“My milk fat has dropped in the last two weeks – should I be worried?”
This is a frequent question during May.
The common assumption is that low milk fat is caused by ‘very lush grass’ not having enough fibre.
This will mean low rumen pH and acidosis — cows loose in the dung is taken as evidence!
The stock answers are usually: ‘feed silage/straw at milking’ or ‘feed a rumen buffer’.
However, responses to these measures are variable in practice.
Cows in most cases are milking very well, have good appetites, and look otherwise healthy.
So is there a problem at all?
It is certainly the case that low milk fat on an indoor diet is cause for concern.
It indicates rumen acidosis, driven by high starch content and insufficient fibre intake, which causes significant health issues.
On the other hand, low milk fat percentage as an indicator of rumen health problems does not directly translate for a grazing context.
Why? Because the mechanism for reduced milk fat in grazing cows is somewhat different.
It is caused by fatty acids (conjugated linoleic acid. CLA) derived from rumen metabolism of grass. The change to milk fat production occurs within the udder itself, not in the rumen.
High quality second rotation grass increases the effect, and with cows also at peak yield, the drop in milk fat can be quite noticeable.
For this reason, it is a false assumption that rumen pH must be lower, if milk fat is low.
Indeed, studies have shown rumen pH to be similar for herds at 3.9% fat and 3.45% milk fat.
Differences were due to lipid content, not fibre in the diet.
This is not to say that good management of rumen pH and adequate fibre at grass are not important, however, such as:
* high intakes (17+kg DM) of quality grass, preferably on 24-hour allocations;
* post-grazing residuals of 4-4.5cm;
* maintaining pre-grazing covers 1,200kg DM;
* ensuring minimum NDF requirement (33-35% of DM) is met, where grass is in deficit;
* and feeding slower degradability concentrate ingredients (maize preferred to wheat, hulls/beet pulp instead of citrus).
When all of these conditions are met, and milk fat still is low, it is likely a CLA/grass composition effect.
What to do? Continue to meet the points listed above.
Experience has shown that such cases resolve themselves in two to three weeks, when rumen conditions adapt, and/or grass composition changes, but cows remain healthy throughout.
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