Most dairy farmers are into their second grazing rotation, with slow grass growth rates still remaining a concern.

However, cows on most farms are milking well, have good appetites and look otherwise healthy as we enter the beginning of the breeding season on many farms.

So, why the low milk fat?

According to Teagasc, a number of dairy farmers have reported low milk fats in the last week. Teagasc has outlined a number of possible causes for current low milk fat levels on farms.

Low milk fat caused by lush grass

According to Teagasc, the common assumption is that low milk fat is caused by ‘very lush grass’ and not having enough fibre in the diet, but it can also be a sign of rumen acidosis, driven by high starch content and insufficient fibre intake.

This can cause significant health issues due to production of excess lactic acid by the rumen bacteria. 

Low milk fat in this context would be less than 3.25%. To try rectify the issue, many farmers feed silage/straw at milking, or feed a rumen buffer, even where grass supply is adequate.

However, responses to these measures are often poor in practice.

A temporary change within the udder

Low milk fat percentage can be an indicator of rumen health problems, but according to Teagasc this does not directly translate for a grazing context.

The cause of reduced milk fat in grazing cows is somewhat different than that associated with classical acidosis on grain diets.

The mechanism involves specific fatty acids (conjugated linoleic acid [CLA]) derived from digestion of quality grass in the rumen.

These CLAs change milk fat by reducing the activity of key enzymes controlling fat synthesis in the udder. Fibre digestion and rumen health are generally not affected. 

Therefore, the temporary change to milk fat production on high-quality grass diets occurs within the udder, not in the rumen.  

High-quality, second rotation grass increases the affect, as some of this grass can contain high levels of lipid (fats). 

In cows at peak yield, the drop in milk fat can be quite noticeable also. For this reason, it is not the case that rumen health, and therefore cow health, is poor.

Some key guidelines include:


There is a strong association between cow/herd genetics for milk fat percentage and it’s production in May.

A herd with zero to negative PD for fat percentage on their economic breeding index (EBI) report could expect milk fat to be around 3.6% in May.

Stage of lactation

Milk fat percentage at peak yield is generally expected to be 0.4-0.5% lower than the lactation average.

So a herd delivering 4.3% fat for the year will usually be expected to be 3.8-3.9% in May.

High intakes

High intakes (17kg/DM [dry matter]) of quality grass on 24 hour allocations in good grazing conditions.

Maintain pre-grazing covers >1300kg/DM and post grazing residuals of 4.0 to 4.5cm. 

Ensure minimum fibre content

Ensure minimum fibre content (neutral detergent fiber [NDF]) requirement (33-35% of DM) is met where grass is in deficit.

High-quality grass is typically 37-42% NDF, providing adequate fibre. Around 75% of total diet NDF should come from forage. 

Feeding slower degradability concentrate

Feeding slower degradability concentrate ingredients when feed deficits arise. Maize is preferable to barley or wheat, as the starch in it is degraded more slowly.

Soya hulls/beet pulp are good sources of NDF. Citrus pulp has low fibre content and increases acidosis risk at high inclusion rates.

Experience has shown that such cases resolve themselves in two-three weeks when rumen conditions adapt, and/or grass composition changes, but cows remain healthy throughout.