Why Does Butterfat Drop During Summer?

For many dairy farmers, one of the first signs of summer heat stress is not a dramatic drop in milk yield. Instead, it is often a gradual reduction in butterfat percentage.

This can be frustrating, particularly when temperatures do not appear excessively high. However, temperature alone does not determine heat stress risk. The Temperature Humidity Index (THI) combines temperature and humidity to provide a more accurate picture of the conditions cows are experiencing. As humidity rises, cows become less effective at dissipating heat, meaning heat stress can develop long before many producers expect it.

Many nutritionists use THI as an early warning indicator of heat stress, with more noticeable impacts on performance often seen once THI exceeds 74. By this stage, reductions in dry matter intake, milk yield and milk constituents can already be occurring.

The recent spell of warm weather experienced across many parts of the UK would have pushed THI values into the 80s at certain times of the day, creating conditions where milk solids and butterfat can come under increasing pressure.

Heat Stress And Butterfat Depression

Reduced dry matter intake is often the first explanation given for falling butterfat levels, and it is certainly part of the story. As cows consume less feed, fewer nutrients are available to support milk production and milk solids. However, heat stress does more than simply reduce feed intake.

Heat-stressed cows often alter their feeding behaviour, consuming smaller and less consistent meals. These changes can affect rumen function and fibre digestion, both of which play an important role in milk fat production. At the same time, cows divert more energy to cooling mechanisms such as panting and increased respiration, thereby increasing maintenance requirements and altering how nutrients are utilised within the body.

For this reason, butterfat depression is often one of the first indicators that cows are struggling to cope with summer conditions. The impact of heat stress on performance is frequently greater than reduced feed intake alone would suggest.

Supporting Butterfat Through Nutrition

When butterfat is under pressure, maintaining milk composition becomes a priority, particularly for herds supplying milk under composition-based contracts.

Supplemental fat is often part of the nutritional response because it provides a concentrated source of energy without generating excessive metabolic heat during digestion. However, not all fats are nutritionally equal. Different fatty acids support different production outcomes, which is why understanding the fatty acid profile supplied can be just as important as the amount of fat included in the ration.

Palmitic acid (C16:0) has a recognised role in supporting butterfat production and can be a valuable nutritional tool when milk fat percentages begin to fall. For high-yielding and late-lactation cows, C16-rich supplements are often used to support milk constituents and maintain milk contract value during periods of heat stress.

The desired outcome should determine the fatty acids supplied. When the objective is supporting butterfat production, understanding the role of C16:0 becomes increasingly important.

The UFAC Approach

At UFAC, we believe producers should look beyond fat percentage and focus on the fatty acids delivering the response.

While some fatty acids are associated with supporting milk yield, milk solids, and body condition, palmitic acid (C16:0) plays an important role in supporting butterfat production. Rather than treating all fat supplements as interchangeable, we focus on how individual fatty acids influence specific production outcomes.

This approach allows nutritionists and producers to make more informed decisions based on the challenges they are trying to solve. During periods of heat stress, when butterfat depression can quickly affect milk value, understanding the role of fatty acids within the ration becomes increasingly important.

Responding early and matching the nutritional strategy to the desired outcome can help protect performance throughout the summer months.

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