Butterfat remains a major commercial driver on Irish dairy farms. Milk is paid largely on an A+B-C basis, where processors pay for kilograms of fat and protein and deduct a volume charge. Protein is usually the more valuable component, but fat still makes up a large part of final milk value. For spring-calving herds, this matters most when butterfat falls during peak milk supply.

Milk fat depression, or MFD, is more than a low butterfat test. It is best understood as a reduction in milk fat concentration or fat yield while milk yield remains normal or may even increase. Protein often holds closer to expectation. Irish field work has commonly used repeated bulk-tank fat at or below 3.3%, with protein at or above 3.2%, as a practical herd-level warning sign. Other definitions use fat below about 3.2%, or a fall of more than 0.4 percentage points lasting 10 days or more.

The Irish context is important because not every late-spring fat drop is true MFD. Historic Irish herd analyses found that around 10% of herds had bulk-tank fat below 3.3% and around 40% were below 3.6% during April to June. MFD can occur anytime from April to September. Teagasc reported a 0.44 percentage-point reduction in national milk fat from spring to summer in 2023.This timing is commercially painful because it coincides with high milk volume.

Devenish have designed an innovative product which is designed to significantly enhance milk solids. This product is very effective in reducing the impact of MFD. Devenish Milk Solids Booster is composed of four products each having significant enhancing effects on both butterfat, milk protein and total milk solids output. Devenish Milk Solids Booster when used in line with their definite guidelines has also shown significant cow fertility benefits with increased conception rates and reduced embryo deaths. Significant cow health benefits have also been reported on many farms.

The graphs below show responses obtained on Irish family farms when Devenish Milk Solids Booster was included in these farmer’s concentrates at 80 g/cow/day in 2025 and the graph highlights that butterfats remained significantly higher throughout 2025. The graph clearly shows MFD occurring in 2023 and the typical seasonal variation in butterfat when Devenish Milk Solids Booster was not included.

• Both herds show a clear spring trough, with butterfat lowest around April to May.
• Farmer A shows a sharper 2023 dip, followed by recovery from late summer into autumn.
• Farmer B holds butterfat higher overall, but still shows the same seasonal spring pressure.
• Both farmers included Devenish Milk Solids Booster in 2025  and obtained significantly higher butterfats and kgs of milk solids.

True diet-related Milk Fat Depression is caused by altered rumen biohydrogenation. Under certain nutritional  conditions, particularly low effective fibre, high concentrate feeding, high starch intake, or elevated unsaturated-fat supply, rumen fatty-acid metabolism can shift away from the normal biohydrogenation pathway and towards the  trans-10 pathway. This results in increased production of specific intermediates, most notably trans-10, cis-12 conjugated linoleic acid. These intermediates directly inhibit de novo fatty acid synthesis in the mammary gland, reducing butterfat production. 

The outcome is classic milk fat depression: milk yield may be maintained, but milk fat percentage and milk fat yield fall.

Irish grazing herds add another layer of complexity. Lush leafy grass, high unsaturated-fat intake, selective grazing, and low rumen pH may all contribute in some herds. Also, time of year and herd fat genetics explain much of the recurring spring-summer pattern. Low butterfat at grass should not automatically be treated as a simple fibre shortage.

Butterfat is margin. When it falls, first identify whether the herd is experiencing normal seasonal decline, true nutrition-driven MFD, or another health or energy-balance problems. Consult our nutritional team for the best advice for your farm.