11.9: Manure as a Pollutant; stewardship farming.

As we all know, there can be too much of a good thing. Excessive manure applications may cause plant growth problems. Here we are mostly referring to raw animal manure (not composted manure) applied to farm fields. It definitely causes problems in the water-rich region we live in, especially since dairy cattle carrying capacity typically exceeds the amount of acreage available to receive the poop as fertilizer. Manure is typically spread across the surface of a crop field. But, manure runs off fields and gets into streams and lakes. Nitrogen in manure can also leach down through the soil profile and seep into groundwater. Both are big pollution problems and are becoming worse in many areas where livestock, (poultry, pigs, or cows) are confined and congregate.

It is especially important not to apply excess poultry manure because the high soluble salt content can harm plants. Plant growth can also be retarded when high rates of fresh manure are applied to soil immediately before planting. This problem usually doesn’t occur if the fresh manure decomposes for a few weeks in the soil, and it can be avoided by using solid manure that has been stored for a year or more. Injection of liquid manure by large-scale farms sometimes causes problems when done on poorly drained soils in wet years. The extra water applied and the additional use of oxygen by microorganisms may mean less aeration for plant roots. Loss of readily available nitrate for plants occurs by denitrification when too much water saturates a field.

Nutrient Imbalances and Buildup

When manures are applied regularly to a field to provide enough nitrogen for a crop like corn, phosphorus and potassium build up to levels in excess of what a crop needs. It is often mistakenly believed that buildup results from the fact that manure nutrient ratios are out of balance with crop uptake requirements (especially more P). Nutrient ratios for most manures are actually equivalent to a crop’s needs (roughly a 2:1 ratio for nitrogen and phosphorus). If farmers are smarter about how they apply manures, most nitrogen can be conserved through good timing and application methods. Apply manure immediately prior to the growing season rather than winter for example. Or inject liquid manure and incorporate it right away. Then the manure rate necessary to meet crop nitrogen requirements can be substantially lowered and the accumulation of P and K in soil reduced!

Erosion of phosphorus-rich topsoil contributes both soil sediments and phosphorus to streams and lakes, polluting surface waters. This is how Green Bay gets its name, alot of P naturally occurs in the clay soils in the watershed. If soil P levels are built up even more with manure applications the problem will only worsen as the 'loose' P feeds algae in the receiving waters. The sediment from plowed fields for annual corn and soybeans also contributes to ecological problems in bays and lakes. Green Bay used to have wild rice growing on its margins but sediment (and the introduction of bottom-feeding carp, and paper mill PCBs) killed this natural wild food.  The wild rice was a staple crop of the native people who inhabited the region before European contact.  The people were named "Menominee" by French voyagers  - it means people of the wild rice.  Sadly, the wild rice disappeared completely in about 1950.

When very high P buildup occurs, it may be wise to switch the manure application to other fields or to use strict soil conservation practices to trap sediments before they enter a stream. Crops that do not need manure for N, such as alfalfa (its a legume), allow a “draw-down” of phosphorus that accumulates from manure application to grains. (However, this may mean finding another location to apply manure.) In the last 50 years, our Green Bay region has lost 50% of its former hay crop acreage to corn and soybeans because of the federal subsidies for these crops and the lack of consequences. The manure and sediment from plowing for annual crops have suffocated the Bay waters and created a hypoxia zone.

Figure \(\PageIndex{1}\): Milk wagons in front of the Lessor-Maple Grove Townline Cheese Factory in Wisconsin.

Case study in farm modernization and the resulting nutrient flow on the farm: Full Circle Farm, Seymour WI., Valerie Dantoin-Adamski

This farm has been in my husband's family for over 125 years. The farm has seen a lot of changes over five generations. It started with a homestead of 160 acres carved out of a cedar tree swamp by Poland-born Valentine and Barbara Adamski. My husband Rick Adamski (and I) bought the farm from his parents in 1990 and promptly bumped dairy cow numbers up from 35 to 80. We put in a new milking parlor and bought a new tractor. We were on the path of the 'modern' dairy mantra to get ever-bigger (or get out). But instead, we took a turn to a more sustainable farming system. We chose to use managed grazing and perennial hay fields rather than growing annual crops. We chose to sell organic milk and now organic, grass-fed beef as better choices for the environment. Our son Andrew (and partner Heather) are downsizing even more and growing organic vegetables on just 16 acres. We will likely put the farm in a trust that will allow it to return to its pre-settlement state. The farm has come full circle.

