US - Two new studies at the University of Illinois have helped determine how much calcium growing pigs require and have illuminated the mechanisms by which they absorb it.
Calcium must be fed in adequate amounts and in the right balance with phosphorus to optimise pig performance. "We can use different measures to determine requirements for calcium," says Hans H. Stein, professor of animal sciences at Illinois.
"Different amounts may be needed to maximize growth performance, mineral deposition in bone, or calcium and phosphorus retention."
Mr Stein, in conjunction with other researchers from the U of I and AB Vista Feed Ingredients, conducted two experiments to determine responses to graded levels of calcium in diets fed to pigs from 11 to 25 kilograms. In both experiments, pigs were fed diets containing different levels of standardized total tract digestible (STTD) calcium, ranging from 0.32 per cent up to 0.72 per cent. All diets contained 0.36 per cent STTD phosphorus.
Pigs fed diets containing 0.48 per cent or more STTD calcium had the greatest concentrations of bone ash, bone calcium, and bone phosphorus.
However, on measures of growth performance, average daily gain started to decline at 0.54 per cent STTD calcium, and gain:feed ratio started to decline at 0.50 per cent STTD.
The optimal levels of dietary STTD calcium for retention of calcium and phosphorus were at or above 0.60 and 0.49 per cent respectively.
Taken all together, the different measures point to an STTD calcium requirement of 0.49 per cent or less for growing pigs from 11 to 25 kilograms.
"Based on these results, the requirement for STTD calcium for 11 to 25 kilogram pigs is likely around 1.35 times the requirement for STTD phosphorus," says Mr Stein, adding that further experiments need to be conducted to verify this value.
The researchers also studied the expression of certain genes involved in transcellular transport of calcium at the different levels of dietary calcium. Transcellular transport requires more calcium channel proteins, calcium binding proteins, and energy than passive paracellular transport, so the latter is preferred if enough calcium is available to be absorbed that way.
As the calcium levels in the diets increased, the mRNA expression of genes for the calcium channel proteins TRPV5 and TRPV6, calcium binding proteins CALB1 and S100G, and the vitamin D receptor protein VDR decreased in the kidneys.
Expression of genes for TRPV6 and VDR, as well as plasma membrane protein ATP2B1, also decreased in the jejunum as dietary calcium increased.
"The main site for regulation of calcium balance appears to be in the kidneys," Mr Stein says. "When dietary calcium is adequate and luminal levels of calcium are high enough to allow for paracellular transport, transcellular uptake in the kidneys and jejunum is reduced."
Funding for this research was provided by AB Vista Feed Ingredients, Marlborough, UK.
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