How to turn around a collapse in milk butterfat levels

Stock image.
Stock image.
Joe Kelleher

Joe Kelleher

I have received numerous phone calls over the past few weeks from farmers who have seen their milk butterfat percentages drop. The two main questions they have are: what is causing it; and how can I fix it?

Dropping butterfat is usually caused by one of three things - genetics, nutrition or a health-related issue.

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The first question I will ask a farmer when they tell me their herd's butterfat has collapsed is: did it drop at the same time last year by a similar amount?

If they answer yes then the likelihood is that the herd is genetically low in butterfat and a quick look at an EBI report will quickly back up if this is the case or not.

In the superlevy era, some herds intentionally bred for low butterfat and coupled with this, some breeds, especially Montbeliarde, are notoriously low in fat, all which will contribute to a low butterfat at this time of year.

If the farmer is experiencing a drop in butterfat that is unusual to the farm, then we would have to look at nutrition and health related issues.

Acidosis is often pointed to as a cause of depressed butterfat and as acidosis is often caused by a lack of fibre in the diet, this can generally be overcome by feeding long fibre or by feeding additives including yeasts.

However, most of the farmers I have been speaking to will have cows that are grazing covers in excess of 1200kg dry matter. Grazing cows are generally not short of fibre when grazing to appetite on 1200+kg DM swards.

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Cows need at least 35pc fibre (NDF) in their diet (with ideally 75pc of this coming from a forage source) to avoid encountering acidosis issues.

Fibre added above this level will likely reduce energy intake and performance.

High quality ryegrass sward samples taken in May from research farms generally range from 38-42pc. This will rise to 45pc after heading date.

The fibre versus milk fat is well known and indeed if fibre is lacking then milk fat pc is likely to drop. This can be very evident in TMR situations with a high percentage of starch in the diet. However, the situation may be a bit different at grass.

This is because fresh pasture can contain a high level of short chain fatty acids - polyunsaturated fats (PUFAs) and these tend to be highest in late spring (second or third grazing round).

There may also be differences between ryegrass varieties (previous research has shown Tyrella to have higher than average PUFA levels).

Why is this relevant to milk fat depression?

When PUFA enter the rumen, the rumen microbes commence a process called bio-hydrogenation, which essentially means conversion of unsaturated to saturated fats.

However, if PUFA levels in the diet are high and rumen pH is in the 5.8 to 6.0 range then there can be an increase in the production of some by-products.

These by-products are not harmful to the cow. However, if they enter the bloodstream and reach the mammary system, they can act to depress milk fat synthesis by the mammary cells.

This explains that while the rumen can be functioning well and cows are perfectly healthy, milk fat percentages can be reduced. The 'culprit' is small amounts of PUFA intermediate products not a lack of fibre.

Finally, I asked Teagasc dairy specialist Joe Patton for his views on managing and resolving low butterfat levels and his analysis can be read below.

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