Milk urea

Milk urea can be used alongside other information as a management tool to support decisions to:

• Optimise nitrogen management and reduce nitrogen loss risks from the farm system.
• Review feed management, including grazing management, and the type and amount of supplementary feed.

Milk urea concentrations are a near real-time indicator of the herd’s dietary crude protein status and their efficiency of converting dietary nitrogen into milk production. Consistently high or low milk urea provides a prompt to review nitrogen use and feed management across multiple aspects that affect the farm system’s nitrogen loss risk.

Management options to improve farm efficiency and performance can be identified using milk urea data and complementary annual and seasonal information like purchased nitrogen surplus, farm insights reports, nutrient budgets and Farm Environment Plans.

How urea is produced

The cow’s liver produces urea from ammonia. This ammonia mainly comes from the breakdown of dietary crude protein in the rumen, as well as from the metabolism of absorbed amino acids and body protein.

Excess ammonia typically occurs when degradable protein in the diet exceeds what the rumen microbes can use, which is influenced by the energy available from fermentable carbohydrates and by how quickly ingested feed moves through the rumen (the passage rate).

The resulting urea is excreted in urine, with smaller amounts excreted in milk. When the diet does not contain enough protein, less urea is excreted, and more is recycled back into the rumen. There, it is converted back to ammonia and reused by the microbes. Read more on protein metabolism here.

High milk urea concentrations

High milk urea (>33 mg/dL) indicates surplus dietary crude protein and risk of higher urinary nitrogen excretion, especially when milk urea is above 40 mg/dL. Pasture-based systems naturally tend to show higher bulk milk urea than in housed systems. However, these higher values are generally not a sign of poor cow health or reproductive performance.

High milk urea concentrations can help identify opportunities to improve dietary nitrogen use efficiency and manage nitrogen loss risks across the farm system. This is especially useful at key times of the year, such as autumn.

In autumn, nitrogen leaching risk increases because higher dietary protein and urinary nitrogen loads often coincide with cooler temperatures, slower pasture growth, and higher rainfall. These conditions reduce plant nitrogen uptake and increase drainage. Therefore, focusing on management and mitigations during this period can have the greatest impact on reducing nitrogen loss.

Management options include refining grazing management, nitrogen fertiliser and effluent applications, and optimising the profitable use of supplements. Alternatively, excess urinary nitrogen being deposited onto soils can be reduced by using off-paddock areas or by lowering stocking density – for example, through timely culling. Read more about reducing nitrogen loss here.

Low milk urea concentrations

Low milk urea (<17 mg/dL) may indicate a dietary crude protein deficit. However, lower values are common during spring and can simply reflect the early lactation status of cows.

If milk urea concentrations remain consistently low (especially below 10 mg/dL), it is important to check other indicators such as milk production, pasture residuals and body condition score to understand the situation.

You could test your pasture and supplements, while using the FeedChecker calculator and the Supplement Price Calculator to assess the complete diet and marginal cost-benefit of possible options.

Focus on trends

Look for trends in average milk urea concentrations and compare with regional averages (if available) as values typically vary day-to-day. Investigate large differences relative to regional averages and any extreme values to understand potential contributing factors.

Farm system context

The type and amount of dietary protein and energy relative to animal requirements are key drivers that affect milk urea. However, it is also influenced by other factors like stage of lactation, time of year, body condition loss, milking frequency, water intake, breed, and timing of feeding. Milk urea values should be interpreted in the context of the wider farm system.