). As indicated above, the required concen- tration in the diet may be influenced by interactions with other constituents of the diet; and one important example of this type of interaction is the increase in the requirement for Cu caused by high concentrations of Mo and S (see p. 247). In general, two approaches can be used to assess the dietary requirement for a nutrient element: (i) feeding trials in which graded amounts of the element are present in, or added to, the diet, and (ii) calculations, for a particular class of livestock, involving a factorial approach based on the needs for particular physiological functions (Grace, 1984; Underwood and Suttle, 1999). In making an assessment based on feeding trials, it is necessary to relate the dietary supply of the element to measurements of live-weight gain or of some aspect of body composition, or to the appearance (or non- appearance) of characteristic symptoms. However, any feeding trial has the disadvantage that the results apply precisely only to the type of animal and conditions of the trial: to obtain results that apply widely, trials must be repeated for various classes of livestock under a range of conditions. The factorial approach to assessing the requirement for a nutrient element involves adding together the components for different physiological needs, i.e. for maintenance and for live-weight gain, fetal development and/or lactation and/or wool growth, as appropriate (Price, 1989; Grace and Clark, 1991). The component for maintenance at a constant physiological state is equal to the inevitable endogenous loss in faeces and urine that occurs even when the intake of the nutrient element is extremely low. The components for live-weight gain, fetal development, lactation and wool growth are derived from the concentrations of the element in these products. Summation of these various items then provides the total net requirement. However, as most nutrient elements are not completely absorbed, it is necessary to apply an absorption coefficient in order to convert the net requirement to the dietary requirement. The dietary requirement can then be converted to a dietary concentration on the basis of a known or an assumed intake of DM
Nutrient Elements in Ruminant Animals 83
Occurrence and Diagnosis of Deficiencies in
Ruminant Animals
When grassland receives fertilizer applications that produce a high yield of herbage, the concentrations of N, P and K in the herbage are normally more than sufficient to meet the requirements of animals that are grazing. However, where livestock are fed largely or entirely on hay (which is more mature than the herbage normally used for grazing), N and/or P may be less than adequate, due to the decline in these elements with increasing maturity of the herbage. With grassland that is managed extensively and receives little or no fertilizer, herbage that is grazed is liable to contain less than optimal amounts of N and P. In contrast to N and P, deficiencies of Ca, Mg and S are more likely to occur with intensive than with extensive management. Calcium deficiency usually occurs only in cows producing high yields of milk and only at the beginning of a lactation period, when the physiological demand is greatest and when the animal may not be able to meet its needs from a combination of dietary supply and mobilization from bone reserves. The effect of intensive management on Mg deficiency is due partly to the plant uptake of Mg being reduced by high rates of fertilizer K, and partly to the absorption of Mg by the animal being reduced by high concentrations of K and N in the herbage. In addition, the herbage concentrations of both Ca and Mg are lower in all-grass than in grass–clover swards. A deficiency of Na is most likely to occur in areas where atmospheric inputs are low, but the risk
Nutrient Elements in Ruminant Animals 85
Heifer, growing
1 year old
Cow, pregnant, non-lactating
Cow (600 kg) yielding 30 kg milk day−^1
N (%) P (%) S (%) K (%) Na (%) Ca (%) Mg (%) Cl (%) Fe (mg kg−^1 ) Mn (mg kg−^1 ) Zn (mg kg−^1 ) Cu (mg kg−^1 ) Co (mg kg−^1 ) I (mg kg−^1 ) Se (mg kg−^1 )
Table 4.7. Dietary concentrations of nutrient elements for three types of dairy cattle recommended in the USA (from National Research Council, Subcommittee, 1989).
