Is growing greens a greener option than grazing cows? Glen Herud says no:
. . .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.
Fertiliser is expensive and it costs time and money to apply it. Farmers have both financial and environmental incentives to apply no more than is absolutely needed.
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.
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. . .
Herud points out that organic farmers might not use nitrogen fertiliser but their crops get nitrogen in different forms. Organic farmers leave fields fallow for long periods and cultivate several times a year to get rid of weeds without spraying so leak more nitrogen.
He also explains the role of legumes.
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. . .
Radical environmentalists would have us believe that growing fruit and vegetables would be better for the environment, and particularly water health, than dairying.
The science shows it’s not that simple and that grazing dairy cows could result in cleaner water than growing vegetables.