Along the way, farming with five generations, the nutrient concentration on the farm has ebbed and flowed. The land was fresh and full of minerals and nutrients built up after the last glacier retreated from the Green Bay, Wisconsin area ten thousand years ago. The land was home to the Menominee people who moved from one forest area or waterway to new areas every ten years. There was no title to the land back then, before colonization. It belonged to everyone. Then the French, British, and Americans had a little war over who would own the land. The English/American system of government claimed the land, surveyed it and designated townships - six miles by six mile size units of government.  The townships were further divided into 40 acre parcels that were then 'titled' so the land could be sold to homesteaders to occupy.  The Menominee people were confined to a reservation about 45 miles west of Green Bay, about a 1/10th of the land they formerly occupied for the last several thousand years.

The township of Maple Grove where our farm is located was surveyed and platted in 1853 and the first immigrant farmer/settlers came in the 1880s, cut the cedar and white pine tree, and blew up the stumps with dynamite (some of which we discovered in an old granary as late as 1996). They turned the soil with plows and horses. An acre was just about how much land a man could turn over (plow) in a day. Full Circle was just a homestead in its first generation under the Polish immigrants, Valentine and Barbara. Very few nutrients were imported or exported.

The second generation (Stanley and Tekla) fed their 13 children from a large garden and sold eggs, pork, and a little milk to the corner cheese factory that Stanley helped organize in 1905 (Figure \(\PageIndex{1}\)). They hand-milked a dozen cows, keeping milk in cans cooled by groundwater or springs. Electricity and refrigeration came to their farm around 1940. That meant hand-pumping water for livestock could be replaced by an electric pump. Milking machines replaced the children's labor. Herd size tripled to 36 cows. They still kept fertilizer purchases very low because there was still a good supply of natural minerals in the soil.

Two boys stayed on the farm, Frank and Dick (the 3rd generation). They bought a tractor in 1958. The last horse, Prince, left the farm in 1958. They continued to use dairy animals and pigs as a source of manure nutrients. They were considered 'low input' farmers by the neighbors because they never got on the modernization kick. They bought some fertilizers and started using some chemical herbicides, but not much. They used old-time organic methods of crop rotation and animal manure as ways to retain organic matter and natural soil fertility. The Adamskis were organic before it was a 'thing'.  They milked about 36 cows and bought the neighboring 80 acres so Dick and his bride Evelyn could establish their own household in 1953.

When Rick and Val (4th generation) bought the farm, it was not a difficult decision to choose the track of low-input, only organic, fertilizers. The soil tests show the farm didn't need phosphorus because of all the animal manure over the years. Nitrogen was needed, but only about 1/3 of what most farmers would consider the 'right' amount. Corn yields on this farm were above the county average. What started to show depletion was potassium. Naturally mined potassium sulfate (K2SO4) or gypsum was brought in to bring K levels up. The number of dairy cows increase to 80 head.  That did not include replacement heifers or steers.  This farm is an example of one that buys fertilizer in the form of purchased hay from other organic farmers. The farm quit dairying in 2014 and now 90 beef cattle graze 160 acres of pasture. Sun and rain, hay and pasture, build the beef bodies. They poop out the excess nutrients, and the soil fertility cycle continues, Full Circle.

