At this stage of the grazing season – from late spring to early summer – questions often arise around maintaining milk fat content when spring calving herds are grazing lush high-quality grass at this time of the year.
It is often perceived that low milk fat indicates rumen acidosis, with negative consequences for herd health; however, low milk fat when it comes to rumen acidosis would have to be as low as 3.25%.
When a herd’s milk fat content drops, farmers often feed silage/straw at milking or feed a rumen buffer, even when grass supply is sufficient.
However, responses to these measures are often poor in practice, as cows in most cases are often milking well with no indication of poor health and with a good appetite.
On an indoor diet, where the herd is on a high starch/low fibre diet, this drop in milk fat content would be concerning as it would indicate the production of excess lactic acid by the rumen bacteria.
However, low milk fat content as an indicator of rumen health problems does not directly translate for a grazing diet as the cause of reduced milk fat in grazing cows is somewhat different than that associated with classical acidosis on grain diets.
Drop in milk fat content
This drop in milk fat involves specific fatty acids (CLA) derived from the digestion of quality grass in rumen, which changes milk fat by reducing the activity of key enzymes, controlling fat synthesis in the udder.
In general, fibre digestion and rumen health are not affected ,and the temporary change in milk fat content on high quality grass diets occurs within the udder and not the rumen.
Studies have shown that differences in milk fat content from herd to herd were due to lipid content, which was added to the diet, rather than fibre in the diet, while rumen pH was similar in all herds.
Genetics has a huge say on milk fat percentages, especially in May, as a herd with zero predicted difference (PD) for fat % on the economic breeding index (EBI) report could expect milk fat to be around 3.6% in that month, according to Teagasc.
Milk fat content at peak milk yield is generally expected to be 0.4% to 0.5% lower than the lactation average, so it is not unnatural for a herd delivering 4.3% fat for the year to be sending out milk at 3.9% fat in late April to May.
It is important for farmers to maximise the herd’s energy intake at this point of the lactation as cows are reaching peak, ideally grazing 36-hour allocations at pre-grazing covers of 1,400kg dry matter (DM)/ha and achieving a post grazing residual of 4-4.5cm.
Fibre content of the diet is still going to be important, as a minimum fibre content (NDF) of 33-35% of DM is met where grass is in deficit, whereas when grass quality is lush and optimal, a diet consisting of 37-42% NDF will provide adequate fibre of which 75% of the total NDF should come from forage.
When feed deficits arise, feeding a slower degradability concentrate like maize is an option that is preferable to barley or wheat, as the starch in it is degraded at a slower rate.
Soya hulls or beet pulp are also good sources of NDF, and are a popular choice for farmers trying to rectify feed deficits and milk fat content deficits.
Farmers should also limit the inclusion of high oil content feed ingredients in concentrates and ensure that cows are well covered for macro minerals in the diet such as salt, phosphorus, magnesium, and calcium.
If all of these diet requirements are met and milk fat is still low, just continue feeding, meeting the above criteria, as studies have shown that low milk fat cases eventually resolve themselves in two to three weeks.
When rumen conditions adapt and when grass composition changes, milk fat will eventually rectify itself and the cow should remain healthy throughout these milk composition changes.
