At a Hawera Feedright roadshow in April Dr Jim Gibbs, senior lecturer in livestock health and production at Lincoln University, outlined how a rumen in a grass-fed cow differed from one in a cow used to being fed total mixed ration (TMR).
The fluid passage rate – the flow of water through the rumen – is much greater in grass-fed cows than those on a TMR diet. In TMR confinement systems, Gibbs said dietary drymatter (DM) levels tended to be about 50%. A cow eating 26kg DM in a TMR system would be eating about 26 litres of water. In contrast, a high intake, grass-fed cow would be eating 80-100l of water.
“That water has to go somewhere. There is a limit to how much can go through the walls of the rumen – 18l/hour over that period – and the rest of it has to go somewhere.”
Generally, that “somewhere” is the dung.
The rumen of a grass-fed cow is also much larger than that of a cow on a TMR diet. The former will account for 20-25% of the animal’s liveweight while in the latter the rumen is 10-12%. For a 700kg North American Holstein Friesian in a TMR situation, the rumen would weigh about 70kg against a 550kg NZ Friesian with a 140kg rumen.
“It’s a very, very large rumen with a high water flow that’s pushing through, and it’s a very different shape.”
Gibbs identified another crucial difference – the rumen structure. For cows on a TMR diet, there are three identifiable layers within the rumen – a fluid layer at the base, a floating layer, and a gas cap at the top. Not so in a high intake, grass-fed system.
“What you have is one very large pot of lawn clippings that’s rolling over and over again – there’s no structure or shape to it.
Buy that yacht
Farmers should put the money they were planning to spend on straw or hay to feed cows during lactation towards a yacht instead, Dr Jim Gibbs says.
They would have more fun and animal performance wouldn’t suffer.
“When you add straw to a pasture-based diet, you do not get a layer floating on top [in the rumen]. What you get is pieces of straw through your 140kg of lawn clippings rolling over and over and over in the rumen.”
Gibbs said while adding fibre to a total mixed ration (TMR) diet did stimulate rumination, in a pasture-based system other triggers including extension of the rumen, weight of the rumen, and metabolite concentration came into play.
“There are some very clear results around adding straw to a pasture-based diet,” he said.
“There’s not one study alive that says in a pasture-based system if you add fibre to the diet in the form of hay or straw, you increase production. There are many studies showing that if you add fibre to the diet in the form of hay or straw, that you reduce production or make no effect at all.”
Gibbs said the practice had no effect on rumen function and effectively replaced a high quality feed – pasture at about 12MJ ME/kg DM – with a low quality feed of 6-8MJ ME/kg DM.
“The straw that you put in there has a rumen retention time of between 48 and 72 hours. So, on a rolling basis your 1kg of straw becomes 3kg in the rumen that only holds 14kg DM. There’s no practical advantage to it and there is an immediate cost.”
Another repeated reason to include hay or straw in a pasture-based diet was to protect against rumen acidosis. Gibbs likened the incidence of rumen acidosis in pasture-based systems to yeti sightings – everyone will say they have seen one but actual evidence is elusive.
He said while New Zealand dairy cows in general had a lower rumen pH than those on a TMR diet, it was a physiological difference rather than a symptom of acidosis.
Unless pasture made up less than half a cow’s diet, adding fibre would have no impact on rumen pH.
