Maintaining butterfat levels during the grazing season is one of the main challenges facing milk producers, and not just during the spring, claims Dr Derek McIlmoyle, AB Vista’s Technical Director for GB and Ireland.
“Spring calvers are particularly affected, but even later lactation cows will typically produce milk with a lower butterfat content when grazing, despite yields naturally declining at a steady rate,” he explains. “What’s critical is to make sure the right amount and type of energy is being supplied to the cow, and that any changes in energy supply to support butterfat production don’t adversely affecting milk yields, body condition or fertility.”
The high sugar, low fibre content of modern ryegrass swards now typically persists right through until late summer, resulting in an excess of quickly available energy whenever cows are grazing. Supporting high milk yields also requires diets with an overall energy density of at least 12MJ ME/kg DM, leading to rations that are often high in rapidly fermentable starch and potentially high in oils.
“Unless properly balanced in the rumen, these factors combine to create conditions in the rumen that are detrimental to fibre digestion and butterfat production,” states Dr McIlmoyle. “As a result, it should be no surprise that milk fat levels often drop rapidly as soon as cows are turned out to grass, and stay low for most of the summer.”
A new feeding strategy
According to Dr McIlmoyle, what’s needed is a feeding strategy that balances energy release in the rumen and limits the dramatic rumen pH drop associated with high intakes of grazed grass and starchy concentrates. It also needs to promote the production of milk fat pre-cursors to support butterfat production in the udder.
“The aim is to provide as stable a rumen pH as possible, optimising fibre breakdown in the rumen by minimising the time spent below pH 5.8,” he continues. “This maximises production of the volatile fatty acids (VFA) acetate and butyrate, both of which are important pre-cursors for milk fat production.”
Dr McIlmoyle’s advice is to start by feeding additional forage that contains long fibre as part of a buffer feed. Also include a good supply of digestible fibre, such as in sugar beet feed, to further promote production of acetate and butyrate whilst still keeping overall ration energy density high.
Benefits of a rumen conditioner and live yeast
“Even those farms supplementing grazing with just compound in the parlour can switch to a feed based on digestible fibre energy, rather than starch energy. Feeding concentrates ‘little and often’ will further help maintain the more stable rumen pH needed for good fibre fermentation, as will addition of a slow-release rumen conditioner or a metabolically active live yeast,” he adds.
Both act to reduce the rate and extent of any rumen pH drop, and so minimise the negative impact on fibre fermentation. For example, in a recent trial using cows receiving a 65:35 forage-to-concentrate ratio diet, addition of a slow-release conditioner and an active live yeast together 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).
“This was due to a substantial reduction in the time rumen content spent at low pH (see Figure 1), leading to improved fibre fermentation and a significant rise in the acetate concentration in the rumen (63.1% vs. 61.9%),” explains Dr McIlmoyle.
Figure 1 – Effect on rumen pH (Source: Schothorst Feed Research, 2014)
“These are big changes, and it shows how important it is to optimise rumen fermentation if butterfat production is to be maintained when grazing, whether that’s achieved through changing the diet, adding a rumen conditioner, including a live yeast, or all three!”
Read more about Acid Buf
Read more about Vistacell