Maintaining butterfat levels during grazing is a great challenge facing milk producers and it’s a particular issue for spring calving herds, where cows are hitting peak yield and need to retain sufficient condition for good fertility.

“But even later lactation cows will typically produce milk with a lower butterfat content when grazing, despite yields naturally declining at a steady rate,” pointed out Dr Derek McIlmoyle, AB Vista’s ruminant technical director.

“Although these cows aren’t facing the same energy demands as those in early lactation, yields are often still in excess of 15 litres/cow/day. Plus it’s an important time for replenishing body fat reserves prior to drying off.”

Balancing energy supply

Supplying the right amount and type of energy to the cow is, therefore, critical if butterfat production is to be supported without adversely affecting milk yields, body condition or fertility.

“Grazed grass can be low in fibre with high sugar and oil contents,” Dr McIlmoyle explained. “And for modern ryegrass swards, this can persist right through until late summer, not just during spring when most expect it.

“Unless properly balanced in the rumen, the rapidly fermentable carbohydrates, together with the higher oil content, can have a negative impact on rumen function and fibre digestion, resulting in depressed butterfat production.”

Stabilising rumen pH

Achieving the correct energy release in the rumen is particularly important as this helps limit any rumen pH drop, and promotes fibre digestion and the production of key milk fat key pre-cursors, such as the volatile fatty acid acetate.

“Aim to keep rumen pH as stable as possible and to minimise the time spent below pH 5.8 when fibre digestion is compromised,” advised Dr McIlmoyle.

“Start by ensuring an adequate forage-to-concentrate ratio in the overall diet, and make sure any buffer feed contains sufficient long fibre to stimulate cudding – to release acid-neutralising saliva – and good rumen function.”

Hay or big bale silage are ideal, recommended Dr McIlmoyle, with chopped straw a good alternative if these are not available.

Correct concentrate choice

“In addition, make sure a good proportion of the energy supplied by concentrates comes from feeds containing digestible fibre," he said.

"Dry feed options include sugar beet feed and soya hulls, along with many of the moist feeds, and this will help both maintain rumen pH and boost the supply of milk fat pre-cursors.

“If only feeding compound in the parlour, switch to a concentrate based on digestible fibre rather than starch, and feed ‘little and often’ to avoid overloading the rumen. And always make sure concentrates are high quality with good palatability to encourage full intake alongside grazed grass,” he adds.

Dr McIlmoyle also highlighted the value of using of a slow-release rumen conditioner, like Acid Buf, or a metabolically active yeast, such as Vistacell. Both reduce the rate and extent of any rumen pH drop, and so minimise the negative impact on rumen fermentation.

Increase butterfat production

In a recent trial, the addition of a slow-release conditioner and an active live yeast to cows receiving a 65:35 forage-to-concentrate ratio diet increased both milk fat production (1649 g/cow/day vs 1462 g/cow/day) and fat-corrected milk yield (38.9kg/cow/day vs 37.1kg/cow/day).

The result was due to a substantial reduction in the time the rumen spent at low pH (see table1), which improved fibre fermentation and produced a significant rise in the acetate concentration in the rumen (63.1% vs 61.9%).

“These results show how important it is to properly support rumen fermentation if butterfat production is to be maintained when grazing. It doesn’t matter whether that’s achieved through changing the diet, adding a rumen conditioner or live yeast, or all three. The overall improvement in butterfat percentage, and subsequent milk value and profitability, can be substantial,” he concluded.