Agricultural pollution is essentially too many nutrients in a place where they are not wanted.
Cows and crops have adverse effects on the environment in different ways.
The Inefficient Cow
Cows are not very efficient at using the nutrients that they consume.
A small amount of nutrients are used by the cow to produce milk.
Most nutrients go through the cow and out again via effluent and urine. The environmental impact of cows is due to too many nutrients being applied to an area via cows’ effluent.
Let’s say a dairy farm grows enough grass to naturally support a stocking rate of 2.3 cows per ha and the cows produce 20 litres of milk per day.
A small number of nutrients are removed from the farm via the milk.
Most nutrients are returned to the pasture via the cow’s dung and urine.
This is pretty much a self-replenishing system.
But if the farmer decided to buy maize and canola meal from the neighbour’s farm and starts to feed them to the cows.
The farmer is now importing nutrients from the neighbour’s farm and feeding them to the cows.
Because most of these additional nutrients will not be used by the cow (because she is an inefficient user of nutrients) those additional nutrients will be deposited onto the land via the cow’s effluent.
The result is the stocking rate on the farm could increase to 2.6 cows per ha with the cows producing 30 litres per day. The farm is producing more milk but it has also become more intensive and the possibility of too many nutrients leaching out of the soil is greater.
The Efficient Crop
Plants are very efficient at utilising nutrients.
They absorb a lot of nutrients from the soil as they grow and when the plant is harvested, those nutrients are effectively removed from the soil and carried away with the plant.
These nutrients need to be replaced by importing nutrients from somewhere else in the form of fertiliser.
Fertilisers can be mulch, manure, minerals or synthetic compounds.
But the principle is that the farmer needs to import nutrients to replace the nutrients removed by the crop.
This is where cropping farmers can end up with adverse environmental impacts.
Most nutrients in the soil are not in a form that plants can absorb.
When a crop is planted the plants absorb large amounts of the available nutrients in the soil.
If a crop removes 90kg of nitrogen from the soil, it’s not a simple matter of applying 90kg of nitrogen fertiliser to replace it.
Nitrogen needs to be converted to nitrate by microorganisms in the soil before a plant can access it.
Cropping farmers are constantly in a balancing act trying to replace nutrients and ensure there are enough plant-available nutrients for the next crop. But also not wanting to add too much fertiliser.
How Much Nitrate is Lost?
The table below shows the amount of nitrate that is lost below the root zone.
It’s no longer available to the farmer.
This lost nitrate will continue down the soil profile until it gets into the water table.
As you can see, the losses range widely among the different farming types and soil types.
But if you look at the “NZ dairy cows on silt loam soils with zero nitrogen fertiliser”. It losses 25kg of N.
The “NZ dairy cows on silt loams with 200kg of N fertiliser”. It lost 59kg of N. I suspect that the additional N loss is not due to the fertiliser being applied to the farm, but that the farm can support more cows and those additional cows are adding more effluent.
Also, look at the variation in N loss for the mixed cropping 14-102 kg, and cereal rotation 17-87 kg.
The variation is due to numerous factors but it shows that cropping in some cases is much lower than dairy and much higher than dairy in other cases.
Finally, look at the “USA corn, carrots” with 155kg of N loss.
This is 100% plant-based agriculture with significant N loss.
The main feed to dairy cows in the USA is corn.
In the USA dairy system, we have a super-intensive cropping system with high emissions and N losses. Feeding a super intensive dairy system. When we see figures comparing dairy to other foods, the dairy figures are usually derived from the US dairy system.
One last point, just to make sure my message is loud and clear.
Plant-based does not mean more sustainable!
The graph below shows nitrogen losses from vegetable production are much higher than in other forms of agriculture.
That’s because market gardening is high-intensity agriculture.
I’d like you to think of this graph next time you hear someone proposing we replace cows with plant-based agriculture.
You can read the report that I sourced these graphs from here.
Long-time readers will know, I always finish off by saying a good solution to these issues is to integrate ruminate animals into cropping systems and vice versa.
Crops take nutrients out of the ground and animals put nutrients into the ground. They are opposites and they complement each other.