Suckler farms are urged to check that pre-calving rations are balanced and magnesium supply is sufficient following recent revisions to magnesium absorption rates in cattle.
US research suggests magnesium availability from feed sources could be higher than previously expected, but absorption from rock sources (such as magnesium oxide) could be lower.
New indications on magnesium availability to dairy cows have been issued by the National Research Council (NRC) in the US. Bill Weiss from the University of Ohio is working on new dairy recommendations.
The work has found that magnesium from rock sources – the most common form of magnesium supplementation in the UK – has half the absorption level as previously thought.
SAC Consulting nutritionist Karen Stewart recommends consulting with a nutritionist about balancing diets according to the farm setup and the forage available to cows.
Factors limiting magnesium absorption
- High-potassium rations Check silage analysis – high potassium levels might follow slurry application. Potassium is positively charged like magnesium and competes for absorption in the rumen.
- Rumen concentration Potassium concentration in the rumen can be much higher than magnesium, with estimates suggesting absorption can decrease 50% with a high-potassium diet (Source: Bill Weiss, Ohio State University)
- Ration is important A lot of straw in the diet can mean less magnesium going into the cow, but straw is also lower in the antagonist potassium. Magnesium and potassium levels in rations are often unknown and potassium levels might be underestimated.
- Feed access Ensure cows have adequate feed access to support intakes. Feed barrier access needs to be 600mm (260mm) for 600kg cows, 700mm (300mm) for 700kg cows and 800mm (340mm) for 800kg cows. Ad-lib access in brackets. (Source: Red Tractor
- Avoid fat cows: Appetite is reduced and intake falls, potentially decreasing magnesium intake.
Why magnesium matters
Ms Stewart stressed that grass staggers and metabolic problems around calving are the two critical health implications of undersupplying magnesium.
Furthermore, she said ample forage stocks and good silage increase the risk of metabolic problems around calving, as cows have higher body condition scores.
“Magnesium plays a vital role in helping cows mobilise their own reserves of calcium to help with muscle contractions. Low magnesium is associated with slow calvings.
“If staggers is considered a particular risk or the silage has particularly high potassium levels as a result of slurry applications, further supplementation may be considered.
“Getting a full silage analysis, including minerals, and planning pre-calving rations with nutritional advice will be particularly important ahead of spring calving 2020.”