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
there was every indication that they still err on the generous side (Wan Zahari et al., 1990). The important features of the estimates shown in the table are as follows:
1. Requirements for growth decrease with age but increase with growth rate.
2. Requirements for pregnant ewes rise rapidly in late pregnancy to a level equal to that of the lactating ewe.
3. No figure need be met on a day-to-day basis and it would be surprising if performance suffered on any diet providing an average of 3 g Ca kg21 DM throughout the year.
Table 4.6. Requirements of sheep for dietary calcium (AFRC, 1991) and phosphorus (modified from AFRC, 1991) at the given dry-matter intakes (DMI).
Live weight Production Diet DMI Ca P
(kg) level/stage quality (kg day21) (g kg21DM) (g kg21DM)
Growing 20 100 g day21 L 0.67 3.7 2.6
lambs H 0.40 5.7 2.8
200 g day21 L U – –
H 0.57 7.0 3.9
40 100 g day21 L 1.11 2.4 2.0
H 0.66 3.4 2.0
200 g day21 L 1.77 2.6 2.3
H 0.93 4.0 2.4
Pregnant 75 9 weeks L 1.10 1.4c 1.6c
ewe H 0.71 1.6c 1.0c
carrying 13 weeks L 1.28 2.0 2.0
twinsa H 0.85 2.6 1.6
17 weeks L 1.68 2.9 2.3
H 1.13 3.9 2.0
Term L 2.37 3.2 2.2
H 1.62 4.3 1.8
Lactating 75 2–3 kg L 2.8–3.7 2.8 (m) 2.7
ewe milk day21 L 2.3–3.2 3.1 (bm)b 3.0
nursing H 1.8–2.4 3.8 (m) 2.7
twins H 1.5–2.1 4.3 (bm) 3.0
aThe requirements for small ewes carrying single lambs are similar, assuming that they will eat proportionately less DM.
bRequirements are influenced by the ability of the diet to meet energy need and prevent loss in body weight (m); diets which allow loss of body weight at 0.1 kg day21(bm) are associated with higher requirements.
cSufficient for dry ewe.
L, poorly digestible diet with ‘q’ value (q = ME/GE, the metabolizability of gross energy at maintenance) 0.5; H, highly digestible diet, q = 0.7; U, unattainable performance.
In view of evidence that cattle can absorb calcium very efficiently when necessary (Van Klooster, 1976; Van Leuwen and De Visser, 1976), the AFRC (1991) estimates are again commended for general use (Table 4.7). The important features are as follows:
1. Requirements for growth decrease with age but increase with growth rate and are similar to those of lambs.
2. Requirements increase slowly during pregnancy and in high-yielding dairy cows increase by a further 33% or more with the onset of lactation (the table does not allow for secretion of calcium-rich colostrum).
3. Because there is little time for cows to replenish lost calcium reserves between lactations, the average calcium concentrations in the annual diet should be approximately 4.5 g Ca kg21DM, 50% higher than for sheep.
4. When cows ‘milk off their backs’ (i.e. lose body weight to sustain production), requirements in concentration terms increase substantially (by 80% in the example given).
Empirical feeding trials have been extensively used to define the calcium requirements of pigs. ARC (1981) tabulated the results of 56 trials published between 1964 and 1976 and concluded that requirements had increased substantially (by up to 50%) since 1967, as judged by the average ‘optimum requirement’. They attributed the changes to increases in pig performance on diets of increased nutrient density. There are, however, two reasons why the average ‘optimum requirement’ will overestimate needs for calcium. Firstly, many of the trials contained only two widely spaced treatments, one less than and one probably in excess of requirement, the upper value being taken to be ‘optimal’. If the data set is restricted to trials with at least three calcium levels, the mean optimum concentration is 7.9 ± 1.7 g Ca kg21 DM (n= 18) for pigs of 20–90 kg LW as opposed to 8.5 g Ca kg21DM for unselected data.
A further source of bias is that many trials were based on the assumption that, as dietary calcium concentrations increased, those of phosphorus must be similarly increased to maintain a constant Ca : P ratio. Since inorganic phos- phorus supplements lower the degradation of phytate (see Chapter 5) and phytate lowers the availability of calcium, this assumption will increase the apparent calcium requirement. High calcium ‘requirements’ (> 10 g kg21DM) were invariably accompanied by high phosphorus provision (≥10 g kg21 DM). If the data set is further restricted to trials using the factorially derived minimum phosphorus requirements (5.9–8.8 g P kg21 DM: Table 4.8), the mean ‘optimum’ calcium requirements declines further to 7.6 g kg21DM.