is accentuated by fertilizer K, which tends to depress the plant uptake of Na and also increases the leaching of Na from the soil. Deficiencies of the micronutrient elements are generally restricted to localized areas, though, in some parts of the world, deficiencies of certain ele- ments, including Cu, Co, I and Se, are widespread. Micronutrient deficiencies occur most frequently in areas where the soils are generally infertile and, if the intensity of management is increased by the application of fertilizers, the increase in herbage yield will inevitably accentuate any deficiency. However, most soils have large reserves of micronutrients and, in general, there is little reduction in the concentrations of micronutrients in herbage as a result of fertilizer use. Despite the generally localized occurrence of micronutrient deficiencies, a study of such deficiencies in New Zealand found that only with Co and Se was there a clear relationship between the total concentration in the soil and the incidence of deficiency in animals; with other elements, various soil, plant and management factors masked the relationship (Grace and Clark, 1991). Even the concentration in the herbage is not always closely related to the incidence of deficiency in grazing animals, as a deficiency may be due to poor absorption rather than low concentration (Spears, 1991). Mild deficiencies or excesses of nutrient elements in animals are difficult to identify, because their effects are often similar to those due to under- nutrition or the presence of internal parasites (Underwood and Suttle, 1999), and, although pathological symptoms may occur, these are often not specific for any one element. For example, anaemia is a symptom of deficiencies of Fe, Cu and Co, and reproductive disorders are liable to occur with deficien- cies of Cu, I, Mn, P, Se and Zn (Jones and Thomas, 1987; Underwood and Suttle, 1999). Although some symptoms are specific, they may become apparent only when the deficiency is serious and the effects well advanced. Examples include goitre resulting from I deficiency, white muscle disease from Se deficiency and tetany from Mg deficiency. An important research objective is therefore to develop better means of predicting the incidence of deficiencies and of diagnosing subclinical deficiencies before visible symptoms appear. Although the chemical analysis of herbage or soil for a particular nutrient element may be useful in indicating the likelihood of a deficiency in livestock, it cannot provide definite evidence of a deficiency. Also, if herbage is used, it is important to analyse samples that are as similar as possible to the ingested herbage, since grazing animals tend to select herbage of higher nutritional quality than the average of that available (Little, 1981). A more reliable indication of a subclinical deficiency in livestock is often provided by the analysis of blood, saliva or milk. Such an analysis may be either for the nutrient element itself, or for a metabolite or biologically active form of the nutrient, such as an enzyme, a hormone or a vitamin. Two examples of critical concentrations of actual nutrient elements in the blood serum that have been proposed for cattle and sheep are, for Cu, a concentra- tion of 0.6 mg l−^1 and, for Se, a concentration of 50 μg l−^1 (Suttle et al ., 1984). Examples of biologically active molecules whose concentration in blood has
86 Chapter 4
developed for providing supplementary nutrient elements where deficiencies are likely. These measures include: (i) treating the water supply, which is effective for cattle if a pressurized supply of water is available; (ii) dosing individual animals with soluble glass boluses containing the required element(s) in a slow-release form; (iii) dosing with other slow-release forms of nutrient elements; and (iv) injecting the animals with appropriate compounds, such as copper glycinate for Cu, or sodium selenite for Se (Thompson, 1980; Suttle et al ., 1984; Underwood and Suttle, 1999). When the animals receive concentrate feeds in addition to grassland herbage, it is often effective to incorporate supplementary nutrient elements into this component of the diet.
