Pay attention to vitamin D in your pigs' diet

Saturday, August 8, 2015

Vitamin D deficiency can cause metabolic bone disease and even sudden death in nursery and finishing pigs, posing the question whether current ration formulations provide enough dietary vitamin D for animals raised in confinement

by JANICE MURPHY

Vitamin D is a fat-soluble hormone responsible for enhancing intestinal absorption of calcium, iron, magnesium, phosphate and zinc. Within the body, it is stored in adipose tissue, muscle and the liver.

Vitamin D is essential for calcium homeostasis. Mammals maintain serum calcium concentrations within a stable range in order to support normal muscle contractions, nerve activity and facilitate the release of various other hormones. In order to maintain calcium homeostasis, calcium can be mobilized from bone reserves, stored in the kidney or absorbed from the diet. Vitamin D's utility does not stop there, since it is also used within other body systems in the control of immune system functions.

Hypovitaminosis D is the term used to describe a vitamin D deficiency. This deficiency can result from inadequate vitamin D intake from the diet or inadequate exposure to sunlight, disorders that limit vitamin D absorption and conditions impairing the conversion of vitamin D into its active metabolites (these could include various liver, kidney, and hereditary disorders).

Sustained hypovitaminosis D can result in metabolic bone disease, a general term used to describe multiple nutritional diseases related to bone growth or modelling. When growing pigs experience hypovitaminosis D the open growth plates become widened due to failure in bone mineralization, a disease process known as rickets. Hypovitaminosis D in mature animals is referred to as osteomalacia, since the growth plates have already closed and the primary issue is defective bone metabolism.

A recent study at Iowa State University has focused attention on vitamin D levels in the swine industry. In 2010, the university's Veterinary Diagnostic Laboratory (ISU-VDL) became aware of several cases of sudden death in nursery and finishing pigs. Upon investigation these deaths were attributed to vitamin D deficiency and hypocalcemia. While feed mixing errors were evident in several of the cases, still others could not be explained despite the diagnosis of hypovitaminosis D.
In light of this, the researchers set out to further investigate vitamin D-related issues in swine by:

• Examining the range of serum 25-hydroxyvitamin D [25(OH)D] concentrations in pigs of different age groups
• Comparing these values to published reference values
• Determining if 25(OH)D concentrations varied with season
• Assessing the quality of vitamin D supplements in feed premixes from multiple commercial suppliers.

The study included nursery (two to four weeks), grower (10-14 weeks), finisher pigs (six to eight months), sows and mature boars. Fifteen case submissions in each age category were selected for inclusion from a three-week period in both January and June 2011. A total of 1,200 serum samples were selected, 600 for each time period.

In June 2011, serum samples from pigs raised outdoors or that had access to open lots were also secured. The additional pigs included nursery, grower and finisher pigs and sows. Ten serum samples were collected from each age group in order to provide a comparison of 25(OH)D concentrations in the samples between confined and outdoor pigs.

Finally, through collaboration with feed companies, 45 samples of vitamin D premixes were obtained from suppliers and analyzed at two laboratories over a nine-month period (November 2011 through July 2012).

The serum results revealed that, in all age categories, 25(OH)D concentrations in numerous samples were lower than reference values (Table 1). Although there was considerable variation within all age groups, there were individual pigs within each group exhibiting deficient 25(OH)D  levels compared to historical reference values.

In the nursery, finisher and boar age categories, there was a significant difference between the months of January and June, with June samples exhibiting higher levels of circulating 25(OH)D. (See Table 2.) Grower pig values were, however, significantly higher in January than in June. Overall, mature animals had higher 25(OH)D concentrations than younger animals in both time periods.

As has been reported in previous studies, serum samples from pigs raised outdoors had significantly higher 25(OH)D concentrations than samples from pigs raised indoors. (See Table 3.)

Of the 45 premix samples received, 23 were manufactured outside the United States and 22 samples came from two U.S. manufacturers. Although the vitamin D level varied in the samples tested, no individual premix had a concentration of 25(OH)D statistically lower than 500,000 international units IU per gram. However, differences were detected based on the month of sampling; premixes collected in the spring (February, March, and April) had lower levels of 25(OH)D than those collected during summer months.

Since pigs are primarily raised in modern confinement facilities, without exposure to sunlight, the provision of dietary forms of vitamin D is essential. The results of this study suggest that 25(OH)D levels are highly variable across all age categories of confined swine. Based on the ranges reported in the serum data, individual swine within a population may be functioning at suboptimal serum vitamin D levels without exhibiting clinical signs of deficiency. However, hypovitaminosis D can cause clinical metabolic bone disease if serum 25(OH)D concentrations are low for extended periods of time, potentially leading to low blood calcium and phosphorous levels, tremors, weakness, seizures and sudden death.

Of particular interest in this study were the results of the subset of outdoor pigs, which showed significantly higher 25(OH)D levels than their counterparts raised indoors. This certainly raises questions of whether current ration formulations provide sufficient dietary vitamin D to support the physiological requirements of animals raised in confinement.

The National Research Council (NRC) recommends that dietary vitamin D in swine diets range from 150 to 220 IU (3.75 to 5.50 milligrams) per kilogram of diet (depending on the stage of production). While the 2012 NRC requirements for sows were increased to 800 IU per kilogram (20 milligrams), there was no adjustment made for growing pigs. Typically, the swine industry uses three to five times the NRC-recommended level of vitamin D in the diet, yet the serum levels of 25(OH)D in confined herds are still well below that of pigs raised outdoors.

Research into the implications of subclinical hypovitaminosis D in swine has not been reported. However, from studies in human medicine, there is evidence that vitamin D plays a vital role in anti-cancer regimens and in the immune system. It is therefore possible that inadequate levels of vitamin D could compromise the pig's immunological response to invading pathogens or to vaccination.

It is important to be aware that vitamin D is susceptible to degradation by heat and moisture, especially if direct contact occurs with certain minerals like ferrous sulfate and manganese oxide. Several reports have made the link between feedstuffs contaminated with mould or mycotoxins and outbreaks of rickets. It is therefore essential that quality control measures are in place to properly manage premixes and their inclusion in complete swine diets to prevent hypovitaminosis D. The researchers reported that this particular quality control concern was identified as a contributor in two metabolic bone disease cases that occurred in Iowa.

It is a worthwhile exercise to assess specific aspects of individual nutrition programs to ensure that target nutrient levels are being achieved. Not only will this give the industry a benchmark, it is important to ensure that producers are getting what they pay for, especially when it comes to expensive ingredients like premixes. BP

Janice Murphy is a former Ontario agriculture ministry swine nutritionist who now lives and works in Prince Edward Island.

Source: Arnold J, Madson DM, Ensley SM, Goff JP, Sparks C, Stevenson GW, Crenshaw T, Wang C, and Horst RL. Survey of serum vitamin D status across stages of swine production and evaluation of supplemental bulk vitamin D premixes used in swine diets. J Swine Health Prod. 2015;23(1):28–34.