Creating a stink: part two

What is animal waste from farms doing to our waterways? (15 minute read)

Welcome to The Turnstone. Here, I help people understand important issues such as climate change, water quality and conservation. I send my articles out every Sunday - if you’d like them emailed to you directly, you can sign up to my mailing list.

I have a vivid memory of my closest encounter with cow manure. It was summer, and I was helping in the milking shed. It was on a farm north of Whangārei, a place where I had many happy holidays as a child. I’m not quite sure how old I was, maybe around nine. I was wearing a bright sundress my mother had sewn and a pair of gumboots.

The shed was filled with the rhythmic sound of the milking machine. Cups were attached to each cow’s udder – four cups, one for each teat – and a vacuum pump sucked out the milk and carried it along hoses to a big metal vat. I couldn’t put the cups on the cows, but I could remove them and hang them up when the cow was done. That was my job in the shed. I felt useful and I loved it.

In fact, I loved everything about the farm. I loved the gentle cows, always curious but never pushy. I loved the sight of the lush green pasture. I loved riding around the farm on the back of the motorcycle (no quad bikes in those days, no helmets either). Best of all, I loved the farm dog, a level-headed collie named Ron. I was a townie kid having a great time.

But I wasn’t paying attention in the milking shed. I didn’t notice a cow lifting her stumpy tail. Then I felt something warm and liquid running down my back...

A good hot shower sorted me out, but I was never quite so confident in the milking shed again.

Manure is an inevitable consequence of cows and they produce astounding quantities of the stuff. A human produces around 120-250 grams of faeces per day, and around 1.4 litres of urine. For humans, scientists measure the two types of waste separately, but for livestock they usually count the combined volume. A sheep produces 1-4 kilogrammes of waste in the form of faeces and urine, per day (different studies have produced very different results). Beef cattle produce an average of 37 kilogrammes of waste per day. A dairy cow is larger and produces more waste, 62 kilogrammes.

That adds up to a colossal quantity of waste. I actually did the maths. New Zealand’s 10.2 million cows and nearly 27 million sheep produce at least 570,000 tonnes of waste per day, perhaps as much as 620,000 tonnes. I couldn’t comprehend the numbers, so I converted it to something I could visualise. The amount of waste produced by cows and sheep, each day, adds up to about 20,000 full concrete mixers – that’s the weight of the concrete and the truck combined. Although dairy cows make up less than 20% of our livestock in terms of numbers (there are 6.3 million of them), they contribute around two thirds of the waste, or 13,000 concrete mixers. Beef cattle contribute just under 5000 concrete mixers, while sheep contribute somewhere between 1200 and 3600 concrete mixers, depending on which figure you use for how much waste a sheep produces.

But the volume of waste isn’t the only thing we need to know about to understand the problem New Zealand has with livestock waste. It matters where that waste is deposited.

Sheep and beef farms cover a third of New Zealand’s land area, around 8.8 million hectares in 2017 (because farms are often mixed, the two are reported together). This means that the sheep and beef waste is spread out, at around 20kg per hectare per day. However, there are only 2.5 million hectares of dairy farms. As a result, two thirds of the livestock waste is ending up on 9% of New Zealand’s total land area. That works out at 160kg of waste per hectare per day.

This concentrating of New Zealand’s livestock waste is relatively recent. Back in 1990, there were fewer dairy cattle but twice as many sheep. The total amount of waste produced by cows and sheep hasn’t increased by much over the last few decades, a few thousand concrete mixers at most, but it’s having more of an impact because it’s now landing on a much smaller area.

Image credit: Melanie Newfield

To understand why this is a problem, it helps to understand a natural process called the nitrogen cycle (there’s a good video explaining the process here). I’ve written about it before, but that was some time ago, so I’ll go over it again. Nitrogen makes up nearly 80% of our atmosphere, and it is abundant in all living things, but there are some interesting complications in the way nitrogen moves between the atmosphere and living things. Nitrogen in the atmosphere is in its pure form, and is stable and non-reactive, which means that very few living things can get the nitrogen they need from the atmosphere.

There are only two natural ways that nitrogen can be converted into a form plants can use – a chemical reaction caused by lightning, or through the activity of certain microbes. Of the two, microbes do most of the work. Until two chemists, Fritz Haber and Karl Bosch, worked out how to reproduce the process and scale it up to industrial levels, nitrogen inputs were a real limitation on how much food farmers could produce. Farmers had two main ways to get nitrogen into the soil. The first was to grow plants, such as peas, beans and clover, which harbour nitrogen-converting bacteria in their roots. The other way was to apply human or animal waste, which was recognised as a precious asset. But the cheap fertiliser created as a result of the Haber-Bosch process changed all that. From the 1950s onwards, when chemical fertilisers came into common use, farmers could produce much more from their land.

