Understanding the seasonal reduction in milk fat percentage on Irish dairy farms
Within Irish dairy systems, farmers consistently face a seasonal reduction in milk fat percentage from spring to summer. In this article, Michael Dineen and Chris Heffernan from Teagasc Moorepark share recent findings on milk fat percentage.
Milk fat contributes to the economic value of milk, as it can be processed into various food ingredients, including butter, cheese, cream and whole milk powder.
Within Irish dairy systems, farmers consistently face a seasonal 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).
This milk fat reduction often coincides with peak milk yield. Research has demonstrated 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 might have a greater influence on milk fat than the stage of lactation. Therefore, this seasonal reduction in milk fat percentage might be associated with dietary factors (e.g. pasture fibre concentration) or environmental influences (e.g. day length) during this specific time period.
Nutritional factors
The reduction in milk fat percentage observed has often been attributed to nutritional factors such as low fibre and high fat concentrations in pasture. However, 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 to maintain milk fat production across the high-risk period. It is 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 (e.g. > 5 kg/day) or significant changes in concentrate formulation (e.g. high starch content).
Genetics
Selection for high milk fat percentage predicted transmitting ability (PTA) has proven effective in increasing average milk fat percentage. Experiments have demonstrated that for every 0.1% increase in milk fat percentage PTA, there is an associated 0.25–0.29% increase in actual milk fat percentage. 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, suggesting that genetics can raise the baseline but might not prevent the observed seasonal reduction.
There is growing evidence that the seasonal reduction in milk fat percentage across the year might be related to changes in photoperiod length and natural endogenous circannual rhythms of the cow. These rhythms seem to operate independently of environmental factors, such as heat stress, or variations in forage quality and could explain a considerable proportion of the seasonal variation in milk fat.
Decision support tool
A large commercial-farm experiment was conducted over 2021 and 2022 to investigate factors associated with bulk tank milk fat percentage on pasture-based dairy farms. The experiment identified grazing rotation number (i.e. time of year) and the herd’s milk fat percentage PTA as the primary factors associated with bulk tank milk fat percentage. Incorporating these two factors into a decision support tool enabled accurate and precise prediction of bulk tank milk fat percentage on Irish dairy farms. It is envisaged that this tool will assist dairy farmers, nutritionists and researchers in understanding the consistent seasonal patterns of milk fat percentage throughout the year, thereby supporting more informed on-farm decision making.
Overall, it is crucial to determine when specific nutritional intervention strategies are required to increase their effectiveness and return on investment. Emerging evidence suggests that 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.
For further insights, Mike Dineen and Chris Heffernan joined a recent episode of the Dairy Edge podcast.
Listen in below:
Moorepark 2025
Farmers will be given the opportunity to learn more about the research ongoing at Teagasc Moorepark at the upcoming open day on Wednesday, July 2, 2025. At the event, researchers and advisors will showcase the latest technologies and best practices.