31 August 2001

Scrutinise mineral supply to

tackle sub-clinical milk fever

By Richard Allison

SUB-CLINICAL milk fever is a widespread problem leading to poor dry matter intake, displaced abomasums and retained cleansings, even in herds without any clinical cases.

Dry cow mineral nutrition has traditionally focussed on preventing milk fever, but paying more attention to mineral supply has shown additional benefits in a recent study, says Frank Wrights ruminant technical manager David Wilde.

"Sub-clinical milk fever impairs muscle function increasing risks of displaced abomasums, retained cleansings and prolonged calvings. Potentially every cow suffers from it around calving time. It can cost about £40/cow, mainly due to losses in milk production.

One common precaution against milk fever is to feed low calcium, high magnesium diets during the last three weeks of the dry period. But this only prevents full-blown milk fever and does not solve the underlying problem of insufficient calcium mobilisation just after calving.

He suggests a strategy which allows high levels of calcium to be fed during the dry period without causing milk fever. This involves formulating rations to correct the diet-ary cation/anion balance (DCAB).

"Most UK dry cow rations have a positive DCAB, but the aim is to achieve a negative balance before calving to acidify blood."

To investigate the benefits of the high calcium DCAB approach, Frank Wright sponsored an experiment at the Welsh Institute of Rural Studies.

"Post-calving dry matter intake was 0.7kg higher for cows fed the DCAB diet. This resulted in 100 litres/cow extra milk over the first four weeks of lactation."

Conception rates were also improved and the calving to first service interval was 2.5 days shorter. This alone is equivalent to a saving of £6-£7/cow, explains Mr Wilde.

Velcourts livestock director Richard Snow has also found improved intakes when using the DCAB approach. "But this system is not totally foolproof and requires full commitment.

"Since adopting DCAB three years ago, post-calving intakes have increased and there are fewer cases of displaced abomasums. Occasion-ally, there will be blips when a couple of milk fever cases occur."

The reason for this is not clear, but urine pH is checked regularly to ensure the DCAB diet is working to acidify blood. To do this, however, indicator strips must be carried to catch cows urinating.

A pH value of 8.0-8.5 is expected for a typical dry cow ration and 6.0-6.5 for a DCAB diet, says Mr Wilde. "Values above 8.5 will lead to cases of milk fever, while values below 5.5 indicate excess chlorides and sulphur in the ration."

Timing is critical because cows should be fed DCAB diets 12-18 days before calving, says Mr Snow. "This small window requires regular selection of cows to move them into the DCAB group."

He has adopted an additional third group of dry cows because moving them straight from the far-off group to the DCAB diet is too great a shock. &#42

DRY COW NUTRITION

&#8226 Aim to avoid milk fever.

&#8226 Consider DCAB approach.

&#8226 Maximum intakes cut problems.