We all know the optimal intake for a cow is 18kg drymatter (DM) of grass but why 18kg? Why can’t they eat more?

New Zealand research has shown again and again a cow can graze about 3.4% of its bodyweight at peak intake. For a 500kg cow that is 17kg DM of grass. For a cow that’s a bit heavier it’s 18kg DM at peak intakes.

And there are some good reasons why they can only eat this much, even if you offer them more.

For starters, have you ever really watched how a cow eats?

If you haven’t, spend some time with the girls in the paddock and take a good look. It’s also on YouTube of course.

For a cow, tearing, grinding and swallowing the grass is only the beginning of the process. For the rumen to do its thing, the grass particles need to be very small so cows “unswallow” the grass and chew it again with their molars.

They don’t actually bite the grass with their teeth. Instead their tongue sweeps out in an arc, wraps around the stalks and then tears it, pulling it between the teeth in the lower jaw and the pad in the upper jaw.

Cows have fewer teeth than other animals and they don’t have upper incisors at all and, compared to sheep, their upper lips don’t do a lot of work – it’s all in the tongue action and a swing of the head to help with the tearing.

It then grinds the grass with its upper and lower molars, mixing it with saliva before swallowing.

Cows can take about 60 bites a minute and they can do that for eight to 10 eight hours a day. They then need 12 hours to ruminate that feed.

Because for a cow, tearing, grinding and swallowing the grass is only the beginning of the process.

For the rumen to do its thing, the grass particles need to be very small so cows “unswallow” the grass and chew it again with their molars.

You will see your cows doing this – after grazing they will sit down and do what is called “chewing their cud”. Their jaws are working flat out.

After being re-swallowed, the tiny particles are then worked on by a whole host of bacteria and other bugs in the rumen before passing through the cow’s other three stomachs and intestines for further digestion.

The interesting thing about cows and grass is, because of the way they eat it, they can’t get closer to the ground than about 5cm.

They eat most efficiently when grass is about 15cm tall – more than 2000kg DM/ha. The bite size gets smaller as the covers get closer to the ground.

Even if you give them a larger paddock, with the same amount of grass in it but it is all short, they often won’t make the effort to get it down.

If the grass is pushed into the ground they won’t eat that either.

Instead they will stand at the gate and moan.

Also, even if your grass is at the correct height, you need to give them enough time to eat the grass.

If it takes an hour to walk the cows to the dairy, and then three hours standing waiting to be milked and then another hour back to the paddock and you do this twice a day that is 10 hours out of the day the cow doesn’t have time to eat grass.

And remember it has to ruminate and sleep as well.

Your cow’s intake could actually be a lot lower than 18kg DM.

But do they need this much grass?

A lactating dairy cow needs energy for maintenance (so it doesn’t lose condition), pregnancy, walking to and from the dairy and the rest for milk production.

Energy is calculated as megajoules of metabolisable energy per kilogram of drymatter – MJ ME/kg DM.

Good lush spring dairy pasture is about 12MJ ME/kg DM and can fall to about 9MJ ME/kg DM in the summer dry.

A 500kg cow needs 59MJ ME/day for maintenance (5kg DM good grass), about 2MJ ME/day for walking on the flat to the dairy (0.2kg DM grass) and about 6MJ ME/day for pregnancy during lactation (0.5kg DM grass).

That’s almost 6kg DM of grass at 12MJ ME/kg DM just to keep the cow doing its thing. Heifers which are still growing also need energy to do that and cows which are light will use energy to put weight back on.

And then there is finally milk production.

So the more you feed your cows above that 6kg DM grass, the more energy they have for making milk.

Closer to calving, and when the cows are dried off, the requirement for pregnancy goes up. Four weeks prior to calving they need about 40MJ ME/day (3kg DM of grass) and two weeks prior it reaches about 50MJ ME/day (4kg DM of grass).

So how do you get your cows to eat more?

Advocates of pre-graze mowing believe that cutting the grass before putting the cows in the paddock allows them to hoover it up without having to spend all that time and energy tearing it with their tongues.

Plus it is of a consistent bite size rather than a reducing bite size as covers get lower.

The mower also puts it in nice rows so they don’t have to go looking for it – they can just start at one end of the paddock and stop at the end of the row.

The other way is to top up that 18kg DM/day of pasture with concentrates.

While cows are going around on the rotary platform for eight minutes they can slurp up 2kg DM of grain per milking which is about 52MJ ME per day if twice-a-day milking.

Or they can chow down on it in a feeder in the paddock or on a standoff pad after milking.

Trials have shown that the 18kg DM of grass daily can be stretched to 20-22kg DM for a 500kg cow if concentrates are included and with maintenance, pregnancy and walking already taken care of, that extra feed goes into one thing – milk production.

So does it make sense to feed cows more?

It’s the big debate. Should we be grass farmers only or should we feed a mix of grass and grain or why don’t we just put them in a barn?

Traditionally, New Zealand dairy farmers are known as grass farmers and we are told our milk is valued because of it.

The idea that our cows are outside eating green grass and enjoying the sunshine (or the wind and the rain) which makes them happier and healthier is part of the NZ dairy story.

Grass has also always been a cheap feed although with the price of dairy land the last few years this is now debatable. However, as the cost has already been incurred when you buy the farm it makes sense to use the pasture first.

Feeding cows well, so they are healthy and at the right body condition score for the time of year is a no brainer but adding in that extra couple of kilograms of feed to make more milk is up to you and the way you want to farm and the infrastructure you have to enable you to do it.

If the feed you are buying in is paid for by the extra milk production with money left over, and you are not leaving grass behind in the paddock which should have been eaten, then it might make your financial budgets work a lot better.

Whatever you do, and however you do it, your cows will tell you what they think of it.

If they are moaning at the gate wanting more feed and rushing back to paddocks after milking then they are hungry.

If they are settled, doing their cow thing, then you know you have it about right.