Replacing cows with vegetables wouldn't be better for our waterways at all

Friday, 30 November 2018

OPINION: Nitrogen and our waterways: It's a topic that is never far from the headlines in New Zealand.

Nitrogen leaching is arguably the biggest pollutant of our waterways and it's a huge issue globally, too. It's basically a result of intensive agriculture.

But what constitutes intensive agriculture may surprise you.

* Canterbury's Happy Cow Milk Company placed into liquidation

How does nitrogen get into our waterways?

Nitrogen is needed to grow plants, plants absorb nitrogen from the soil. If we continually harvested the plants, the soil would become depleted of N and eventually nothing would grow. So we need to replace the nutrients that the plants absorb.

Nitrogen can be added to the soil via modern fertiliser, animal manure or decomposing foliage (compost).

Simply put, nitrogen leaching is when more nitrogen is being applied to the soil than is being absorbed by the plants.

Dairy farming

The way we get excess nitrogen from dairy farming is via the cow's urine. Urine contains 69 per cent nitrogen. The greater the concentration of cows per hectare the more N leaching you get (more or less).

Averaged out, dairy farms leach about 60kg of nitrogen per hectare per year.

Surprisingly, nitrogen fertiliser does not contribute greatly to N leaching on dairy farms. This is because farmers (generally speaking) only put on what the plants can absorb.

Vegetables

Many people will be surprised to know that market gardening leaches three times more nitrogen than dairy farming.

There are plenty of papers out there showing leaching of over 170 kg N/ha/yr for various vegetable crops.

How can it be, that vegetables leach more than dairy cows?

Market gardeners apply quite high rates of nitrogen fertiliser to each crop.

Vegetables are generally fast-growing crops. That means they harvest multiple crops and therefore they are cultivating their paddocks multiple times every year.

Cultivation increases N leaching.

Glen Herud is the founder of the Happy Cow Milk Company.

The other important thing to note with market gardening is there are a lot of times in the year where there is no plant in the ground. So there is nothing to absorb the N in the soil. So it leaches down.

Contrast this to a sheep/beef/dairy farmer's paddock. That paddock has grass in it all year, which is absorbing N for about 10 months of the year. That paddock will be cultivated once every five to eight years.

A combination of high N fertiliser inputs, multiple cultivations per year and portions of the year where there are no roots in the ground, mean that vegetable production 'leaks' a lot of nitrogen.

What about organic vegetables?

Organic vegetable production shares all the characteristics of conventional growers listed above. Organic vegetables still need nitrogen, they just get it in a different form. There are still periods of the year where there is no crop in the ground and they also cultivate multiple times per year. In fact, they tend to cultivate more than conventional growers as cultivation is used to control weeds as opposed to sprays.

Organics has many advantages, but from what I have seen nitrogen leaching under organics is not greatly reduced. I realise it's a controversial thing to say.

An additional note, N leaching under kiwifruit is also very high.

Horticulture and market gardening occupy a very small land area in New Zealand. So the impact at a catchment level is not huge.

You would think the waterways would benefit if we replaced the dairy farms with vegetable growers. That seems logical.

But it's not the case.

Legumes

The third way nitrogen gets into the soil is via legumes. Soy, peas, chickpeas, clovers and beans are all legumes.

The air we breathe is 78 per cent nitrogen. Legumes are able to take the N from the air and 'fix' it via their roots into the soil. This nitrogen now becomes available to other nearby plants to absorb.

Legumes are fantastic, they create free nitrogen for other plants to absorb.

The basis of New Zealand pasture-based agriculture is clover and ryegrass pastures. The clover fixes the nitrogen into the soil and the ryegrass plants absorb it (more or less).

Organic and biodynamic farmers take this principle one step further, by planting much more diverse pasture/legume swards.

But when you plant a whole paddock in just legumes we potentially have a problem. These legumes are fixing nitrogen at a great rate and there are no other plants to absorb the nitrogen. Often, the result is the legumes are producing more nitrogen than is being used and it gets leached.

I found a number of studies that showed peas to be leaching between 80-120 kg N/ha.

It would seem logical that replacing cows with plant-based proteins such as legumes that go into the Impossible Burger would be good for our waterways. But there's enough science to suggest it wouldn't be a better outcome at all.

This is a very basic overview of the science and there will be many exceptions to what I outlined above. But it shows that the full story is often not black and white.

I believe the way things occur naturally is how they are designed to work in an ecosystem.

We wouldn't go into the wild and just see intensive herds of cows. We wouldn't see hectares of just legumes growing or fields of vegetables all in one area.

Instead, we would see fields of diverse grasses and herbs mixed with legumes. These pastures would be filled with trees and amongst them, a range of animals grazing.

All these different elements are designed to work together, one is not better or worse than the other. They all play a role and we need all of them.

Sustainability is about mimicking nature.

It's possible to take cows, legumes and vegetables and grow them in a more intensive manner than we see in nature without bad environmental effects.

One possible example is if a farmer wanted to grow peas. The crop could be followed by a wheat crop and then followed by pasture which is grazed for 12 months and then replanted into peas again.

This mixed farming system is what farmers used to do 50 years ago. But, as with all industries, farmers have specialised. We, therefore, get farmers who just grow crops or only have dairy cows.

We could see dramatic sustainability improvements if farmers and growers reverted to mixed farming systems.

But for that to happen we need to change the economics of farming.

This is the first of a weekly column. I hope to explain in simple terms some of the complexity around the food and agricultural sector.

In our modern world, we see so many black and white debates. But that's too simplistic. All subjects have many grey areas to contend with. So much depends on so many fluctuating factors with many different results.

I hope to explain the grey areas a little better and while we won't all agree, hopefully, we can at least understand each other better.

If you have any questions for me, please leave them in the comments below.

Glen Herud is the founder of the Happy Cow Milk Company.