Calculating swine diets using digestibility ratios may support growth, feed intake

Designing swine diets using the ratio of digestible calcium and phosphorus may support growth performance and feed intake as overfeed calcium can slow both, says researcher.

The team of researchers from the US and UK explored the use of varying levels of digestible calcium on pig performance and bone ash concentrations with differing amounts of digestible phosphorus for swine ranging from 50 to 85kg.

The project addressed a missing piece in a series of studies exploring the ratio of digestible calcium (Ca) and digestible phosphorus (P) in swine diets, said Hans Stein, corresponding author and professor of animal science at the University of Illinois.

“We had some data that that ratio between calcium and P may be more important than the absolute values,” ​he told FeedNavigator. “If you overfeed calcium you get reduced growth performance – [we] think its really important to not overfeed calcium, but to not overfeed it, we need to really understand what is the amount.” ​

A requirement of the standard total tract digestibility (STTD) of calcium had not been set for pigs ranging from 50 to 85kg, the researchers said in the study. The experiment sought to test a hypothesis that the “that the requirement to maximize growth performance of 50- to 85-kg pigs for STTD Ca, expressed as the STTD Ca:STTD P ratio, is less than 1.35:1.”​

The researchers found that as more STTD Ca was added to the diet the average daily feed intake (ADFI) declined, regardless of the concentration of STTD P.

The highest bodyweight and average daily gain (ADG) were reported when 2.7g/kg STTP was used suggesting a ration of STTD Ca:STTD P of 1.2:1 and 1.25:1 based on respective body weight and ADG scores.

Excess calcium can slow the growth performance of 50 to 85kg pigs if the amount of dietary phosphorus is near or below requirement, they said.

However, the growth performance of pigs receiving diets with phosphorus levels above the requirement is only “maximized”​ if STTD Ca also is higher than required.

“The STTD Ca to STTD P ratio needed to assure adequate bone mineralization without affecting growth performance is about 1.23:1 if the concentration of P is at the requirement,”​ they added in the paper.

Why examine calcium and phosphorus use?​

Calcium plays a role in several physiological processes, and, reduction in growth performance along with bone and kidney diseases have been linked to having too much or too little calcium (Ca), the researchers said. The level of calcium in an animal’s diet also alters phosphorus (P) digestibility.

The dietary requirement for Ca typically is established on the basis of total Ca, they said. But, data on the standardized total tract digestibility (STTD) of Ca for several feed ingredients has recently been determined.

Feed formulators now have the ability to design diets using the STTD of Ca, they said. The change may mean that it is possible to find the Ca requirements based on values for STTD of Ca which “may reflect Ca utilization more accurately than values for total Ca.”​

In previous research looking at the use of calcium in feeds for pigs from 25 to 50 kg and 100 to 130kg it was noted that increasing Ca lowered ADFI and ADG, the researchers said. That research also found that the “requirement for STTD Ca should be expressed as a ratio between STTD Ca and STTD P, and that a ratio greater than 1.35:1 may be detrimental to pig growth performance if STTD P is supplied at the requirement.”​

Adding too much calcium to swine diets can reduce the efficiency of phosphorus if it is not overfed, said Stein. Producers try not to overuse phosphorus.

“We can create the phosphorus deficiency by overfeeding calcium,” ​he said. “That’s why we believe it's important to focus on the ratio between the two.” ​

Calcium is an inexpensive feed ingredient so there has been a tendency to overfeed it, he said. However, the use was not fully understood.

“This issue is more important than it was in the past because we might have been overfeeding p as well,”​ Stein added.

However, the ingredient is too expensive to overuse and can have negative environmental effects when excreted in large amounts.

“There has been a push to make sure that we don’t have too much phosphorus in the diet and then it becomes really important that we also understand how much calcium there is in the diet,”​ he said.

Feeding trial details​

During the 30-day feeding trial, 90 pigs received one of 15 trials corn-soybean-meal-based diets, the researchers said. All diets had the same amounts of corn and soybean meal to maintain the same level of phytate and they had equal concentrations of sodium.

The diets included 1.4, 2.7 or 4.1g/kg standardized total tract digestible (STTD) phosphorus (P) and 1.3, 2.5, 3.8, 5 or 6.3g/kg STTD calcium (Ca), they said. Concentrations of P went from 0.5 to 1.5 times the requirement and C levels ranged from 0.3 to 1.7 times what is required for total Ca.

Pigs were weighed at the start and end of the trial and feed amounts were noted daily, they said. At the end of the feeding trial, all swine were harvested and the right femur was collected for analysis.

Feed and ingredients were analyzed, phytate concentration was calculated along with non-phytate P, the researchers said. Swine ADG, ADFI and the gain to feed ratio (G:F) and the quantity of Ca and P in bone also were determined.

Results and implications ​

The highest body weight and average daily gain at STTD P concentration of 2.7g/kg were 87.2 and 1.23kg for STTD Ca levels of 3.3 and 3.4g/kg, the researchers said. The findings were linked to STTD Ca:STTD P ratios 1.2:1 and 1.25:1.

The model also had a negative linear result for STTD for ADFI, they said.

The predicted highest bone ash in the femur for STTD P levels of 2.7 and 4.1g/kg were 55.8 and 60.1 at STTD Ca levels of 5.5 and 6.5g/kg, they said. This “correspond to STTD Ca:STTD P ratios of 2.03:1 and 1.59:1.”​

Having the highest amount of bone ash needs a higher STTD Ca:STTD P ratio, the researchers said. That finding shows that pigs can still use Ca and P to develop bone tissues after growth performance needs are met.

The results of the feeding research indicate that there needs to be a focus on the ratio of STTD Ca and STTD P, said Stein. “It’s most easily understood by the ratio,”​ he added.

“Now we can formulate dieted based on digestible calcium and determine what is the requirement,” ​he said. “It is possible to find the digestible calcium.”​

The next step for the feeding research is to finetune the work with the ratios to make sure they are correct he said. “The other thing that we [will] focus on in the future is determining the requirement of calcium and phosphorus for sows,”​ he added.

Source: Animal Feed Science and Technology​

Authors: L. Lagos, C. Walk, M. Murphy, H. Stein