Milk-fever
Intramuscular injection of 0.4 g of 1,25-(OH)2D35 days before predicted calv- ing date, with reinjections every 5 days until calving, has maintained serum calcium and phosphorus during the critical period, 24 h before and 48 h after calving (Gast et al., 1979). Intramuscular injections of PTH are also effective but impractical (Goff et al., 1989a).
Dietary prevention
One early dietary approach was to feed diets low in calcium prior to calving to prime the homeostatic pathways, notably synthesis of PTH and hence 1,25- (OH)2D3(Boda, 1956; Boda and Cole, 1956; Green et al., 1981), but the nec- essary diets are not easily formulated on many farms and at some point the calcium deficit has to be made good. The most manageable and effective treatment for milk-fever is to feed acidic diets around calving (for review, see Block, 1994); they work by enhancing the sensitivity of the kidney to PTH and thus raising 1,25-(OH)2D3 concentrations in plasma (Gaynor et al., 1989;
Abu Damir et al., 1994) and calcium absorption (Freeden et al., 1988). Acidic diets are those in which the cation : anion balance – essentially (potassium (K+) + sodium (Na+)) 2 (chloride (Cl2) + sulphate (SO422)) – in milli- equivalents (atomic weight ÷ valency) is below 100. Dietary acidity can be monitored via the pH of urine, which should be below 7.5. This line of treat- ment was instigated by Ender et al. (1971), who noted that milk-fever rarely occurred in cows given silage preserved with acids. The feeding of silage rather than hay as roughage will itself decrease the incidence of milk-fever.
Large anion inputs are needed to offset the high potassium levels in the herbage of spring-calving herds, but they can be achieved with large calcium chloride (CaCl2) supplements (providing 50 g Ca: Goff and Horst, 1993). The associated risks of acidosis and aspiration pneumonia can be decreased by giving CaCl2 in gel or emulsion form (in polyethylene glycol) and/or as calcium propionate gel (providing 75 g Ca: Goff and Horst, 1994). The use of calcium salts to ‘acidify’ diets is commended because ‘acidification’ per semay be ineffective if the diet is low in calcium (Oetzel et al., 1988). The acidic diets should be gradually discontinued after 21 days, because they may depress milk yield (see Chapter 7). Since the primary source of excess cations is potassium-rich forage, steps should be taken to minimize the potassium content of pastures (see Chapter 8).
‘Hypocalcaemia’ in pregnant ewes is also treated with calcium borogluconate, and response to treatment confirms the diagnosis. The subcutaneous dose is 50 ml of a 40% solution of calcium borogluconate, with added magnesium.
Preventive measures have not been studied in ewes, but those which reduce the incidence of milk-fever in dairy cows should be just as effective in ewes, provided they begin well before lambing. The use of acid-preserved silage, 1a-OHD3 injections and oral supplementation with CaCl2 and/or calcium propionate are suggested, the latter having particular potential where there is a parallel risk of pregnancy toxaemia. The same treatments are indicated for
Hypocalcaemia in ewes
transport tetany. Treatments that rely on increased synthesis of 1,25-(OH)2D3 require a supply of the unhydroxylated vitamin; this may be in short supply in early spring and require simultaneous supplementation with vitamin D3 (Smith and Wright, 1981).
Since the clinical effects of osteodystrophy caused by calcium deprivation are mostly irreversible and weak eggshells irreparable, preventive measures must be adopted early in development. Prevention is simply a matter of seeing that calcium requirements are met from one or more of a wide range of suitable calcium sources. With housed ruminants receiving regular con- centrate supplements, additional calcium is either incorporated into the whole mixed diet or into the concentrate portion of the ration, usually as calcium carbonate or dibasic calcium phosphate (CaHPO4). The same salts are routinely added to free-access mineral mixtures for housed and grazing livestock; although they are rarely needed continuously as far as the grazing animal is concerned, they are essential components of all rations for non- ruminants.
