There are plenty of changes going on at this time of year. Daylight hours and temperatures are dropping, forages are changing, and one thing I always tend to see is a drop in butterfats at the start of the winter.
It's not the udder, it's the rumen
When butterfat drops, most people assume the cow is short of something and start tinkering with the diet. The problem usually starts in the rumen.
Grass, silage and cake all contain unsaturated fats. The rumen bugs don't like them, so they convert them step by step into saturated fats. It's a normal, healthy process. But when the rumen is under stress, that process gets knocked off course and stops halfway. One of the products left over is a particular fatty acid called trans-10 CLA.
This fatty acid does the damage. It gets absorbed, goes to the udder and switches off the genes and enzymes the cow uses to make milk fat. Small amounts are enough to alter the butterfat produced. The cow is still eating, still milking, but her udder has been told to stop making fat.
What knocks the rumen off course
The same few things, over and over:
- Too much starch, especially fast starch like wheat
- Not enough effective fibre, or a TMR that can be sorted by the cows
- Too much unsaturated oil. Extruded rape, biscuit, wheat distillers and brewers grains are the most common oily feeds on farm, but there are plenty more
- Anything that drops rumen pH for long stretches of the day
Extruded rapeseed is a good example. The extrusion process opens up the seed, so the oil is released in the rumen all at once rather than slowly. Feed enough of it alongside a high starch ration and the rumen gets pushed down the wrong pathway, more of that fatty acid gets made, and butterfat drops.
Where sugar comes in
It's commonly overlooked, but sugar can affect butterfat.
Fresh grass is full of sugar. Once it's ensiled, most of that sugar gets used up in the clamp during fermentation. So a housed cow on a silage-based diet is often getting far less sugar than one out grazing.
I aim for at least 5% sugar in the overall diet. Getting there brings some useful benefits:
- Better fibre digestion. Sugar feeds the fibre-digesting bugs, so the cow gets more out of her forage and eats more
- Stable rumen pH. Balancing some of the starch with sugar has been shown to hold pH up and cut the time the rumen spends in the danger zone
- Better use of protein. Quick energy from sugar helps the bugs mop up spare nitrogen. You often see milk urea come down
- Palatability. Useful when silage is a bit off or intakes are down
Put that together and you've got a rumen that stays on the right pathway. Less of that fatty acid gets made. Butterfat holds.
It isn't a buffer. Buffers patch the problem after the pH has dropped. Getting the starch and sugar balance right helps stop it dropping in the first place.
What I'd check first
If butterfat has slipped:
- Look at the ration's starch and sugar, not just the energy figure
- Check your silage analysis for residual sugar. Low sugar, high lactic silages are the ones to watch
- Look at the overall oil level in the diet
- Walk the feed passage. Is the TMR getting sorted? Are cows ruminating?
Butterfat is one of the best early warnings you've got. When it drops, the rumen is telling you something.
If your butterfat has slipped since housing or a change of forage, it's worth a look.