The factorial derivation of calcium requirements overcomes some of these difficulties but generates others, such as the choice of absorption coeffi- cient to convert net to gross requirements (see Chapters 1 and 2). ARC (1981) assumed that the efficiency of absorption declined from 67 to 47% as pigs grew from 25 to 90 kg. However, the growth requirement declines markedly
Cattle
Pigs
over that weight range and the decline in reported absorptive efficiency reflects an increasingly generous calcium supply. It is more likely that pigs will maintain a high efficiency of absorption for predominantly inorganic cal- cium throughout life if fed to requirement for calcium and phosphorus. The requirements given in Table 4.8 use the components of net calcium require- ment given by ARC (1981) but a uniformly high coefficient for ACa (0.7) (see Chapter 5) and agree well with the requirement indicated by the restricted set of feeding trials. Early calcium provision is higher and later provision slightly lower than that currently advocated by the National Research Council (NRC, 1988). It is becoming increasingly important to define and feed to minimum calcium requirements in order to make the most efficient use of phytate P.
Table 4.7. Requirements of cattle for dietary calcium (AFRC, 1991) and phosphorus (modified from AFRC, 1991) at the given dry-matter intakes (DMI).
Live weight Production Diet DMI Ca P
(kg) level quality (kg day21) (g kg21DM) (g kg21DM)
Growing 100 0.5 kg day21 L 2.8 5.2 3.6
cattle H 1.7 8.0 3.6
1.0 kg day21 L 4.5 6.3 4.1
H 2.4 10.8 4.8
300 0.5 kg day21 L 5.7 3.0 2.3
H 3.4 4.4 1.8
1.0 kg day21 L 8.3 3.5 2.6
H 4.7 5.5 2.5
500 0.5 kg day21 L 10.9 2.6 1.5
H 6.1 3.6 1.1
1.0 kg day21 L 11.6 2.8 2.2
H 6.5 4.3 1.9
Pregnant cow, 600 23 weeks L 6.3 2.1 1.6
calf weight 40 kg H 4.0 2.7 0.9
at birth 31 weeks L 7.2 2.3 1.8
H 4.7 3.0 1.1
39 weeks L 9.1 2.7 2.0
H 6.1 3.5 1.4
Term L 11.2 2.8 2.1
H 7.5 3.6 1.6
Lactating 600 10 kg day21 L 12.0 2.9 (m)b 2.5
cow L 9.9 3.3 (bm) 2.7
20 kg day21 H 11.4 4.0 (m)b 2.0
H 10.1 4.7 (bm) 2.3
40 kg day21 H 19.3 4.9 (m)b 2.8
H 17.8 5.3 (bm) 3.0
L, poorly digestible diet with ‘q’ value 0.5; H, highly digestible diet, q = 0.7; m, fed to maintain bodyweight; bm, fed below maintenance.
The calcium requirements for growing birds according to NRC (1994) are summarized in Table 4.9; they decline with age for all types of bird and are highest for turkey poults. Considerable variation in the estimated calcium requirements of hens for egg production and quality is apparent from reports published over the last 60 years, values ranging from 2.9 to 6.2 g day21 (Roland, 1986). Expression of requirements as dietary concentrations narrowed the reported requirement range to 28–48 g Ca kg21DM. This varia- tion stems from: differences in the basal diets with regard to phosphorus, vitamin D and other interacting nutrients; the strain and age of the birds; the sources of the supplemental calcium; the ability of hens to raise food intake to acquire sufficient calcium; exaggerated safety margins; environmental conditions, including ambient temperatures; and differences in contribution of calcium from the skeleton. Tolerance of low calcium provision early in lay was demonstrated by Hamilton and Cipera (1981), who found no reduction in eggshell strength when hens were given only 20 g Ca kg21DM during the first month of lay and 32 g Ca kg21 DM thereafter. The basis for increasing need during lay is illustrated in Fig. 4.9. In addition, a decline in efficiency of calcium absorption in older hens is probably allowed for in the requirements given in Table 4.10, which continue to increase to the end of lay.
Temperatures well above 20°C tend to reduce feed consumption and hence to increase mineral requirements when expressed as a proportion of the diet.
The use of energy-dense diets, e.g. with added fat, increases the calcium concentration needed and the latest NRC (1994) estimates give three require- ments for white-egg laying hens – 40.6, 32.5 and 27.1 g Ca kg21 DM to allow for food intakes of 80, 100 and 120 g day21, respectively (i.e. decreasing Table 4.8. Factorially derived estimatesaof the calcium and phosphorus requirements (g kg21 DM) of growing pigs for the development of fully mineralized skeletons (modified from ARC, 1981). Corresponding NRC (1988) recommendations are given in parentheses.
