Storage is obviously a good way to keep manure on the farm, but that doesn’t mean all the nutrients are staying put.
Storing manure provides opportunities for dairy farms to maximize manure’s value as a fertilizer, reduce handling costs, and minimize its potential to pollute the environment.
However, it also presents the potential to lose the nitrogen in manure to the atmosphere through volatilization as ammonia.
“Knowing the quantities of nitrogen lost is critical to inform the decision-making process related to developing mitigation strategies,” according to research published by Virginia Tech on an on-farm dairy manure storage structure and supported by the United States Department of Agriculture NIFA (grant no. 2017-67019-26399).
The study quantified the nitrogen lost to the atmosphere during manure storage on small dairy farms as influenced by management and meteorological parameters.
The study identified wind speed as the meteorological factor with the greatest effect on ammonia loss. It also found that 25 percent of ammonia loss occurred during the agitation process to break the crust and homogenize manure before pumping it out for land application..
“The data from our study area suggested wind speeds were higher during the late winter and early spring months,” says Jactone Ogejo. Ojego, an associate professor in Virginia Tech’s department of biological engineering and an extension specialist, is the lead author on the study. “This period, incidentally, is about when the manure storages are full, and farmers are getting ready to apply manure to their fields.”
Crusting
Ogejo, an associate professor and extension specialist with Virginia Tech’s Department of Biological Systems Engineering, says that crusting is an eventuality in dairy farms manure storages.
How does crusting happen? The formation of crust on dairy manure slurry during storage is influenced by various factors. These include high levels of organic matter, solids content, and additional fibrous bedding materials. These components contribute to crust formation as they float and accumulate on the surface. Microbial activity, driven by temperature and nutrient content, produces gases that move lighter materials upwards toward the surface, further promoting crust development. When there is no agitation or slurry mixing, material stratification occurs, leading to the formation of a distinct crust layer. Storage conditions, such as open versus covered structures, also play a significant role; open storage is more prone to crusting due to increased evaporation.
Crusting is one factor that plays a significant role in reducing ammonia emissions.
“Ammonia emissions were markedly reduced during periods with substantial crust coverage – 80 to 100 percent of the surface – such as late summer to early fall,” says Ogejo.
He explains that the crust acts as a physical barrier, inhibiting the volatilization of ammonia by limiting its exposure to air.
“The wind speeds may not have much impact on ammonia loss until it is agitated and pumped out for land application,” says Ogejo.
“However, the high wind speeds may accelerate loss during mixing and when the manure is applied on fields, depending on the method used.” As an example, he points out that if the manure is spread by broadcasting, more losses may be experienced than if injected.
He also notes that the correlation analysis shows a strong negative correlation between crust thickness and ammonia flux: “So, the thicker the crust, the less ammonia is volatilized.”
Using the data from the study, farmers can inform their manure management strategies, specifically when to schedule their manure applications, such as picking times when wind speeds are relatively low to minimize ammonia loss and associated odor nuisance that may arise because of manure application, Ogejo says.
Manure management steps
There are a number of management steps farmers can take to reduce ammonia loss.
For example, applying manure when conditions are less conducive to ammonia volatilization — during cooler temperatures and low wind speeds — can reduce the losses, Ogejo says.
He adds that covering the surface of stored manure can also reduce ammonia loss.
“Our study showed low ammonia losses when the crust was on the surface. One conclusion we can draw from this finding is that if manure can be removed from storage with minimal disruption of the crust, that may be beneficial,” says Ogejo.
It is not clear, however, whether that approach is practical, he notes.
But nevertheless, it does present an interesting engineering challenge of maintaining the crust and mixing the contents beneath it to get homogenous nutrient distribution in the bulk of manure to be used as fertilizer, says Ojego.
Agitating the manure during cooler, cloudy, and less windy days may also be helpful management. As well, avoiding over agitation will help to minimize the ammonia losses.
Other management steps
There are other management steps to consider given that it’s no longer common for small farms to spread manure daily.
For these farmers, they will have to invest in some short-term storage, and devise a plan to reduce the frequency of application, says Ogejo.
Farmers are also advised to consider designing manure storages that are deep rather than wide in order to minimize the surface area of exposed manure where ammonia is lost.
They may also wish to consider adjusting the protein content in their animals’ diet.
“Optimizing the protein content in the diet will minimize the nitrogen lost in the manure and, therefore, ammonia lost,” he says.
Another of Ogejo’s management tips is controlled aeration to promote microbial activity that converts ammonia to forms of nitrogen that are less volatile.
He adds that some people are also in favour of adding chemicals that acidify manure. •
