Phosphate is intrinsically a well-tolerated ion and, for this reason, animals can allow circulating levels of phosphate to fluctuate widely. This, coupled with ready excretion of excess phosphate via the urine, means that livestock generally tolerate excessive intakes of phosphorus. Problems arise through adverse interactions or cumulative effects with other minerals. High phos- phorus intakes predispose animals to urinary calculi in sheep, this is more likely to happen when the diet also provides excess magnesium, because for- mation of magnesium phosphates is integral to growth of the phosphatic cal- culus (Suttle and Hay, 1986). Dietary excesses of phosphorus (5.5–8.3 g npP kg21DM) predispose broilers to TD (see Chapter 4), but the damage can be greatly restricted by concurrently feeding excess calcium (15–17 g Ca kg21 DM) (Edwards and Veltmann, 1983). Excesses of acidogenic sources, such as monobasic phosphate, disturb the acid–base balance in laying hens (Keshavarz, 1994; see also Chapter 6). In the horse, the feeding of excess phosphorus in diets low in calcium can cause secondary hyperparathy- roidism.
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