Dynamics of Bone Mineralisation in Sows and Phosphorus Excretion

Sows mobilise body reserves of phosphorus during lactation and replenish these reserves during gestation. Considering this dynamic allows us to envisage a reduction in dietary intake of phosphorus during lactation, and hence, of excretion into the environment.

Calcium (Ca) and phosphorus (P) play a major role in bone constitution. Inadequate dietary intake of Ca and/or P limits bone mineralisation, while an excess increases P excretion, which can have an adverse impact on the environment.  
In swine production, it is important to understand mineral-use mechanisms in order to maximise the efficacy of dietary P use and develop new feeding strategies. Sows go through a period of very high P requirements – lactation – and one of very low P requirements – gestation. Agroscope conducted a first study with sows to learn more about Ca and P bone mineralisation dynamics, i.e. the quantities mobilised from the animals’ bones during lactation, then recovered during the following gestation.

Four diets compared during lactation

The bone mineralisation of 24 primiparous sows was quantified 2, 14, 26, 70 and 110 days after parturition by X-ray scanner. Four lactation diets were formulated: 100% (Lact100) with 9.9 g Ca and 3.0 g digestible P/kg according to nutritional requirements, 75% (Lact75), 50% without added phytase (Lact50) and 50% with added phytase (Lact50 + FTU), an enzyme that increases dietary phosphorus digestibility. The following gestation diet was formulated to cover nutritional requirements for Ca and P (8.2 g Ca and 2.6 g digestible P/kg).

Recovery even with high mobilisation

During the 26 days of lactation, all sow mobilised part of their body mineral reserves, regardless of diet. The average amount of mobilised bone mineral content (BMC) was 664 g, representing 240 g Ca and 113 g P. At the end of the lactation, BMC of the Lact50 sows tended to be lower than that of the Lact100 sows (-12.8%), while BMC of the Lact50 + FTU sows remained similar to that of the Lact100 sows. During the following gestation, BMC returned to similar values for the different treatments. Consequently, sows fed the Lact50 diet managed to recover despite the higher bone mineral mobilisation during lactation. As a result, P excretion (ingested less retained P) was reduced by 40% and 43% in sows fed a Lact50 and Lact50 + FTU diet, respectively, compared to the sows fed a Lact100 diet.

Possible reduction of P excretion

In conclusion, changes in body composition during the first lactation and second gestation of primiparous sows show that bone mineral reserves were mobilised and replenished, and that the dynamics depended on the P content of the diet as well as on phytase supplementation during lactation. In future, the consideration of not only dietary P but also body reserves of P in the evaluation of nutritional requirements during lactation could substantially limit the use of inorganic phosphates in feed, and consequently P excretion.