Live weight Assumed DMI Phosphorusc
(kg) (kg day21) Calciumb High phytate Low phytate
5 0.3 11.6 (8.8) 9.2 (7.2) 9.2
25 1.05 8.6 (6.9) 8.8 (6.0) 6.3
45 1.8 6.1 (6.0) 5.9 (5.0) 4.2
90 2.8 4.5 (5.0) 4.2 (4.0) 3.0
aUsing ARC (1981) requirements for growth (g kg21LWG) (i.e. P = 10.0 20.1 LW and Ca = 12.5 20.1 LW for birth 250 kg; 5.0 g P and 7.5 g Ca for 50–90 kg) and maintenance (20 mg P and 32 mg Ca kg21LW).
bAbsorption coefficients for Ca: baby pigs, 0.98; weaned pigs, 0.70 for all weights.
cAbsorption coefficients for P: baby pigs, 0.80; weaned pigs, 0.50 for high phytate, 0.70 for low phytate diets; the lower value (0.50) is appropriate for diets which provide phytin P (availability 0.15) and non-phytin P (availability 0.8) in equal proportions and no added phytase.
Poultry
Table 4.9. Dietary requirements (g kg21DM) of calcium and phosphorus for growth in Leghorn (L) or broiler (B) chicks and turkey poults (T) (from NRC, 1994).
Growth stagea
Early Middle Late
Calcium Chick L 9.0 8.0 20.0
B 10.0 9.0 8.0
Poult T 12.0 8.5 5.5
Non-phytate Chick L 4.0 3.5 3.2
phosphorus B 4.5 3.5 3.0
Poult T 6.0 4.2 2.8
aGrowth stages:
Dietary energy density Age (weeks) (kcal ME kg21DM)
L B T L B T
Early 0–6 0–3 0–4 2850 3200 2800
Middle 6–18 3–6 8–12 2850 3200 3000
Late > 18 6–8 20–24 2900 3200 3300
ME, metabolizable energy.
Fig. 4.9. Daily calcium deposition in eggs by laying hens (solid circle) peaks after about 3 months of lay due to the combined effects of increases in rate of egg production, the amount of shell per egg and the Ca concentration in the shell (data from Roland, 1986).
energy density). Recommended values were some 10% lower for brown-egg laying hens, which consume more food, and are much lower than those advocated by Roland (1986; Table 4.10), but they carry the proviso that they may not give maximum eggshell thickness. It has been suggested that calcium requirements should take account of the size of egg produced, increasing from 3.8 g to 4.5 g Ca day21 as egg size increases from 50 to 60 g (Simons, 1986). For pullets entering lay, the optimum transitional feeding regimen for calcium is to increase levels from 10 to 30 g Ca kg21DM 1 week before the first egg is anticipated (Roland, 1986). Thereafter, the need to meet calcium requirements for peak egg production on a daily and even an hourly basis at times is in contrast to the needs of any other class of livestock for any mineral, with the possible exception of calcium for the recently calved cow.
Indeed, the laying hen will voluntarily consume more of a calcium supplement on laying than on non-laying days (Gilbert, 1983).
Calcium is not generally regarded as a toxic element, because homeostatic mechanisms ensure that excess dietary calcium is extensively excreted in faeces. However, doubling the dietary calcium concentration of chicks to 2.05 g kg21 DM caused hypercalcaemia and growth retardation, fast-growing strains being more vulnerable than slow-growing strains (Hurwitz et al., 1995). The adverse nutritional consequences of feeding excess calcium are generally indirect and arise from impairments in the absorption of other elements when the digesta are enriched with calcium; thus deficiencies of phosphorus and zinc are readily induced in non-ruminants. Dietary provision of calcium soaps of fatty acids (CSFA) represents a convenient way of increasing the fat (i.e. energy) content of the diet without depressing fibre digestibility in the rumen. Significant improvements in milk yield can be obtained in dairy cows (Jenkins and Palmquist, 1984) and ewes (Sklan, 1992)
Table 4.10. Requirements of laying hens for dietary calcium and phosphorus (g day21) (Roland, 1986).
Weeks in production
Time in lay (weeks) 21 to 8a 9–16 10–25 26–moult
Ca 3.75 3.75 4.00 4.25
P
Total 0.70 0.70 0.60 0.50
Non-phytate 0.50 0.50 0.40 0.30
aIt is recommended that for this first period figures be used as %DM fed without adjustment for food intake; thereafter, dietary concentrations should be adjusted for estimated food intake to give the desired daily provision.