In an effort to reduce nutrient losses from manure applied to cropland, a number of best management practices have been created. They were crafted using the best available information at the time and, ever since, researchers continue to analyze if they are the best or if they could be better. Three soil scientists were at the North American Manure Expo in Prairie du Sac, Wisconsin, recently to share what they’ve learned about nitrification inhibitors, timing of manure application and type of application to better explain their roles in preventing the loss of nitrogen and phosphorus.
University of Wisconsin – Madison Associate Professor of Soil Science Carrie Laboski began by explaining the nitrogen cycle. The organic matter from manure mineralizes into ammonium, which is consumed by the plant, fixated into clay minerals or converted to nitrates. The nitrates can then be consumed by the plant or lost through denitrification into the air or leached into the groundwater.
The role of an inhibitor is to temporarily stop the process of converting ammonium to nitrate. “You can’t have nitrate losses if you don’t have nitrates,” Laboski said.
Nitrification inhibitors available on the market today include Instinct/N-serve (nitrapyrin) and Guardian (DCD). Similar products also available are urease inhibitors, which Laboski said do not work well in manure, and slow-release materials.
To evaluate the effectiveness of a nitrification inhibitor, Laboski performed a study on saybrook silt loam soil at the University of Wisconsin Arlington Agricultural Research Station using dairy manure.
She applied 7,083 gallons per acre on Oct. 21 and 8,500 gallons per acre on May 3. The manure was broadcast applied, with or without nitrapyrin and followed by tillage. The corn planted on this plot was then harvested for grain and for silage.
There was no significant interaction between the time of application and the use of the inhibitor, Laboski reported. There was also no significant increase in grain yield. However, there was a significant yield increase for silage harvested from the inhibitor-applied plot.
Laboski also looked at what was happening with the nitrogen in the soil. Soil samples were taken on Nov. 16 and April 12 to see how the fall application changed over time. While not statistically significant, she did report about 20 pounds of nitrate had not been converted due to the inhibitor.
On average, 30 pounds of nitrates were lost throughout winter, including some from the inhibitor plots. Laboski said it’s a misnomer to believe no nitrates will be lost. The use of an inhibitor only has the potential to reduce those losses. However, not all scenarios result in such losses to begin with.
“If you have a situation for nitrate loss, you should consider using an inhibitor,” she said. “You may not have a situation for a loss and then you won’t see the benefit.”
Timing of manure application
Phosphorus losses from runoff can cause algae and other plant growth in surface waters, said Peter Vadas, research soil scientist with the USDA Dairy Forage Research Center . While phosphorus can come from many sources – urban, residential, agricultural, etc. – Vadas focused on manure that was surface-applied and not incorporated.
“With every first rain event there is the opportunity for the greatest loss,” he said. Producers have been told to avoid application if runoff is likely from a storm or snow melt in the immediate future – but what is the optimal timing between application and a rain event?
According to Vadas, nine studies have been done to show the effectiveness of timing manure applications. Most of the studies used swine and poultry manure; one used dairy manure. Manure was surface-applied and a rain simulator was used.
Among the studies, there was a large variation in the amount of wait time before the simulated rain event. Some studies with a longer wait period did show decreased runoff – but other studies with long wait times did not differ in runoff, Vadas said.
Therefore, he began looking at the storm hydrology and the percent of phosphorus released.
He found that soil moisture levels before a rain event played a role. On really dry soils it took a long time before runoff began, about 50 to 60 minutes. That is compared to the 10-minute timeframe on very wet soils.
Vadas also learned that about 50 percent of the rain would move across wet soils, whereas dry soils would absorb more of the rain.
The beginning of a storm is when phosphorus releasing from manure is at its peak. This lessens as the storm continues.
As Vadas looked at the data from the nine studies, he saw a low runoff-to-rain ratio in dry soils. The earlier the rain, the higher the ratio. A longer wait time on the rain experiment provided time for the soil to dry, resulting in a lower ratio.
This runoff-to-rain ratio was directly correlated with phosphorus losses. “If you can get more rain going into the soil, phosphorus loss from runoff is a lot less,” Vadas said.
Unfortunately farmers have a limited ability to control when they can spread manure. In Wisconsin, it is typically only 11 to 13 days per month, he said.
Vadas still recommended to avoid manure application if runoff is likely to occur and acknowledged that more time will help assimilate manure into the soil. However, that could take months to achieve.
“When runoff occurs, phosphorus loss is controlled by the amount of rain and how much rain infiltrated the soil – not timing,” Vadas said, “especially if the rain event occurred in the first few weeks.”
The bottom line is that a farmer can’t use timing as a reliable best management practice; it mostly depends on the type of storm received.
Type of application
The incorporation of manure is advocated to reduce odor and ammonia emissions, but is the practice impacting nitrogen losses via other pathways?
J. Mark Powell, research soil scientist with the USDA Dairy Forage Research Center, set out to answer this question when he set up a four-year study to see how incorporation impacts the amounts of nitrogen taken up by the crop, volatized as ammonia, leached as nitrate and remained in the soil.
Manure was applied at 8,000 gallons per acre using surface application, partial incorporation (AerWay system) and incorporation by injection. The same type of application was repeated on the same plot every year.
Oats were planted in the plots in year one. Year two it was corn for silage followed by cereal rye, and years three and four had corn for silage.
Ammonia emissions were measured from zero to 120 hours after application. Year one had very little loss at 10 pounds per acre. There was a tremendous difference in year two at 50 to 60 pounds lost where the manure was surface applied.
Incorporated plots recorded one-third less than that and the partial incorporation settled between the two. Emissions were lower overall in year three but significantly higher in surface application. Year four was similar to year two with high losses.
For years one, three and four there was very high nitrate leaching, especially in plots where it was partially incorporated.
Powell found incorporation helped conserve the ammonium with less of it converting to nitrate than in the other two application methods.
Looking at all four years combined ( see Table 1 ), surface application and partial incorporation have similar nitrogen losses, with ammonia being the greater loss for surface application and nitrate the bigger form of loss for partial incorporation.
Incorporating the manure recorded total nitrogen losses less than one-third of the other two methods with neither ammonia or nitrate greater than the other.
The loss of nitrates through leaching was highest in partial incorporation for all four years. This application method also recorded the highest residual nitrate leaching, measured one year after the study.
Looking at yields, Powell said the crops didn’t respond to any of the conserved nitrogen. There was approximately 40 pounds per acre more nitrogen available to the plant in the incorporated plots, yet the yields were not any higher. Therefore, he found no significant difference in nitrogen uptake by the plant but noted that the soils were giving off a lot of nitrogen even in the control plots.
Overall, he found that incorporating manure decreases total nitrogen loss by 18 percent for partial incorporation and 65 percent for incorporation. PD