But the natural nitrogen cycle still continues. Plants absorb nitrogen, in the form of nitrates or ammonia, from the soil, and convert it into living tissue, including protein. Animals eat the plants, and the nitrogen is absorbed into their tissues. When they excrete waste, they are excreting nitrogen in various forms in their urine and manure. When they are eaten by other animals, nitrogen is absorbed into those animals, including us. When living things die, the nitrogen compounds are broken down to form ammonia.

Nitrogen occurs in many different forms in the environment, but the most important ones to know are nitrate, nitrite, ammonia and nitrogen gas. The process to convert one to the other goes by steps, but it doesn’t happen easily. Each step is done by a different group of microbes. I’ve mentioned microbes which can convert nitrogen from the atmosphere – they turn it into ammonia. Then, there are microbes which convert ammonia to nitrite, microbes which convert nitrite to nitrate, and microbes which can take the whole process in the opposite direction, step by step, from nitrate, to nitrite, to ammonia, to nitrogen gas.

The animal waste deposited on pasture contains different forms of nitrogen. In cow and sheep urine, most of the nitrogen is in the form of urea, which is chemically related to ammonia. Soil microbes convert some of it to nitrogen gas and some of it to nitrites then nitrates. In the process of these conversions, small amounts of nitrous oxide gas are released. You may have heard of nitrous oxide as laughing gas, or you may have heard of it as one of the gases contributing to climate change. The quantities in the atmosphere aren’t large, but it has a powerful warming effect, hundreds of times greater than carbon dioxide. Globally, its contribution to climate change is about 5%, but if we look at the figures for New Zealand, it makes up 11% of our contribution. The process of soil microbes breaking down the urea from cow and sheep urine makes up a big proportion of that figure, although I haven’t been able to find exactly how much.

Nitrous oxide is one reason why livestock waste is a problem, but it’s not the only reason. Another problem is that the waste contributes to high levels of nitrates in the soil. While nitrates are essential to plant growth, it is quite possible for there to be more nitrate in soil than plants can use. The problem is worsened because urine and manure are not evenly distributed across pastures – with some areas getting vastly in excess of what the plants can use, and some not getting enough, which means that farmers still need to apply fertiliser.

The extra nitrate doesn’t just sit in the soil and wait until a plant is ready to use it. Nitrates dissolve easily, so they are carried away by water – ending up in streams, lakes and groundwater. Exactly how much nitrate leaches from pasture depends on a lot of different factors such as soil type, soil moisture levels, soil and pasture condition. But it’s impossible to escape the fact that much more waste is landing on dairy farms than on sheep and beef farms. And that, therefore, more nitrate is leaching from dairy farms.

I’ve covered a lot of the problems with nitrates already. They contribute to blooms of toxic cyanobacteria. They contribute to an overgrowth of aquatic plants and algae which can end up sucking all the oxygen from the water, killing fish. They can end up in marine environments, helping to cause “dead zones” in the sea. And, at higher concentrations, they are toxic to people, especially babies.

There’s more than just nitrate in animal waste. There are other nutrients, such as phosphorus and potassium, which are useful for plant growth, but a problem in excess. And, as with human waste, there are billions of bacteria. Cattle and sheep manures don’t contain as many microbes dangerous to humans as human waste does, but they do contain some, for example Campylobacter, which I mentioned in last week’s article. The main risk is surface water contamination – rain washing the waste directly into streams. Some of this is water flowing over land, but disease-causing microbes can also be washed through the soil and into groundwater. The inquiry held into the 2106 Havelock North outbreak of Campylobacter, which was caused by a contaminated aquifer and sickened more than 5000 people, found the likely source to be sheep paddocks on a nearby farm.

But waste doesn’t end up only on paddocks. It ends up on laneways, in yards and in sheds. This is a particular issue with dairy cows, which are in sheds and yards much more often than other livestock. For obvious reasons, this waste cannot be left to build up, so the shed and yards are hosed down, either manually or automatically. This creates a specific type of waste, mostly diluted manure but also containing disinfectants, milk and other substances, commonly referred to as effluent.

Until about 1990, waste from milking sheds and yards was usually treated using effluent ponds, before being discharged into waterways. While the wastewater entering the waterways was better than raw sewage, it was still high in nutrients and live microbes, and harmful to the environment in many ways. However, since the 1990s, this kind of wastewater has been increasingly applied back onto farmland. It’s a win-win, improving pasture growth, and reducing the waste going directly into our fresh water.

Well, that’s the theory, anyway. The reality is a little more complex, because there’s still a limit to how much the land – or more specifically, the plants on the land, can absorb. And that isn’t just a matter of the effluent, it also depends on the amount of fertiliser used and, of course, the waste being directly deposited by the animals on the pasture. Studies have found that some of the nutrients in effluent sprayed on pasture can leach into waterways, but it is mostly the waste directly deposited on pasture, and fertiliser, which contribute to nutrient leaching.

