Monday, July 23, 2012

Water Quality and Dairy Cows

The dairy industry has is coming under increasing pressure around its environmental impact on NZ water ways. I have noticed in the past 2 years the calls to reduce the impact of dairy farming have reached fever pitch.

The term “Dirty Dairying” has been around for a while now and I’ve seen more than a few vehicle’s with the bumper sticker “Fonterra stop shitting in our waterways”. The public perception is that dairy farming is bad for our water ways.

So I thought we would look at the science behind water quality and determine what the facts are. As we all know that facts are often the first casualty in public debate.

This presentation to ministers by the commissioner for the environment Dr Jan Wright is a very good summary of how our water ways are getting contaminated.

I will summarise her presentation, but if you have a spare 20 minutes, give it a quick look.

There are essentially 3 types of pollutants that affect our water ways:
  • Sediment -as a result of erosion and flooding, where large amounts of soil and gravel etc get washed into the water ways.
  • Bacteria- from animal and human waste being discharged into the water ways.
  • Excess Nutrients-when Nitrogen and Phosphorus find their way into the water ways.

Dairy farms main pollutant is Nitrogen from excess nutrients. This graph shows how much nitrogen is leached per ha from the different farming classes. Interestingly Horticulture is the worst offender, but Horticulture makes up a very small area so the overall effect is not as great as the dairy industry. You can see that a dairy farm leachs on average 50kg of N/ha/yr, which is less than horticulture. I have seen data that show vineyards leaching 80kg of N/ha/yr.

Public perception is that cows standing in unfenced water ways and waste from effluent systems is the biggest cause of nitrogen pollution from a dairy farm. But it’s not. It would be great if it was, because those two issues are relatively easy to fix.

This graph shows 85% of dairy Nitrogen pollution is a result of Urine. But this is not urine from cows standing in a stream urinating or urine from dairy effluent.
We hear so much about dairy shed effluent, but dairy shed effluent is collected while the cows are being milked, which accounts for 4-5 hours of the day. The remaining 19-20 hours the cows are in the paddock, urinating.

Research has shown that the cow’s urine from the unrine patch is the main culprit. When a cow urinates the urine lands in an area about the size of a dinner plate. The nitrogen in that urine patch is equivalent to 800kg nitrogen/ha. To put that into perspective a dairy farmer would apply around 200kg of nitrogen fertilizer per ha per year and that would be done over 4-5 applications. Many farmers apply much less.

The grass within that dinner plate sized area is only able to utilise a small fraction of the nitrogen in that urine patch. The nitrogen then converts into nitrate form which is water soluable. The nitrate attaches to the water molecules and leach through the soil profile.
Once the nitrates are below the grasses roots then there is very little that can stop the nitrates filtering through the soil profile and into the water table, which in turn reaches our water ways.

There is a lot of research going on by scientists to better understand nitrate leaching and ways of combating it. Nitrogen inhibitors are a recent development. The inhibitor is sprayed onto the land. Trials show that considerable leaching reductions can be achieved, but on farm results are reported to be less effective. 

The reason the councils are concerned about the increase of dairy farming is that the nutrient leaching will increase. If a sheep farm is converted to dairy the average nitrogen leaching will increase by 30kg/ha.
If the current level of growth in the dairy industry continues at 3-4% per year, then cow numbers could quite possibly be double their current levels in 15-20 years. I don’t think the wider community will allow there to be 12,000,000 dairy cows without major restrictions.

At this point in time the industry does not have a commercially viable answer to the nutrient leaching issue. There are certain farming practises that are worse than others, that can be minimised. But as a whole there is no silver bullet.

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