Friday, May 19, 2017

Compost, Stockpiling, and Fresh Manure – What is happening?

Handling systems affect manure nutrient levels and forms by influencing gaseous emissions, exposure to runoff and leaching, and as a result can influence a manures ability to supply crop nutrients when land applied. Traditionally, on feedlots in Iowa, pens are cleaned periodically and then the manure is stockpiled, either near the lot or in a field and then eventually land applied. In some cases, lots might be scraped frequently and the manure land applied almost immediately. So what difference does each of these make to the manure properties?

 Freshly excreted manures are often very wet, which is especially true during wet periods of the year, like after rainstorms or snowmelt. These wet conditions generally make long-distance transport difficult or almost impossible. This also requires field be available for land application throughout the year, and as such, has generally fallen out of favor as it means the ground can’t be used to support crop production throughout the summer. Though in some cases, through the use of varied rotation, usually including some hay ground, land application can be accomplished. As a result, stockpiling manure becomes more prevalent. In this practice, manure is cleaned form the pen surface and then heaped into stacks, or stockpiles, to await reloading, hauling, and spreading. Stockpiling is generally a passive process but helps in matching manure application timing to better align with typical crop production practices. A third alternative is composting. Composting is an aerobic treatment where the manure is managed so the pile continues to have oxygen in it, providing conditions for microbes and bacteria to break down the material. Composting uses mixing to make a more uniform pile and causes the material to heat, often killing pathogens within the manure and inactivating weed seeds.
Figure 1. Stockpiled beef cattle manure.

When looking at the results, there are two things to look at. The first is the nutrient concentrations and the second is the mass balance.  When it comes to the concentrations, this tells us how far we can afford to move the manure. Higher concentrations mean we can afford to haul it a bit further as it is more nutrient dense. The results indicate that both stockpiling and composting increase the nutrient density relative to fresh manure. Much of this change is due to water loss, so we just aren’t hauling around as much water.

However, if you look at the results a bit closer, you’ll see we actually end up with less nitrogen and phosphorus to land apply from both stockpiling and composting. This occurs because some of the nutrients are lost due to volatilization of the nitrogen during the storage process.  Some of the loss of is due to dust and rainfall runoff during stockpiling and composting for phosphorus. Taken together, these results would show that if we need to transport manure long distances, composting might be a good option, but if we are using the manure on farm and want all those nutrients, stockpiling might be a better choice for your operation. Of course additional factors like consistency of the manure, killing pathogens, or inactivating weed seeds might be additional factors to consider. (these results are a summary of Larney et al., 2006 – Fresh, stockpiled, and composted beef cattle feedlot manure: nutrient levels and mass balance estimated in Alberta and Manitoba).
Table 2. Nutrient concentration of fresh, stockpiled, and composted beef cattle manure.

Dry Matter
Water
Total Carbon
Total Nitrogen
Inorganic Nitrogen
Total P

lb/ton
lb/ton
lb/ton
lb/ton
lb/ton
lb/ton
Fresh
698
1302
216
11.2
2.6
3.2
Stockpiled
856
1144
212
13.2
3.8
4.6
Composted
1280
720
208
18
1
6.6

Table 3. Mass comparison of fresh, stockpiled, and composted beef cattle manure.


Initial Mass
Final Mass
Dry Matter
Water
Total Carbon
Total Nitrogen
Total P

lb
lb
lb
lb
lb
lb
lb
Fresh
1000
1000
349
651
108
5.6
1.6
Stockpiled
1000
636
272
364
67
4.2
1.5
Composted
1000
328
210
118
34
3.0
1.1