12 June 2025

Getting into the rhythm of milk fat

The factors contributing to the annual reduction in milk fat percentage are likely to have consistent annual patterns, rather than variable patterns such as pasture nutritive value or weather conditions.

Credit: Teagasc

Valuable for butter, cheese and cream, the fat content of milk is also key to a farm’s bottom line. Yet each spring and summer, Irish milk fat levels consistently decline. What’s driving this seasonal dip? Researchers are exploring how diet, genetics and daylight interact in this complex annual rhythm.

Milk fat contributes substantially to the economic value of milk, because it can be processed into a variety of food ingredients, such as butter, cheese, cream and whole milk powder. It’s also considered the most variable component of milk, with many nutritional and non-nutritional factors thought to influence its production.

In Ireland, there is a consistent reduction in milk fat percentage from spring (i.e. February/March) to summer (i.e. May/June), with a reduction of 0.44% observed nationally during 2023 (CSO, 2024; Figure 1).

In such spring-calving pasture-based production systems, the herd’s stage of lactation and the occurrence of peak milk yield are believed to contribute to this reduction. However, previous research has shown that the greatest reduction in milk fat percentage typically occurs in May for both spring- and autumn-calving dairy cows, suggesting that the time of year may have a greater influence on milk fat than the stage of lactation. Therefore, this seasonal decline in milk fat percentage may be associated with dietary factors – for example, pasture fibre concentration – or environmental influences, such as day length, during this high-risk period.

A cow’s biological cycle – circannual rhythms – could account for up to 0.30% of the fluctuation in milk fat percentage. 

Dietary factors

Low fibre and high fatty acids in pasture during the high-risk period have been suggested as factors contributing to reduced milk fat.

Senior Research Officer Michael Dineen explains: “Fibre digestion provides a key source of acetate and butyrate for milk fat synthesis, with fibre also stimulating rumen buffering through saliva production. High unsaturated fatty acids, such as C18:2 and C18:3, are biohydrogenated within the rumen and, if rumen conditions are unfavorable, milk fat-inhibiting bioactive isomers can be produced.”

Two recent observational experiments conducted on commercial Irish dairy farms did not find evidence linking these factors to reduced milk fat percentage. In both experiments, pasture fibre and fat were maintained at satisfactory concentrations across the high-risk period. It’s possible that milk fat percentage is reduced when this pasture nutritive value interacts with concentrate supplementation. However, the impact of concentrate supplementation on milk fat in pasture-based systems remains inconsistent, with substantial effects likely requiring high levels of concentrate supplementation – more than five kilogrammes per day – or significant changes in concentrate formulation, such as high starch content.

Teagasc Postdoctoral Researcher Christopher Heffernan gathers pasture samples for analysis. Credit: Teagasc

Environmental factors

Researchers from Penn State University have proposed that the variability in milk fat percentage across the year might be related to changes in photoperiod length and natural endogenous circannual rhythms – in other words, the cow’s yearly internal biological cycles or processes.

These rhythms seem to operate independently of environmental factors, such as heat stress, or variations in forage quality. It’s estimated that these annual rhythms could account for 0.15 to 0.30% of the fluctuation in milk fat percentage, with the magnitude of the effect possibly varying by latitude and hence change in photoperiod length.

“Interestingly,” says Michael, “in dairy-producing countries in the southern hemisphere, a reciprocal circannual rhythm appears to occur. The exact mechanisms are unclear, but it’s important that dairy farmers, nutritionists and researchers consider these annual rhythms when making management decisions.”

Figure 1. Fat percentage of milk supplied to Irish milk processors each month from 2014 to 2023 (CSO, 2024)

Implications of animal genetics

Experiments have demonstrated the effect of animal genetics on the milk fat percentage of grazing dairy cows. Current estimates suggest that for every 0.1% increase in milk fat percentage predicted transmitting ability (PTA), there is an associated increase of 0.25-0.29% in milk fat.

Selecting animals for greater milk fat percentage PTA is a robust strategy to increase annual milk fat. However, it appears that the reduction in milk fat percentage during late spring to early summer might still occur in herds with high milk fat percentage PTA, albeit from a higher baseline.

The reduction in milk fat percentage from spring to summer represents a considerable financial loss for both milk producers and processors. There is limited evidence to suggest that pasture nutritive value is causing this reduction.

While a positive relationship between milk fat percentage PTA and milk fat highlights the important role of genetics, improvements in genetic merit does not seem to alleviate the reduction. A clear and consistent annual rhythm appears to be emerging, which dairy farmers, nutritionists and researchers should consider, as the factors driving this trend may be beyond the control of management.

The factors affecting milk fat production in dairy cows under pasture-based management systems was the topics up for discussion on this episode of The Dairy Edge podcast. Mike and Chris explained the typical reduction in milk fat during late spring and early summer and outline some results from their recent research project.

ACKNOWLEDGEMENTS

The commercial farmers that participated in some of this research are gratefully acknowledged.

Funding

This research was funded by the Irish Dairy Levy administered by Dairy Research Ireland and the Teagasc Walsh Scholars Programme.

Contributors

Michael Dineen, Senior Research Officer, Grassland Science Research Department, Teagasc, Animal & Grassland Research and Innovation Centre, Moorepark. michael.dineen@teagasc.ie

Christopher Heffernan, Teagasc Postdoctoral Researcher, Grassland Science Research Department, Teagasc, Animal & Grassland Research and Innovation Centre, Moorepark.