Farm Ireland

Monday 23 October 2017

John Donworth: Recovery time for grass is essential

It will take time for grass plant to recovery
It will take time for grass plant to recovery

John Donworth

It's funny the bit of useless information we got to hear about during the heat- wave. For instance, I learned that if you spread 22,000 gallons of water on an acre of ground, it's the equivalent of 25mm (1 inch) of rain.

Spreading this amount of water on an acre of dry land was tickling the minds of some dairy farmers as they saw their grass covers disappear before them.

In previous droughts, and we have had only a few in my lifetime of working with Teagasc, the general understanding was that the last thing you would contemplate was getting out the vacuum tanker and start spreading water on parched ground.

Previous thinking was that the roots of the grass plant would go deeper into the ground in their search for moisture.

Applying water to such a plant would only encourage this root to head to the surface, to get at the easy moisture. Therefore, the idea that someone would spread 22,000 gallons of water on an acre of ground was seen as off the wall.

However, in the recent heat- wave, contractors were able to offer a service where they could apply 30,000 gallons of water an hour.

The charge for this service was €115 plus VAT. I presume this was for an acre of ground.

This volume of water could only be present in quarries or in lakes.

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If one were to go after such a homemade irrigation plan, would it grow 500 to 600kgs of grass dry matter?

That's a difficult question to answer. Speaking of rainfall, at home where I live in Limerick, up to last Friday morning, I measured 12mm (1/2in) of rain for the previous three days.


That's approximately 12,000 gallons of water on each acre.

We didn't get any of the downpours that fell in other areas, but we didn't have the moisture deficit that's present in other areas either.

Before the rain of last week, the moisture deficit nationally was 75mm (3in).

This puts dry soils at 100mm and marginal soils approaching 50mm.

At a moisture deficit of 50mm, grass growth will be slowing down.

Data from the new Teagasc Moorepark grass measuring project (i.e. Pasture Base) gave grass growth rates last week of between 10kg and 30kg DM of grass dry matter on dry soils, while marginal soils bombed along at 45-70kg of grass dry matter. Measurements taken from farms in Kerry last week returned a growth rate of 45kgs, down from 65kg the previous week.

So, last week's rain will have reduced the moisture deficit somewhat, but it will not have eliminated the drought.

The advice last week was to divide the farm into plots for grazing of one thirtieth and one fortieth of the area each day. That's a long rotation and the idea behind it was to prevent you from running out of grass completely.

In this scenario, you hopefully had between 1,250kg to 1,400kg of grass in front of you for the cows.

Once the cows had grazed their allocation, the rest of the diet was made up of concentrate and silage.

For instance, Curtin's farm in Moorepark was on a diet of one third grass, one third concentrate and one third silage.

Up to 6kg of concentrates should be fed before silage is introduced

However, the real problems were occurring where farmers were going into covers of between 500kg to 600kg and also on a 20-day round.

A lawnmower would have no bother cutting a cover of 500-600kg.

If you were in this situation, then no doubt you were also feeding silage and concentrates.

Can you stop feeding the silage now that the rain has arrived?

The short answer is no.

It will take time for the grass plant to recover.

In many cases, the grass ahead of the cows was of the course variety. It was under stress with an amount of seed heads.

The green leaf that was present was beginning to shrivel up in the worst of cases. This leaf is basically dead and will be slow to take up moisture and nitrogen which it will need for growth. It is best grazed off and the whole cycle will start from a new base.

What about nitrogen fertiliser?

If you were spreading away during the dry weather then don't go out again until the next rotation. The nitrogen that went out in the heat is still there.

It didn't go anywhere. It needs moisture to get taken in and that has now happened.

We are going to get a burst of growth now, helped by background nitrogen which is released due to the mineralisation of soil organic matter – but the drought isn't over yet. We will have a lag phase of two to three weeks in grass growth.

Continue to supplement until you have between 1,400kg and 1,600kg of grass dry matter ahead of you.

One other point. I am seeing too many vacuum tankers in fields beside drinking troughs. That's telling me the water infrastructure is not fit for purpose.

John Donworth is a Teagasc dairy specialist and a regional manager in Kerry and Limerick

Indo Farming

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