If used in layer hen diets, limestone must be considered carefully because it might be the source of reduced eggshell resistance.
Calcium is one of those essential nutrients that can be secured cheap. This is true for all kinds of diets for pigs, poultry, ruminants, even pets. A metric ton of calcium carbonate costs about EUR80, when monocalcium phosphate (a major source of phosphorus) is over EUR500 per metric ton. No wonder most focus (research, marketing, advertisement) is on phosphorus (read, phytase), whereas calcium is largely ignored — not unlike salt and the case for sodium.
With calcium being inexpensive, oversupply rather than deficiency is usually the problem. As it stands, calcium carbonate (in pure form or as limestone) is used as a carrier for most premixed products (drugs, vitamins, trace minerals, nutritional additives and even some concentrates). It is even added at the rate of 0.5 percent in soybean meal as a flow enhancer — a fact largely unknown. Thus, most diets usually contain 10 to 20 percent more calcium than the actual target figure used during feed formulation. This causes virtually no problems, if we ignore the cost-price issue, with the only notable exception being that of layer hens.
Whereas most animal diets contain less than 1 percent calcium, layer diets must contain substantially more. Indeed, such diets contain from 3 to 4 percent, only because the formation of each eggshell requires 2 grams of calcium (in the form of about 5.5 grams of calcium carbonate). Thus, maximizing calcium intake is crucial for these animals. Although exceeding 4 percent dietary calcium can cause metabolic disorders (and hence other dietary means are employed to maximize calcium absorption rather than intake), it is the possibility of a calcium deficiency that concerns our discussion here. As it is logical to assume, a reduced calcium intake will cause layer hens first to deplete their bone reserves (with ensuing leg disorders), and then to reduce and eventually cease egg production. All the while, eggshell resistance decreases and cracked egg numbers increase.
Although pure (feed-grade) calcium carbonate is relatively inexpensive, there is another source of calcium that is even cheaper. That is limestone, which costs as little as EUR30 to EUR50 per metric ton. One would be excused to believe that such difference in price is not enough to bother, in the grand scheme of feed formulation. But, when the cost of each egg is considered (say EUR10 cents) it becomes evident that any savings is worth evaluating. In this case, however, one can be penny wise but dollar fool, as my U.S. colleagues said to me when I brought this question up in a recent meeting. Let me explain.
Limestone comes from a wide variety of sources and in very different forms and degrees of hardness. Thus, its solubility, availability, and even its concentration in calcium can be unpredictable. For example, calcium concentration varies between 32 and 38 percent. Quite often, as is true in commercial practice, one batch of limestone might contain lower levels of calcium than assumed. And, as calcium analysis is costly and time consuming, such laboratory tests can only monitor the extent of variability — thus, being of little use if the actual batch is already used up. Assuming a typical layer diet contains 10 percent limestone, this translates to a range of 3.2 to 3.8 percent dietary calcium. The difference (0.6 percent) is considered enormous for layer hen diets to be ignored.
Limestone versus oyster shells. Coarse particles of limestone or oyster shells are often provided during the evening hours for hens to pick at will. This is done independent of normal feeding, and it is in addition to calcium already present in feed. First of all, this practice alone can be a justification or indication that actual calcium levels in feed are lower than those expected when feed is formulated with limestone based on book rather actual values. Second, some producers believe oyster shells are superior to limestone, whereas others consider the two equal and prefer the cheaper one: limestone. This confusing situation is supported by numerous equally puzzling research reports. In my opinion, it is the highly variable concentration of calcium and a problem in picking the wrong particle size in limestone that is behind all such confusion.
Dolomitic limestone. It is often given away so cheap that it entices many nutritionists to consider it as a source of calcium for layers, or as a filler and carrier for premixed products. However, there is (always) a catch in that, dolomitic limestone contains high levels of magnesium (over 10 percent). Magnesium competes with calcium for absorption sites and as such, it is considered a problem in diets for layers. Excess levels of magnesium can also bind calcium in the gut, reducing thus its availability to the animal. For all these reasons, dolomitic limestone should not be used in layer diets.
Limestone does not have to be condemned for being an unreliable source of calcium. After all, it comes with a price tag that agrees with its quality. There are three solutions to this problem:
- Buy large enough quantities of single-sourced limestone to justify a laboratory calcium analysis. If limestone is sourced from different suppliers, this becomes impractical.
- Buy from a supplier that provides a written guarantee on calcium levels and check with a laboratory analysis once or twice per year.
- Buy directly from a single source (mine), bypassing the trader step. You will still need to run a laboratory test at least annually.
At the end of the day, when cracked eggs are a perennial problem, it might be worthy to abandon limestone for pure calcium carbonate — if this resolves the problem.