There is little to choose between the common mineral sources of calcium, when they are compared with calcium carbonate (Soares, 1995). The excep- tions are dolomitic limestone and soft-rock phosphate, which have relatively low availabilities (0.65 and 0.70, respectively) when fed to pigs or poultry (Dilworth et al., 1964; Reid and Weber, 1976). All mineral sources of calcium are likely to have a lower than average availability when fed in high-phytate, low-phytase diets. Claims that the calcium from CaCO3 is much more avail- able to pigs than that from CaHPO4(Eeckhout et al., 1995) may reflect satura- tion of the inhibitory effect of phytate at the high calcium levels which were achieved when adding CaCO3, thus confounding the effect of source and level. Particle size can be important in the choice of calcium supplements.
The high calcium needs of laying hens are commonly met by supplementa- tion with 4% calcium as limestone, a relatively cheap and plentiful source, but replacing one-half to two-thirds with granular sources, such as oyster shell, has a sparing effect, so that 3% calcium can be sufficient in respect of shell quality (Roland et al., 1974). It seems that large particles of calcium remain in the gizzard longer and provide more retainable calcium than finely ground sources (Scott et al., 1971), probably by achieving synchrony with diurnal fluctuations in the calcium requirement for shell calcification (Whitehead, 1995). Particle size of CaCO3 influences the phosphorus requirement of broilers (see Chapter 5). There are other reasons for discriminating between sources, however; phosphorus-free calcium sources are best in grain-based diets for ruminants and monobasic calcium phosphate (CaH2PO4) is unsuit- able for laying hens, because it results in acidosis and poor shell quality (see Chapters 5 and 7). The acidotic calcium chloride or sulphate may be useful for manipulating acid–base balance in dairy cows to avoid milk-fever. When Prevention of bone and eggshell disorders
Calcium supplements
calcium and other minerals are mixed with the feed, it is important to ensure that they are evenly distributed. Inadequate mixing of limestone with cereals may have contributed to hypocalcaemia after drought feeding (Larsen et al., 1986) and can be avoided by using molasses to stick such supplements to the grain (see Chapter 9 for detail). Growing use is being made of calcium complexes with organic acids (‘calcium soaps’) as energy sources in ruminant nutrition, though not as improved sources of calcium. In the poultry industry, supplementation of diets with calcium formate reduces the contamination of carcasses and eggs with pathogenic bacteria.
The principal factors affecting the mineral requirements of farm animals, including calcium, were described in Chapter 1. Several developments in livestock husbandry have combined to steadily raise dietary calcium require- ments. These include: (i) genetic improvement in the animal, resulting in faster growth and higher yields, as exemplified particularly in broiler produc- tion; (ii) the increasing use of energy-dense diets; (iii) the practice of early weaning, which emphasizes the decrease in efficiency of utilization associated with solid as opposed to milk diets; (iv) the breeding of animals at a young age, while they are still growing; and (v) the use of antibiotics, hormones and other feed additives as growth stimulants. Where these growth effects are exerted through higher feed consumption, the requirements – expressed as dietary concentrations – are little affected, but, where feed efficiency is also improved, as is usual, requirements rise correspondingly.
Liberal supplies of vitamin D3 enable the animal to make the best use of limited intakes of calcium and are important to livestock housed for long periods, particularly high-yielding dairy cows and laying hens. The following recommendations assume that supplies of vitamin D3are adequate.
There have been no rigorous attempts to define the calcium requirements of sheep (or cattle) by means of feeding trials and it is therefore necessary to rely on factorial estimates of requirement. Unfortunately, these have varied considerably from one authority to another, largely due to disagreement on a realistic coefficient of absorption with which to generate gross requirements (AFRC, 1991). A major reduction in calcium requirements was first proposed by the Agricultural Research Council (ARC, 1980), on the grounds that ruminants could absorb calcium with a high efficiency when necessary: infor- mation published subsequently and reviewed on p. 68 gives no convincing reason to doubt that assertion. Minimum requirements should therefore be calculated assuming the maximum attainable efficiency of absorption of 0.68.
The AFRC (1991) endorsed that assumption and their estimates are therefore commended and presented in Table 4.6. The requirements for lambs were tested by feeding at 75%, 100% and 125% of the recommended level and