The fine line between cow requirements and the maximum tolerable limit for certain minerals means they should be given the same level of attention as any dietary nutrient.
This is the message from Bill Weiss, professor of dairy cattle nutrition at Ohio State University.
See also: 3 tips on feeding fats in dairy rations
He runs through five key areas for consideration for mineral feeding:
1. Avoid overfeeding copper
There was little justification to the high rates of copper being fed in many dairy cow diets in the UK, said Professor Weiss.
Drawing on results from a UK survey of 50 farms carried out in 2015, he highlighted the average UK farm was feeding 28mg/kg DM of copper. This was markedly higher than his recommendation of 15mg/kg DM.
Copper is essential to promote immunity, which helps reduce metritis, mastitis and the number of retained foetal membranes (RFM), he said.
However, if cows are fed too much, this could lead to accumulation of copper in the liver and even death.
The levels being fed in the UK are putting cows at risk of developing deadly copper toxicity, while adding unnecessary cost. He urged farmers to consider all sources of copper in the diet and question feeding rates.
2. Choose the right source of copper
When high sulphur or molybdenum was a problem on farm, Professor Weiss advised opting for organic copper sources that were more available to the cow than cheaper, inorganic sources.
Where antagonists were not a problem, he said inorganic sources, such as copper sulphate were adequate.
However, even when molybdenum was present at very high levels of 16mg/kg DM, this only created the requirement to feed 20-25mg/kg DM of copper – still well below the UK average.
3. Know your water
A cow’s requirements for sulphur is 0.2% of the diet. However, raise this to just 0.3% and you could quickly experience problems with reduced intakes and milk production, together with reduced copper and selenium availability.
Professor Weiss urged farmers to “know your water”, as water sources could be a significant source of sulphur on farm.
If water included 250mg/litre of sulphur, for example, that is the equivalent of adding 0.1% of dietary sulphur.
“The diet may be perfect at 0.2% sulphur, but cows are getting 0.3%,” said Professor Weiss. “You can handle this by feeding more copper.”
When water was 700mg/litre this could create bigger problems that were harder to deal with.
4. Feed dry cows a mix of selenium
Selenium deficiency can be a problem in calves, leading to sub-optimal immunity.
As a result, dry cows should receive a 25:75 mix of selenised-yeast and sodium selenite (inorganic selenium). Selenised-yeast transfers selenium effectively to colostrum, leading to elevated level of blood selenium in calves. Inorganic selenium also acts on the microbiology of the gut, aiding immune function.
5. Provide vitamin A & E to close-to-calving cows
Vitamin E and A is essential for good immune and general biological function. Calves are reliant on colostrum for vitamins A and E as they are born with virtually no natural vitamin A and E, he said.
Consequently it is worth feeding both of these vitamins to close-to-calving cows.
Feeding high vitamin E in the close-to-calving diet also reduces early lactation mastitis.
Professor Weiss recommended providing twice as much vitamin E in the close-to-calving diet than the far off dry period and about 50% more vitamin A.
Bill Weiss was speaking at Mole Valley Farmers Lifetime Dairy roadshow at Trethorne Golf Club, Launceston.