The needs for calcium and phosphorus by growing pigs have been researched extensively and recommendations for requirements for digestible calcium and digestible phosphorus have been proposed. These recommendations have been validated in performance experiments, and the recommended levels were confirmed to be accurate.
However, there is a need to determine the requirements for calcium and phosphorus for developing gilts and sows, too. Due to the rapid increase in productivity of sows over the last decade, it is believed that requirements for all nutrients including calcium and phosphorus have increased. Still, research to determine the requirements for calcium and phosphorus for reproducing animals is lacking.
What Do Developing Gilts Really Need?
From the extensive work to determine requirements for calcium and phosphorus by weanling and growing-finishing pigs that was conducted over the last two decades, it is possible to estimate the levels of calcium and phosphorus that will maximize bone development. Because developing gilts need to stay in the herd for several years, it is important that they have maximum bone ash and bone strength when they enter the breeding herd.
The requirements for calcium and phosphorus that maximize bone ash should, therefore, be used in diets for developing pigs (Table 1). It appears from these data that the ratio between digestible calcium and digestible phosphorus that is needed during the growing phase increases as pigs get older because less and less phosphorus is needed for soft tissue growth as pigs get older and the optimum ratio in diets for mature gilts is, therefore, close to the ratio in bone ash.
How Are Gestating Sow Needs Different?
The needs for calcium and phosphorus by gestating sows represent the sum of the needs for maintenance, maternal growth, fetal development, and growth of placenta and other reproductive tissues. Unfortunately, there is a paucity of data to demonstrate the needs for calcium and phosphorus for all of these requirements and estimates, therefore, can only be made based on a number of assumptions that may or may not be completely accurate. However, there are a few data for the endogenous losses of calcium and phosphorus by gestating sows and it is likely that these losses are representative of the maintenance requirements for calcium and phosphorus by gestating sows. There are also estimates for requirements for maternal gain, which are believed to be closely associated with the requirements for gain of bone ash from the first to the fourth parity of sows.
Estimates for requirements for fetal growth and growth of reproductive tissues have been published and although these estimates are variable among experiments, these data represent the best possible estimates for these requirements. As expected, requirements for fetuses and reproductive tissues are much greater from day 90 to farrowing than they are before day 90 because of rapid growth of fetuses late in gestation. Based on these estimates it is possible to make estimates for requirements for digestible calcium and digestible phosphorus by gestating sows (Table 2). Because these estimates are based on the daily requirements, the concentration of digestible calcium and phosphorus needed in the diets depends on the daily feed intake and will change with changes in feed intake.
The requirements for digestible calcium and phosphorus can be estimated, but there are considerable difficulties in formulating diets based on digestible calcium and phosphorus because there is a lack of data for the digestibility of calcium and phosphorus in feed ingredients fed to gestating sows. The digestibility has been determined in most feed ingredients fed to growing pigs, but gestating sows have digestibility values that are very different from those observed in growing pigs (Figure 1), and as a consequence, it is not possible to use digestibility values obtained in growing pigs when formulating diets for sows.
It may be necessary to formulate diets for sows using values for total calcium and phosphorus rather than digestible calcium and phosphorus, and requirement estimates in Table 2 are indicated both as digestible and as total calcium and phosphorus. In addition, values for total calcium and phosphorus are indicated both for diets without microbial phytase and for diets with microbial phytase, and it needs to be taken into account that the response to microbial phytase is different in sows than in growing pigs.
Lack of Data for Lactating Sow Digestibility of Calcium and Phosphorus
The requirements for calcium and phosphorus by lactating sows represent the sum of the requirement for maintenance and the requirement for milk production. As for gestating sows, the requirement for maintenance can be estimated from the requirement for endogenous losses and the requirement for milk production is largely determined by the amount of milk produced every day, which is strongly influenced by the number of pigs suckling the sow.
Under practical circumstances, most sows will likely have an average milk production of 12 liters per day during lactation and daily feed intake usually vary between 5 and 7 kg per day. It is possible to calculate the needs for digestible calcium and phosphorus in lactating sows producing 12 liters of milk per day and consuming 5, 6 or 7 kg of feed daily (Table 3).
As for gestating sows, there is a lack of data for the digestibility of calcium and phosphorus in feed ingredients fed to lactating sows, and it may be necessary to base formulations on total calcium and phosphorus rather than digestible calcium and phosphorus, as is also indicated in Table 3. Responses to microbial phytase by lactating sows are different from growing pigs and also different from gestating sows, and there is also a need to generate data for responses to phytase by lactating sows.
More Work to Be Done
Estimates for requirements for calcium and phosphorus by developing gilts are based on strong scientific data, but for gestating and lactating sows, there is a lack of data for which requirements can be based. The above estimates were based on calculations of requirements and a number of assumptions had to be made and there is therefore a need for these numbers to be validated under practical conditions. Likewise, there is a need for generating values for the digestibility of calcium and phosphorus by gestating and lactating sows in all feed ingredients and the impact of microbial phytase needs to be verified as well.
