Lead is tolerated at 1-5 mg kg-1 diet, but a sustained level of about 12 mg kg-1 would be lethal. Lead shot trapped in silage may partially dissolve in fermentation acids to give up to 3800 mg soluble Pb kg-1 DM. The daisy (Bellis perennis L.) can accumulate 60-80 mg Cd kg-1 from contaminated soils, which is 30 times more than grass, therefore herbage growing in the vicinity of derelict mine workings should be analysed for heavy metals. A 500-kg horse grazing rough terrain may ingest 1-2 kg of soil per day, so the amount of any industrial fallout, contamination or seepage should be deter-
172 © 2002 CAB International. Methods in Agricultural Chemical Analysis: a Practical
Handbook (N.T. Faithfull)
mined. Herbage high in molybdenum, iron or sulphur may lower copper absorption and thus depress serum copper. Feed is analysed by the usual procedures already described for mineral, trace and toxic elements.
Frape observes that a horse digests fibre less easily than domesticated ruminants, therefore shorter grass containing a higher proportion of leaf is a more valuable feed than herbage approaching maturity. A suitable fibre method for non-ruminants is the NDF procedure (see Method 7.5. Determination of neutral detergent fibre (NDF) or plant cell-wall constituents, page 133).
Until recently (Moore-Colyer and Longland, 2000), little has been published regarding the feeding of silage and haylage to horses, but haylage is becoming more available commercially as farms seek to diversify. The main problem with silage is the risk of botulism (from Clostridium botulinum) and enteritis (from Cl. perfringens). Clostridial spores affect horses more than ruminants. Silage should be well fermented, high in DM and free from moulding. The problem with hay is dust which causes respiratory problems, so it is sometimes washed before feeding. Haylage lacks the dust, and is safer than silage because it is much higher in DM (approximately 50% DM), although more expensive. It is readily digested by ponies and offers a suitable high-energy alternative to hay in horse rations (Moore-Colyer and Longland, 2000). Before feeding it should be checked that the haylage or silage smells sweet (or analysed for VFAs, ensuring low values for butyric and valeric acids), and the DM and pH (which should be 4.0-4.5) determined (Frape, 1986, pp. 234-235).
Details of the DM, pH, lactic acid and VFA content in haylage are available at:
http://www.dairybiz.com/archiv/cowtalk_41.htm and the nutrient composition of commercial semi-wilted bagged forages can be found at:
The determination of digestibility parameters using methods designed for ruminants are clearly not applicable to horses, which have no rumen, but possess a simple stomach and an enlarged caecum and colon where microorganisms facilitate the digestion of cellulose. Unlike in the ruminant, the products of microbial digestion have less opportunity of being absorbed and no opportunity of being further broken down by its own digestive enzymes (McDonald et al., 1969).
It is the 'apparent digestibility' rather than the 'true digestibility' that is usually determined. This is because substances in the faeces not arising directly from the food lead to an underestimation of the proportion of the intake actually absorbed by the animal, also the fractions of the faeces derived from food or endogenous origins are in most cases indistinguishable (McDonald et al., 1995). Endogenous constituents include sloughed-off cells from the gut mucosa, microbial matter and digestive enzymes. Thus, for the case of crude protein:
apparent digestion coefficient = 100 x
(N intake total faecal N) N intake
.. . ... . „„„ (N intake [total faecal N metabolic N])
true digestion coefficient = 100 x-
The total faecal N is the undigested feed N in the faeces. The true digestion coefficient is not the same as the true protein, because the feed can contain nitrogenous substances, other than crude protein, which can be converted to ammonia in the Kjeldahl digestion. The metabolic N may be estimated by analysing the faeces for non-feedstuff-derived N after feeding a diet containing no protein, or only a small amount of highly digestible protein. Faeces markers have been recommended for this procedure in the case of non-ruminants (Schneider and Flatt, 1975). The addition of a solution of 3% (v/v) sulphuric acid (approximately 0.5 M) to the faeces will help prevent loss of ammonia on drying. In the case of calcium, the equations become:
apparent digestion coefficient of Ca = 100-
Ca intake x 100
true digestion coefficient of Ca = 100 x
Ca intake - faecal Ca +endogenous faecal Ca
The inability to correct for the loss of gases such as methane would lead to an overestimate of food absorption, digestible carbohydrate and digestible energy, but various mathematical modelling techniques have attempted such a correction (Cone, 1997). Earlier work with ruminants by Blaxter and Clapperton (1965) gave an equation linking digestible energy with methane production based on more than 2500 determinations of the 24-h production of methane by sheep and cattle.
Methods to estimate digestibility which avoid the necessity for fistulation include gas production from feeds incubated with faecal inoculums using an automatic pressure evaluation system, and various digestion marker methods (see below). The concentration of marker in the feed and faeces may be used to calculate the apparent digestibility of nutrient components (such as crude protein) by employing the following equation:
% nutrient apparent digestibility = 100 -
% marker in feed % nutrient in faeces 100 x-x
% marker in faeces % nutrient in feed
The rate of passage through the gastrointestinal tract exerts an important influence on the intake and digestibility (Poppi et al., 2000). Thus, digestibility is reduced as the rate of passage increases (Pearson and Merritt, 1991). Finely grinding the feed decreases the retention time and hence the digestibility (Blaxter et al., 1956). Mean retention times may be monitored by administering a chemical marker with the feed and collecting the faeces over a defined period; this avoids the difficulty of total collection procedures. Naturally occurring largely indigestible substances, such as lignin, acid-insoluble ash and indigestible ADF have been tried as markers, but are not very satisfactory. A marker should remain unchanged after passing through the digestive tract, neither should it migrate. Migration is when the marker becomes detached from the food particle and then the unbound marker reattaches to other food particles. The marker should ideally not be attached to the whole continuum of particle sizes, because there may be selective retention of larger or smaller sized particles in some sections of the gut (Cork et al., 1999). It is essential to recover the metal marker from the same food or plant matrix to which it was initially bonded. Chromium (III) oxide (chromic oxide, Cr2O3) powder may be administered in a capsule, but is no longer considered suitable, because it does not associate specifically with either the particulate or liquid component of the ingesta and results in sedimentation and sporadic transfer of marker (Bertone et al., 1989a). The soluble chromium (III) salts bond only slowly to the feed surface, and therefore it is preferred to reduce a chromium (VI) salt (e.g. dichro-mate) with ascorbic acid to bind it on to the fibre by a mordanting process as explained by Marais (2000). Cobalt and titanium have been used, but current opinion, however, favours the use of rare earth elements, cerium, dysprosium, and ytterbium, of which ytterbium gives the most favourable results. The ytterbium solution is best applied by soaking, rather than spraying, the feed with a solution of the ytterbium salt, because this exposes the ytterbium ions to more of the natural adsorption sites (Mader et al., 1984). The salt which has been used for horses is ytterbium (III) chloride hexahydrate (Aldrich, mol. wt 387.54) (Bertone et al., 1989b). Subsequent analysis may be by AAS (Teeter et al., 1979; Bertone et al., 1989a; Luginbuhl et al., 1994) or ICP-MS (Combs and Satter, 1992). Ytterbium as a marker has been used experimentally and reviewed in a thesis by Morrow (1998), where big-bale silage and hay were fed to ponies, and the rate of passage determined. Although little significant difference existed between the feeds, the trend was for the silage to have longer mean retention times than the hays.
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