The fifth generation on the farm, Andrew & Heather, compost the cattle manure and use it to intensively grow 16 acres of organic vegetables which they direct market locally within about a 50 mile radius.  This is a net out-flow of nutrients from the farm as vegetable produce leaves the farm, but it is well-balanced by the inflow from the beef cattle operation.  Other non-family members of the farm are buying shares and becoming new farmer managers and gaining access to the land so that this 240 acres continues to operate as a farm.  About 1/10th of the farm acreage is set aside for perennial plantings, wildlife and wetlands.  The farm was named 2024 Organic Farm of the Year by the Midwest's Marbleseed Organic Organization.  It was also named the 2024 Leopold Conservation award winner by the Sand County Foundation.  The farm is a new model of conservation and productive agriculture that expands the definition of 'family' farm to include new members who share the same values. 

 

Nutrient Imports and Exports

On integrated crop-livestock farms it is commonly possible to produce all or nearly all the feed needs for the livestock. This helps to keep nutrient imports and exports close to balance, one of the advantages of integrated farms. There are different kinds of combinations of cropping and livestock. One extreme is farms that import all the feeds for their animals and then have to remove the accumulating manure. More commonly, farms produce most of their own feed but animal numbers exceed the production from the farm’s own land base. These farmers purchase additional amounts of animal feed and may have too much manure to safely use all the nutrients on their own land. Although they don’t usually realize it, they are importing large quantities of nutrients in the feed that then remain on the farm as manures. If they apply all these nutrients on a limited area of land, nutrients start to build up and nutrient pollution of groundwater and surface water is much more likely. It is a good idea to make arrangements with neighbors to use the excess manure. Another option, if local outlets are available, is to compost the manure and sell the product to vegetable farmers, garden centers, landscapers, and directly to home gardeners. Even when manure is exported from the farm, if there is just too much manure in a given local region, shipping long distances will become very expensive. New manure treatments (like different types of drying and mass reduction methods) may offer ways to make it more transportable to areas of nutrient and carbon deficits.

In Northeast Wisconsin, the number of milk cows in the region greatly exceeds the amount of land available on which to spread manure. Some technologies are being tried out, like making manure bricks and selling them. Many large dairies have bio-digesters which use microbes to digest the manure and release methane. The methane is then used to power electric generators. To move the manure from the large free-stall cow barns, water is flushed across the cement barn floor. It makes a manure slurry that is then scraped from the floors into a slurry pit before being de-watered and then sent to the digester. Seems like a lot of work. If we had right-sized farms more dispersed on the landscape, then the cows would simply walk out to the pasture and deposit their poop. The right amount of cows brings the right amount of manure. Nutrients stay in balance.

Poultry and hogs are routinely fed metals such as copper and arsenic that appear to stimulate animal growth. However, most of the metals end up in the manure. In addition, dairy farmers using liquid manure systems commonly dump the used copper sulfate solutions that animals walk through to protect foot health into the manure pit. The copper content of average liquid dairy manures in Vermont increased about fivefold between 1992 and the early 2000s, from about 60 to over 300 parts per million on a dry matter basis, as more farmers used copper sulfate foot baths for their animals and disposed of the waste in the liquid manure. Although there are few reports of metal toxicity to either plants or animals from the use of animal manures, if large quantities of manure with a high metal content are applied over the years, soil testing should be used to track the buildup.

Another potential issue is the finding that plants can take up antibiotics from manure applied to soil. About 70% of the antibiotics used in animal agriculture end up in the manure. Although the amounts of antibiotics taken up by plants are small, this is an issue that may be of concern when using manures from concentrated animal production facilities that use considerable amounts of these substances.

Nutrient Losses with Grazing

In grazing systems, the animal excrements are directly deposited on the surface (in the case of cattle, we call these “cow pies”). Some of the ammonium/urea is lost to the atmosphere as the manure dries, similar to non-incorporated manure from confined animal systems. Overall, this reduces concerns with N leaching, and runoff tends to be low due to high vegetative cover on pastures. However, because these cow pies are unevenly distributed, they generate small areas with concentrated nutrients while areas in between the “pies” have fewer nutrients and may still benefit from additional fertilizer. For this reason, nitrate leaching may still be a concern with intensively managed and fertilized dairy pastures. Most graziers (the farmers who graze their animals) are adept at having just the right number of livestock on each acre so they avoid unbalanced, built-up nutrients.