The Importance Of Vitamins – Vitamin D

Vitamin D is critical for normal development and growth.  The primary function of vitamin D is to increase blood plasma calcium and phosphorous to a level which will support normal mineralisation of bone.  It also has an important role in preventing hypocalcaemia (milk fever) due to its role in the regulation of calcium.  More recently it has also been found to play a role in the animals’ immune defence system.

During the summer the vitamin D requirement of cattle and sheep can often get ignored as outside on pasture generally they should receive adequate vitamin D either from the sun or ingestion of vitamin D₂ from forages.  Roughly 2 – 3 hours of sunlight a day should meet a beef animal’s requirement and appreciable amounts are consumed through sun-cured forage however the synthesis of vitamin D₃ reduces as daylight hours reduce.  During the winter when cattle are in the shed, they are mainly reliant on vitamin D obtained through grass stores (hay, silage or haylage) or concentrates.  Although forages are a good source of vitamin D₂ it is extremely variable and can’t be relied on as a consistent source of vitamin D.  A standard forage mineral analysis does not tend to include vitamins due to the expense of these tests.

In young animals a deficiency of vitamin D can cause rickets and in older animals causes osteomalacia (softening of the bones that can lead to fractures).  Early signs in calves of a deficiency are poor appetite, decreased growth, stiff gait, weakness and laboured breathing.  Later signs include swollen joints, bowed legs and bent knees.

The EU legislation only permits vitamin D₃ as the authorised source of vitamin D for all animal species, with a maximum permitted level for cattle and sheep of 4,000 IU/ kg of feed (at moisture content 12%).  The requirements from NRC (2016) for beef cattle is 275 IU/kg DM and the requirements for a pregnant ewe is 215 IU/kg DM (NRC, 2007).  Ruminants have a limited store of vitamin D therefore they need a constant supply through sunlight, diet or both.  Commercial products containing vitamin D are provided through synthetic additives in the mineral supplement or feed at regulated quantities.

Mary Young, mary.young@sac.co.uk