Raw milk naturally contains microorganisms. Most are harmless, but higher microbial levels can shorten shelf life, complicate processing and signal hygiene problems that raise costs for farms and processors.
For Oregon’s dairy industry, keeping milk quality consistent matters for profitability and consumer confidence. It also matters for public health systems that rely on strong food safety and quality practices.
The study shows how applied research connects directly to on-farm decision-making and strengthens Oregon’s food and agriculture economy.
At the same time, consumers and policymakers increasingly ask how farming practices — including organic and conventional systems — influence food production. Producers and processors need research that moves past assumptions and points to practical management steps.
There has been limited Oregon-based research that tracks how seasonal conditions and day-to-day farm decisions influence the mix of bacteria in raw milk over time, especially across both organic and conventional dairies.
In response, researchers in Oregon State University’s College of Agricultural Sciences, Oregon Agricultural Experiment Station and OSU Extension Service partnered with seven Oregon dairy farms to examine how raw milk bacteria shift across seasons and management systems.
The study, published in the Journal of Dairy Science, followed four organic and three conventional farms and tracked milk and environmental conditions for one year.
Sampling took place in spring, summer, fall and winter. During each season, the research team visited farms multiple times to collect raw milk from bulk tanks and gather potential environmental sources that can influence milk bacteria.
These samples included current and fresh bedding, feed and towels used during milking. On organic farms during grazing seasons, the team also collected grass and soil from pastures where cows had been before milking. Conventional herds in the study remained indoors year-round, so grass and soil were not collected there.
Across the year, the study found that the overall mix of bacteria in raw milk was largely similar between organic and conventional farms. Still, important differences appeared when researchers looked at bacterial levels and seasonal shifts.
Conventional raw milk showed higher aerobic plate counts in winter and summer, indicating higher levels of bacteria that can grow under standard testing conditions during those periods. Aerobic plate counts are a standard lab test that estimate how many bacteria are present in milk, often used as a general indicator of milk cleanliness and handling practices.
Coliform counts — which estimate bacteria linked to sanitation and environmental contamination in milk — tended to be higher in conventional milk as well, though those differences were not as clear across seasons.
Organic milk, meanwhile, showed greater seasonal variation in microbial diversity, with diversity trending higher in winter and lower by fall. Conventional milk remained more stable across seasons on this measure.
The study also highlighted the strong influence of housing and the farm environment. On conventional farms, where cows were housed indoors year-round, raw milk shared many bacterial types with bedding across seasons.
On organic farms, where cows typically shift between indoor housing and pasture grazing, those connections shifted. When cows were confined indoors, milk shared more bacterial types with bedding. During grazing seasons, milk shared more bacterial types with pasture-related sources such as grass and soil. These patterns suggest that the cows’ immediate environment — bedding, housing conditions and seasonal transitions — shapes the milk microbiota in meaningful ways.
Taken together, the findings point to a practical takeaway: Management and season can matter as much as — or more than — the organic or conventional label when it comes to indicators related to milk cleanliness and microbial stability.
For producers and processors, that supports a more targeted approach to quality improvement. Bedding conditions, moisture management, stall cleanliness, equipment upkeep and seasonal transitions are all potential leverage points, especially during winter conditions and summer heat when challenges can increase.
The study was supported by funding from BUILD Dairy and OSU’s Arbuthnot Dairy Center.
For Oregon dairy producers and industry partners, this research offers Oregon-based evidence that can support quality control planning across seasons, helping farms and processors focus attention where it is most likely to improve outcomes.
The study shows how applied research connects directly to on-farm decision-making and strengthens Oregon’s food and agriculture economy, and helps protect food safety, supports rural jobs and informs science-based discussions about agricultural practices.