We have long known that of all the species tested, piglets are born with the lowest circulating levels of 25(OH)D. Placental transfer of vitamin D from the mother to the piglet is poor. Prior to suckling colostrum piglet serum 25(OH)D concentration is generally below 5 ng/ml (Goff et al, 1984), but can vary depending on sow diets. Colostrum does contain a small amount of vitamin D; piglets receiving a good dose of colostrum will get some improvement in vitamin D status. At 10 days of age the piglet raised indoors typically has a serum 25-(OH)D concentration around 10 ng/ml (Goff et al., 1984), which declines to less than 8 ng/ml by 21 days of age (Witschi et al, 2011). This is not enough to support optimal bone development, but keeps most pigs functioning until they are on diets fortified with vitamin D! After weaning, serum 25(OH)D slowly increases as the piglet consumes starter diet containing vitamin D. However, even though post-weaning diets often contain 6-8 fold NRC requirement levels of vitamin D, piglet serum 25(OH)D concentration at 33 days of age often remains below 15 ng / ml. Even at 77 days of age serum 25(OH)D remains below 20 ng/ml. It may be that the NRC tables need revision with todays piglets raised in confinement. An interesting observation is that piglets raised by sows housed outside with exposure to the summer sun have been found to have serum 25(OH)D concentrations above 50 ng/ml at 21 days of age! This is similar to levels found in humans and other animals exposed to the sun. Perhaps these are the levels that should be considered optimal?
Placental transfer of vitamin D from sow to piglet can be enhanced somewhat by administering a massive dose of vitamin D (5 million IU intramuscularly!) to the sow prior to farrowing. Piglets from sows treated in this way can have as much as 11-14 ng 25(OH)D / ml serum at birth. Also by giving the sow a massive dose of vitamin D prior to farrowing we can also boost the colostrum vitamin D content slightly. These piglets can have serum 25(OH)D concentrations around 15 ng/ml at 10 days of age. Perhaps more significant – if the sow has a poor vitamin D status the piglets may be getting no vitamin D – across the placenta or via colostrum. These piglets have 25(OH)D levels below 2.5 ng/ml by weaning and are most likely to exhibit clinical rickets. They may be reasonably able to maintain calcium homeostasis while on the sow calcium and phosphorus rich milk. But upon weaning they can have difficulty with calcium homeostasis and this is when most are presented to the diagnostic lab.
We have found that a single oral bolus dose of 40,000 IU vitamin D3 given at processing can allow pigs to reach weaning age with serum 25(OH)D concentrations that remain above 20 ng/ml. After oral bolusing, serum 25(OH)D concentration peaks at 80-100 ng/ml and at day 10 of life remains above 50 ng/ml (similar to outdoor pigs!). It declines from that point till weaning. In contrast littermates that do not receive a bolus dose of vitamin D have less than 10 ng 25(OH)D / ml serum. By day 30 of life untreated piglets slowly begin to improve their vitamin D status (typically reaching 12-14 ng/ml) as they begin to consume starter with vitamin D in it. This information suggests that it may also be necessary to boost vitamin D content of early weaning diets for pigs or consider giving another bolus (oral or via drinking water) dose of vitamin D at weaning!
One may want to use caution using vitamin D products that are on the market now that also contain vitamin A. Vitamin A actions can counteract some effects of vitamin D and in calves vitamin A injection was associated with premature closure of physes of the long bones of the hind leg resulting in "hyena" disease in calves (Carroll Woodard et al., 1997). Mixing iron (such as iron dextran) with vitamin D can rapidly oxidize the vitamin D- about 30% of the vitamin D activity is destroyed within 3 days of mixing.