Wheat cropping helps cut nitrates polluting rivers

OPINION: In 2009 I visited Nick Pyke who was the head of The Foundation for Arable Research at the time.

I wasn’t having any luck convincing a Canterbury dairy farmer to trial my wacky “happy cow” dairy farming system. I wondered if cropping farmers would be interested in adding some dairy cows to their system.

What Nick told me formed the basis of our dairy farming system which we loosely call the “happy cow way”.

Nick explained how mixing cows with crops is a mutually beneficial system. Cows put nitrogen into the ground and plants take nitrogen out of the ground.

I was in Nick’s office in 2009, as it appeared to me that nitrate leaching from dairy cows was becoming a big issue and I wanted to find a solution.

I felt that the level of scrutiny and criticism that the dairy industry was experiencing was reaching a crescendo.

Ten years later I find myself at a freshwater consultation meeting. John Penno, the chairman of the freshwater leader’s group and Synlait co-founder was telling the crowd that these proposals “have been well signaled”.

Founder of the Happy Cow Milk Company, Glen Herud, believes farmers will work out a solution to excessive nitrates in rivers.
ALDEN WILLIAMS/STUFFFounder of the Happy Cow Milk Company, Glen Herud, believes farmers will work out a solution to excessive nitrates in rivers.

Indeed they have.

Nitrate gets into our waterways in two main ways, the first is from excess fertiliser leaching through the soil. This tends to be a problem for the horticulture and arable farmers.

The second way is caused by the urine patches of animals and this is the dairy industry’s problem.

Note, it is not from cows standing in waterways.

When a cow urinates, it creates a puddle of urine about the size of a dinner plate. This urine patch contains the equivalent to 1000 kg of nitrogen/ha. That means if you covered one hectare with evenly spaced urine patches, it would be like applying 1000 kg of nitrogen fertiliser in one application.

For perspective, an average dairy farmer applies about 100-200 kg of nitrogen fertiliser over an entire year, in 25kg applications.

It’s up to the plants in the dinner plate-sized urine patch to absorb all that nitrogen and of course, few plants can absorb that amount of N at one time.

Nitrogen converts to nitrate in the soil. Nitrate molecules bond to water molecules, which means when water filters through the soil profile, it takes the nitrate with it.

A cow with a urine sensor, developed by an AgResearch team.
SUPPLIED cow with a urine sensor, developed by an AgResearch team.

There are a few factors which affect the rate of nitrate leaching.

Heavy soils leach at a slower rate than light stoney soils. That’s because heavy clay soils hold water and nutrients in the root zone for longer.

Light stoney soils allow water and nitrates to drain quickly through the soil profile and below the root zone faster.

The amount of water in the soil is important too. A large percentage of nitrate is leached during the autumn and winter periods which tend to be the wettest times of the year.

The other key point is to ensure there are roots in the ground. Nitrate can’t be absorbed if there are no plant roots to absorb them.

The nitrogen loading onto the soils is important too. Obviously the more N applied either by urine or fertiliser will affect the N leached.

Last week I downloaded the river test data from Statistics NZ. I wanted to see how many rivers complied with the new proposed nitrogen limit of 1mg/L.

Of the1506 rivers tested, 192 (or 12.75 per cent) rivers had average readings over 1mg/L of total nitrogen.

Of these 192 rivers, 78 per cent were within 1-3mg/L. So most of the 192 rivers over 1mg/L are just a little bit over the limit. Which could indicate minor changes in practices would start to reverse these rivers N levels over time.

There are 41 rivers with average test results ranging from 3-15mg/L and of those 41 rivers, 20 are in the Canterbury region.

The Foundation for Arable Research has been studying the way in which crops such as wheat can soak up nitrates for more than a decade.
SUPPLIED The Foundation for Arable Research has been studying the way in which crops such as wheat can soak up nitrates for more than a decade.

That’s not surprising because Canterbury has free-draining soils, we’ve increased the N loading by replacing sheep urine patches with cow urine patches. In fact, Canterbury has a 21 per cent higher stocking rate than the New Zealand average. Partly, because irrigation combined with lots of sunshine grows lots of grass.

This is all good news.

Because it means the current state of nitrogen in Canterbury is exactly what the science predicts.

Which means we know how to solve it.

The farmers are the experts and by combining creativity and science they will find the economic solutions.

Recently, science has proved exactly what Nick Pyke told me in 2009.

If you take a deep rooting, nitrogen-hungry plant like wheat and you sow it into the paddocks where the cows spent the winter, you can reduce N leaching by 40 per cent.

Its called a cover crop and it’s just one of the many possibilities available.

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