Soil Ingestion by Ruminant Animals
The ingestion of soil is, potentially, an important influence on the amounts of several nutrient elements consumed and absorbed by grazing animals. When herbage is grazed, there is almost inevitably some ingestion of soil, often amounting to between 2% and 10% of the DM intake. In wet and muddy conditions, soil ingestion by cattle may amount to as much as 20% of the DM intake, and soil ingestion by sheep may be even greater (Healy, 1973; Thornton and Abrahams, 1981). The soil that is consumed generally occurs as surface contamination on the herbage, is attached to plant roots pulled from the ground or is in earthworm casts. Factors that influence the amount of soil ingested therefore include: (i) weather and seasonal conditions; (ii) soil type; (iii) the stocking rate of grazing animals; (iv) the earthworm population; and (v) individual animal differences (Healy, 1973). In humid regions, most of the ingested soil is soil that has been splashed on to herbage by rainfall and the treading of livestock. Rainfall is clearly an important factor in the extent of splashing, but it also has an influence, together with temperature, on the rate of herbage growth, and increased herbage growth may dilute the effect of splashed soil. The effect of soil type is due mainly to there being more splashing when the soil has poor aggregation and is poorly drained. In general, soil ingestion is greater when the number of grazing animals per unit area is high. Soil ingestion is also greater when there is a large population of earthworms, and earthworms tend to deposit more casts on the soil surface in wet conditions, especially if the soil is poorly drained (Healy, 1973). There are often marked seasonal variations in soil ingestion, due some- times to differences in weather conditions and sometimes to differences in the extent to which animals are housed during the winter. Soil ingestion by cattle in New Zealand is generally highest in the winter months, since the animals generally remain outdoors and rainfall is relatively high but grass growth is slow due to low temperature (Healy, 1973). However, in many countries, soil ingestion is highest during spring and autumn, since the ani- mals are housed during the winter and fed silage or hay, which usually have
88 Chapter 4
little soil contamination. In semi-arid regions with extensive grazing, soil ingestion results mainly from the deposition of fine soil particles on to the herbage, a process that is at a maximum at the driest times of year, when plant growth is poor, and especially when wind and the trampling of livestock accentuate the movement of dust. The presence of shallow-rooted plants may also be important, as soil is eaten together with roots when these are pulled from the soil (Mayland et al ., 1975). An approximate estimate of the amount of soil ingested can be obtained from the ash content of the faeces, making allowance for the inherent ash content of herbage, typically about 8%. A more accurate estimate of soil ingestion can be obtained from the content of titanium (Ti) in the faeces, as Ti is present in soils in concentrations of several thousand mg kg−^1 , whereas concentrations in herbage are usually < 10 mg kg−^1 (Healy, 1973; Thornton and Abrahams, 1981). Using one of these methods, a number of estimates have been made of the amounts of soil ingested by livestock on an annual basis. Thus, in New Zealand, dairy cattle have been estimated to consume between 150 and 650 kg soil year−^1 and sheep up to 75 kg soil year−^1 (Healy, 1973). In Ireland, the amount of soil ingested by sheep weighing 60 kg was also estimated to be about 75 kg year−^1 , with rainfall and stocking rate being important factors in seasonal variation (McGrath et al ., 1982). Under semi-arid conditions, the amount of soil ingested tends to increase as the amount of herbage diminishes, and therefore as the severity of grazing is increased. In range conditions in Idaho, USA, cattle were found to ingest 35–550 kg soil year−^1 , with a mean of 180 kg, and the daily rate ranged from 100 to 1500 g with a mean of 500 g (Mayland et al ., 1975). This consumption represented a range of about 1.2–18.8% of DM intake, with a median value of 6.2%. The ingestion of soil is sometimes important in the supply of nutrient elements, especially Co, I and Se, whose concentration in soil is much greater than their concentration in herbage. However, there is little information on the availability to animals of the nutrient elements in soil, though it is likely to be less than the availability of the corresponding elements in herbage. Nevertheless, soil ingestion by sheep has been observed to increase the amounts of Ca, Mg and P retained, suggesting that some absorption occurs (Grace and Healy, 1974). There is also evidence that the incidence of I deficiency in animals is reduced when they consume relatively large amounts of soil (Statham and Bray, 1975). With Se, the ingestion by lambs of 100 g soil day−^1 of two soils resulted in increased concentrations of Se in both blood plasma and liver; and, with one soil, there was also a significant increase in the concentration of vitamin B 12 in the liver. However, neither soil influenced the concentrations in the liver of Fe, Mn, Zn or Cu (Grace et al ., 1996a). In some instances, soil ingestion has been found to have adverse effects on the availability of nutrient elements. For example, it has sometimes, though not always, reduced the availability of dietary Cu (Langlands et al ., 1982; Suttle,
Nutrient Elements in Ruminant Animals 89
