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Determining calcium, phosphorus levels for greater bone mineralization

Kelli Jo/iStock/Getty Images pigs - many young pigs on plastic floor_Kelli Jo_iStock_GettyImages-831729236.jpg
K-State researcher finds sick pigs excreting more phosphorus, creatinine in urine, due to low feed intake and breaking down muscle tissue for energy reserves.

What are adequate calcium and phosphorus levels we should feeding for greater bone mineralization in swine, and what's the best way to measure those levels in a diagnostic setting? Those are two questions Bo Williams, Kansas State University Ph.D. candidate in swine nutrition, recently set out to investigate and detailed during this week's K-State Swine Day.

With the goal to reduce the level of phosphorus in the diet, that will ultimately be excreted into manure that's applied on fields, Williams' first study examined feeding adequate or below adequate levels of phosphorus and how that impacted the calcium phosphorus ratio.

"So, feeding lower levels of phosphorus, if we widen the calcium phosphorus ratio or increase the level of calcium in the diet, we reduced performance anywhere 10 to 12 pounds at the end of the wean-to-finish study," Williams says. "Then when we're adequate in phosphorous and we increase the calcium phosphorus ratio, we start to see an increase in performance, so that would be on growth performance, but we'd see the same result when looking at bone mineralization. Having lower levels of phosphorus and increasing the amount of calcium, we start to reduce the amount of bone mineralization occurring in that animal."

While calcium and phosphorus play a large role in both growth performance and bone mineralization, Williams says the issue is the U.S. National Research Council's recommendations for phosphorus level are set for maximum growth performance, not maximum bone mineralization. For maximum bone mineralization, the phosphorus level is actually slightly higher than it would be for growth performance.

"Usually, producers would feed for  maximum growth performance, not thinking about the bone mineralization part, but slightly higher levels of phosphorus would be needed to maximize that bone mineralization," Williams says.

Williams says there is kind of a "sweet spot of maximizing both growth performance and bone mineralization without causing large excretions of phosphorus."

"It's kind of a fine line of where you want to be and phosphorus is a relatively expensive ingredient," Williams says. "Energy's always the highest and then you got your proteins, amino acids and then phosphorus will be right behind that. It's not included in the diet at high levels, but it is an expensive ingredient to include in the diet."

In his second study Williams examined standardizing the diagnostic tools to measure bone mineralization, since there are several different ways to calculate. The biggest being which bones should be selected for analysis.

"You can measure calcium, phosphorus in the bone, you can measure it in a blood sample that you pull from the pig, or you can measure it in the urine," Williams says.

When Minnesota Pork Board put out a grant asking for research focusing on lameness in finishing pigs, Williams' application was accepted and he decided to break up the research into three sub-studies, one in the nursery, one in the finisher and one collaborating with industry partners.

For the first two studies the research team fed various levels of phosphorus, vitamin D and phytase to look at different bone mineralization in nursery pigs.

"We weren't trying to set a requirement for the phosphorus or vitamin D, but we wanted to have a broad range so we could pick up differences in the bone mineralization," Williams says.

Pigs were fed for 28 days and then on those pigs they collected blood, urine and metacarpals fibulas, second and 10th ribs on the right and left side, so eight bones per pig to analyze for different bone mineralization techniques.

"These can be bone density, bone ash, bone breaking strength," Williams says. "A lot of vet schools look at histopathology, so those growth plates on the fibulas and ribs looking at incidences of bone metabolic disease, so are there fractures in those ribs? Is there cartilage not turning into bone?"

The team found the biggest difference was between the procedures. Since they took the right and left bone from each pig, they were able to analyze two of the exact same bones from the same pig.

"Then when we do bone ash, you can either use a defatted method, so pulling all that fat and water out of the bone before measuring it, or non-defatted, and when you compare the same bone on both procedures, there's anywhere from a 10 to 12% difference just on that same bone based on the procedures used," Williams says. "That can be key because if you don't know the procedures used when you get that value back, interpretation can be much different depending on what that value is. Are you deficient or are you not deficient in phosphorus or vitamin D based on that value that you receive?"

Williams acknowledges not many diagnostic labs look at bones, as it is time consuming to get the bone clean and run it through the different procedures, and then to decide which procedure or bone should be used.

"One key takeaway, we worked with a pathologist and in a textbook that was written 25-30 years ago, they said always collect the second rib for measuring growth plates and histopathology for metabolic bone disease cases, and so they've always done that," Williams says. "With my study we sent them second ribs and 10th ribs and based off my research they found that the 10th rib was a better indicator to find these differences."

That lab is now collecting the 10th rib thanks to Williams' research.

Based on those two studies the research team then collaborated with 14 different production systems across Minnesota and Iowa, 64 commercial sites in total, and at each site collected all the same measurements — bones, blood and urine from a clinically healthy pig, a lame pig and a sick pig. Since they collected data from healthy as well as sick pigs, they were able to see what bone mineralization standards should be for healthy pigs.

With more than 3,000 bones collected the team was also able to examine the differences in procedures across different types of pigs, different age ranges and different health statuses.

"Some pigs came from a healthy sow farm and then they go to a healthy nursery, and they go to a healthy finisher, and they have really high health throughout their life," Williams says. "But we also collected pigs that come from a PRRS positive sow farm and then they get E. coli in the nursery and then they run into flu in the finisher, so then how do those impact bone mineralization as well?"

From that extensive data, the research team found there are differences between analysis when comparing the three pig types. Healthy pigs have higher levels of calcium, phosphorus and vitamin D in the serum compared to lame and sick pigs. Sick pigs are excreting a lot more phosphorus and creatinine in their urine. This is due to the pigs having low feed intake, resulting in the pig breaking down muscle tissue for energy reserves, causing the excess being excreted.

The biggest difference is between the two methods for bone ash, Williams says. When defatting bones, there is a reduction in the amount of variation between the bones, compared to non-defatted bone ash. The team observed the same magnitude of change in bone ash, a 1-2% difference between a healthy and sick pig regardless of the method used, but it is statistically significant within defatted bones due to the reduction in variation across the samples.

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