Irrespective of what type of farm system you are using, cows go through a massive physiological change at the time of calving. One of the main changes their body has to navigate, is a huge increase in calcium mobilisation from the days just prior to calving to the days following calving. This large additional drain on blood calcium gets “re-routed” into the mammary gland to supply the calcium needs of, firstly colostrum, and then increasing milk production during the weeks after calving. This is how a cow avoids getting milk fever (low blood calcium).
A cow, or first calving heifer, is not able to supply these additional calcium needs simply from ingested feed – the significant calcium reserves in the bone need to be used via an increased rate of calcium “shift” from the bone into the blood and onto the udder. Unlike production systems where total mixed rations are used, pasture fed transition cows are at a disadvantage because the pasture they eat has a balance of sodium, potassium, chloride and sulphur which works against efficient calcium mobilisation around calving. This nutrient balance is commonly referred to as the DCAD of the daily ration. A high, positive, DCAD number works against calcium leaving the bone and heading to the blood – a low DCAB number is better for transition cows. However, this is hard on pasture as its DCAD number is often between 350 and 800. It is this fact that contributes to an increased milk fever risk in pasture fed animals.
The use of Mag Oxide or Mag Chloride in transition cow rations assists with lowering the DCAD and raising blood magnesium with both mechanisms being important levers to lower milk fever risk. In addition to using magnesium products, other “anionic salts” can be incorporated into the diet which have a greater ability to lower the DCAD of the overall diet. Remember, that the lower the DCAD number of the total diet pre-calving, the lower the milk fever risk and likely incidence of subclinical milk fever in a herd. These anionic salt products are generally included in the daily ration for around 2 to 3 weeks prior to calving at a rate of between 180 -300 grams per cow per day.
Herds that should consider this feed inclusion approach to reduce milk fever risk are ones that:
- Have the ability to differentially feed cows within 3 weeks of calving from those farther away from calving (hence, running a springer mob)
- Have the ability to blend a pre-mix mineral with the feed being used through a mixer wagon or silage wagon
- Have the ability to mix and deliver the feed (e.g. onto a feed pad) to minimise wastage and time spent with the feed on the ground prior to cow access
If this sounds like your herd, please take the opportunity to talk with your Vet or TSR about feeding options for your farm this coming calving.