Greater emphasis needs to be placed on milk fever prevention on Scottish dairy farms considering the risk posed by high grass silage dry cow diets.

Challenging growing conditions for maize and wholecrop means that grass tends to form the basis of dry cow diets in Scotland. However, compared to other types of forages, grass often contains a high Dietary Cation Anion Balance (DCAB), which influences the metabolic pH status of the cow.

Grass can also be hugely variable in DCAB, ranging from +300 to +800 mEq/kg DM. Critically around calving, a high DCAB in the diet will put the cow into a more alkaline state, reducing her ability to mobilise calcium and increasing the risk of milk fever.

At £50 a case, subclinical milk fever is hugely costly. Based on research that suggests half of second to sixth lactation cows suffer from subclinical milk fever, Mole Valley Farmers nutritionist, Laura Quinn estimates that the ‘hidden’ disease is costing the Scottish dairy industry around £4.5m a year.

“As an industry we need to think about this more, especially in Scotland where we’re at higher risk than the rest of the UK – or even the world – as cows are on grass-based diets,” she says.

The £4.5m may seem like a huge figure, but Laura believes the actual cost could be higher considering clinical milk fever is not included in this figure and the fact the disease is a gateway to other issues like ketosis, metritis and displaced abomasums.

Ideally, dry cow diets should have a DCAB of -40 to +70mEq/kg DM. On systems feeding grass silage or grass silage and dry cow buckets, dietary DCAB could be as high as 450mEq/kg DM. Consequently, a lot of work is needed to drive this figure down.

First and foremost, Laura says it’s important to get a handle on grass silage mineral levels by undertaking silage analysis on every cut and every time clamps are changed. Steps can then be taken to mitigate any issues.

“I don’t think enough farmers are analysing their grass silage,” she says. “You need to know your DCAB. If you’re on a partial DCAB diet, is your total ration between -40 and +70mEq/kg DM? If not, you will be having more subclinical milk fever and associated issues.”

Laura says there are a number of strategies farmers can adopt to hit this DCAB “sweet-spot”:

1. Grow a specific low potash silage

High potash (potassium) levels in grass silage will increase the risk of milk fever. Fields that have had a lot of slurry applied will be prone to high potash levels. With this in mind, consider growing a specific grass silage crop for dry cows on a field that has not received slurry. Cut at about 40-45% dry matter (DM) and big bale. This high DM, low energy silage is perfectly suited to dry cows and usually has a DCAB of 250mEq/kg DM - much lower than ‘conventional’ silage.

2. Incorporate chopped straw

Mixing chopped straw into grass-based rations can help reduce the DCAB, as well as energy. However, this will need to be combined with extra anionic salts to drive the DCAB down far enough. Straw should be chopped short and included at around 4kg a head with an appropriate transition blend or nut.

3. Select an appropriate nut or blend

Selecting an appropriate nut or blend that can “pack a punch” against the high DCAB in grass silage is a must. Not all transition cow supplements include a sufficient levels of anionic salts to achieve the target DCAB so it’s important to check. For example, in a transition cow diet including 6.9kgDM of grass silage, 2.5kgDM of chopped straw and 3.5kgDM of a traditional dry cow nut, the DCAB could be around 167mEq/kg DM. If the dietary components stay the same and the nut is replaced by the same amount of Mole Valley Farmers’ specially designed dry cow nut, TransLink 4000, the DCAB drops to 32mEq/kg DM.

This type of specialist dry cow nut should suit farms with small groups of dry cows where it’s unpractical to make a specific TMR. This nut can be fed alongside grass silage to help achieve the target DCAB and reduce milk fever risk. The blend, Translink 3000 + TCH (rumen protected choline) is also available. This includes a higher level of anionic salts meaning it can be fed at lower levels.

4. Feed wholecrop

Wholecrop and maize silages are lower in DCAB than grass. Obviously these are challenging to grow in most regions of Scotland, but when available, mix into the ration, along with appropriate anionic salts.