Gunz And Rows – Livestock and Soil Fertility
But what if we have livestock on or near the farm? This changes the dynamics somewhat, because the waste from that livestock needs to go somewhere, and oftentimes is has been the nearest field closest to the barn. We’ve seen in various soil sample maps where a field may have fairly uniform phosphorus and potassium levels across most of the field, but will have very elevated values where the old farmstead may have been. Lots where cattle or hogs were kept may be included, and sometimes areas of crop fields closest to the barns where manure was hauled.
Today, due to the size of many modern livestock operations, manure management is more than just finding the closest field to the barn and spreading the “liquid gold”. It involves intensive soil testing, calculating nutrient removal rates of future crops, documenting manure management plans, and hauling tens of thousands of tons (or gallons) of manure to the field to be applied by applicators sized larger than anything grandpa would have imagined.
Manure can be a great source of fertility for crops, but over-applying it leads to its own issues; leaching nitrates into groundwater, runoff of P into watersheds and creating algae blooms in lakes, and in some extreme conditions, yield losses due to excessive nutrients and salts that have built up over the years. Because of these issues, animal agriculture has been under scrutiny, and it is everyone’s best interest to apply manure as wisely as possible.
Because of the variable nature in the composition of manure, it is best to get a manure analysis performed before application. These can oftentimes be run at the same lab that performs soil and tissue testing. By having the manure analyzed, you have a good sense of what is being applied and the nutrient addition that can be provided.
Here’s an example from some composted beef manure from my farm taken in 2018. We feed out a few beef calves every year, bedding the shed with oat straw and periodically cleaning it (and the manure) out and stacking in a pile. We’ll also scrape the lots at the end of each cycle and pile in the same spot. Manure will compost slowly over the next year or two until it is almost the same in look and feel as the soil on the rest of the farm.
As you can see, there are two sets of results – As Received and Dry Basis. It’s important to understand how much water is in the manure to determine how much of it is actual nutrients.
There are two values given for nitrogen: Total Nitrogen and Ammoniacal-N. Both are important; Ammoniacal-N, or “inorganic”, is nitrogen in the ammonium (NH4) form and is readily available to plants. There may be traces of nitrate (NO3) nitrogen in the manure, also plant available, but oftentimes this will be very low as to not be typically measured. Swine manure will oftentimes be fairly high in inorganic nitrogen.
The rest of the nitrogen is “organic”, becoming available to plants through mineralization over time as NH4, but not immediately available. This is one of the key features of livestock manure; it can keep on providing nitrogen to the soil long after the application is made. This can be beneficial if there are crops growing that can take it up, but may be a detriment if the organic N converts to ammonium, then to nitrate, and is lost into the groundwater.
According to this test, most of the nitrogen (about 88%) is organic. The mineralization rate (how fast the organic N converts to inorganic) depends on a host of factors, including weather, incorporation depth and type, and carbon pool makeup of the manure. Ruminant animals’ manure (beef & dairy cattle) typically breaks down slower than non-ruminant (swine, poultry) due to their rations and feedstuffs. The roughage included in ruminant rations typically have higher levels of cellulose and lignin carbon sources which will take longer to decay in the soil. University analysis shows that my composted beef manure may allow 15% of the organic nitrogen to become available the first year, but between 35 and 50% of swine manure’s organic N becomes available at the same time.
The report also shows the amount of nutrients of phosphorus and potassium per ton in the oxide form. This is to provide an equivalent value as commercial fertilizer. Oftentimes these values will be used to determine the value of the manure relative to the commercial alternative.
Crop Removal of Nutrients
What about removing nutrients from the soil for livestock in the form of forage or fodder? For ruminant livestock operations, forage harvesting is a key part of getting feed to the livestock. These forages and fodder don’t come for free; they include valuable nutrients that eventually will need to be replaced for continued soil fertility.
In the spring of 2018 I worked with a few local growers to take weekly samples of their rye, wheat and triticale crops they were growing, either for grain (wheat) or forage for beef (rye and triticale). These samples were sent to a lab for analysis, primarily to understand the nitrogen content and carbon pools, but at the same time, to have the phosphorus and potassium removal rates calculated as well.
The rye crop was harvested later than typical due to wet conditions, but based on estimated biomass size and growth stage, I found that it was removing about 30-40 lbs P2O5 and 170-220 lbs K2O per acre. The soil fertility at this sample point was generally in the “Medium” rating. However, the field where the triticale was being grown was the recipient of many applications of swine and cattle manure, being just outside the operator’s farmstead. These grasses may be “luxury” feeders of nutrients, taking in as much as it can. In this situation, the triticale removed 120-130 lbs P2O5 and 580-600 lbs K2O/acre!
Corn residue may be removed from fields as bales for use in cattle rations and/or bedding. This residue may contain stalks, leaves, husks and cobs, each varying in nutrient levels. Iowa State University states that each ton of dry matter corn residue contains 3 lbs P2O5 and 22 lbs K2O. This would indicate that if 3 DM tons per acre were harvested, about 9 lbs P2O5 and 66 lbs K2O would be removed from being available for recycling into the soil. Of course, these values may vary, so it is important to have removed residue tested, just like the manure test above.
Livestock have traditionally been a part of an integrated farming operation, and even as operations have moved to become more specialized, their importance as a way to convert farm production to protein and fertilizer has not gone away. With the analytical tools available to us today, there’s no reason we need to end up with fertility “hot spots” near livestock buildings. Instead, with proper testing and management, we can get the most out of livestock & crop interactions. Your local Granular Agronomy Certified Service Agent (CSA) has the tools to help you plan and manage your manure and crop residue requirements today.