What is the impact of all this waste on nearby waterways? The news isn’t good. Land Air Water Aotearoa classifies the condition of waterways from “A”, which is the best, though to “D”, which is poor (occasionally they use five categories with “E” the worst). For rivers which run mostly through pasture areas, 30% of the monitored rivers were classed as D and a further 50% were classed as C, in terms of their general environmental health. Only 20% were in categories A and B. In comparison, 50% of rivers running through exotic forest were in categories A and B, as were 65% of the rivers running mostly through native forest. Urban waterways were considerably worse though – only around 3% were in categories A and B.

The picture is similar for other attributes of waterway health, such as levels of nitrate and phosphate. But the figures for microbes show perhaps the sharpest contrast. Most waterways under native and exotic forest look good – around 70-80% were in the top categories. For pasture, that figure was less than 20%. Once again, urban waterways were worse, but they also make up a smaller area. Depending on which figure is used, pasture makes up something like 40% of New Zealand’s land area, so that means a lot of our waterways are in a bad state.

But, when it comes to water quality, not all farms are the same, and it doesn’t just come down to the amount of urine and manure that cows and sheep are producing. How farms are managed can make a real difference to the health of nearby, and indeed more distant, waterways.

One well-recognised action for improving water quality is to keep livestock away from streams and other waterways, and plant the banks back in native vegetation. Doing this can make a real difference to nutrient and microbe levels in streams within the space of a few years. According to a Dairy NZ report from 2018, almost all waterways with a streambed wider than one metre have been fenced off from livestock. However, this will apply only to dairy farms, not the much larger, but less intensive sheep and beef farms.

Other methods are also being used, or tested, to prevent nutrients from getting into waterways. Natural and artificial wetlands are good at absorbing nutrients, and they also reduce sediment, microbes and other pollutants. Keeping livestock out of wetlands is crucial to ensure that wetlands are effective at removing pollutants out of water – this is part of the Dairy NZ plan to reduce the impact of dairy farming on waterways, but there are no data on how well they have achieved this.

One exciting possibility – exciting to me, anyway, because it’s so simple – is using trenches filled with wood chips to extract nitrates from water. That is basically all there is to it, dig a trench or pit, line with polythene, fill it with wood chips and direct drainage water through it. Bacteria decomposing the wood chips do the rest, converting the nitrate back to nitrogen gas which is released into the atmosphere. There’s evidence that this kind of system could halve the level of nitrate in the water. Wood chips have been also been used in other situations to reduce nitrates too, for example by placing bags of wood chips directly in contaminated streams.

These are all methods to clean up water that has been polluted, but are there ways to prevent the pollution getting there in the first place? And what about nitrous oxide emissions from patches of urine – can anything be done about that? Yes, but it’s complicated, especially in New Zealand where we see keeping our animals on pasture as the best way to farm.

When cows and sheep (although it’s most often cows) are grazed on wet pasture, it damages the soil structure and leads to “pugging”, which is where cow or sheep hooves penetrate the soil surface. Pugged soil releases much more nitrous oxide than intact pasture, and also leads to more microbes and phosphate ending up in waterways. The problem is, it’s very hard to avoid pugging in a wet country like New Zealand, particularly in the winter. The only way to avoid it is to reduce the time that livestock are standing on wet pasture. One way to achieve this is to create drained areas of land which cows use for part of the day, so that they aren’t on the pasture all the time.

A more radical solution, at least in the context of New Zealand farming is wintering barns – effectively keeping cows indoors over winter. While this goes against traditional farming practice in New Zealand it does have some real advantages. It’s certainly better for the soil, but it also allows effluent to be collected, treated and returned to the land more evenly than having cows deposit their waste directly. Of course, it is expensive and has to be done properly to manage animal welfare, but there are also animal welfare issues keeping cows on excessively wet pasture. From a practical perspective, a disadvantage is that there is a much greater volume of waste that needs to be managed through an effluent treatment system. However, from an environmental perspective, that’s actually an advantage. It means that untreated waste isn’t deposited directly, and unevently on the pasture, releasing nitrous oxide from urine patches, giving the grass vastly more nitrate than it can use and being washed into waterways.

Then there are more unconventional suggestions, such as toilet-training cows. Cows are trainable, but whether this approach has any practical use in New Zealand is debatable. It’s really more of an option for cows kept indoors, allowing for more effective collection of waste. With cows kept on pasture, where they are moved from one paddock to another, there are significant hurdles for it to be useful.

All of these measures, however, have costs. In some cases, they cost money, in others, they require areas of farmland to be given over to something other than grass and cows or sheep, which has a financial cost too. It’s also not clear exactly how much difference all of these approaches can make. There are signs that farming practices are changing, with improving levels of microbes in Canterbury and Southland waterways and clear reductions in phosphate in most, but not all, areas of New Zealand. But, in terms of their general health, more of our waterways are getting worse rather than better. We’ve still got a lot of work to do.

Most articles in The Turnstone are free, but you can support my work and an receive additional material, including more in-depth interviews, with a monthly or annual subscription. Click the button below for options.

If you would like to support The Turnstone with a one-off contribution, click the “Buy me a coffee” button below.