University of Minnesota soil scientists completed a research study evaluating the application of the nitrification inhibitor Instinct to fall manure applications. First, researchers investigated whether the addition of Instinct to fall-applied swine manure would slow nitrification of nitrogen (N) or stabilize the nitrogen in the manure. The second research question was whether or not early October applications of manure were at greater N loss compared to early November applications when the soil temperatures are cooler.

Once soybean harvest is complete many Minnesota hog farmers begin applying manure to those acres for the next year's corn crop. Manure applications in southern Minnesota begin in early October and usually conclude by mid- November. A significant proportion of the nitrogen in swine finishing manure is in the ammonium-N form. If warm soil temperatures persist after application, the ammonium-N can nitrify and be susceptible to loss via leaching or denitrification. These N losses have negative agronomic and environmental implications. The University of Minnesota recommends fall fertilizer N be applied after soils are less than 50° F at the 6-in. depth. This usually occurs in late October in southern Minnesota.

The research study treatments included two manure application timings (October and November) with three rates of Instinct (0-, 35-, and 70-oz./acre), plus 120 lb. N/acre as anhydrous ammonia with N-Serve in November, and a zero-N control. Swine finishing manure was applied at a rate of 120 lb. of available N/acre, assuming 80% first-year availability if sweep injected. The manure was applied to a Webster clay loam soil on Oct. 5 and Nov. 5, 2010. Four-inch soil temperatures averaged 4.9 and 1.4 degrees F. higher than normal, respectively. Six-inch soil temperatures averaged 55.3 and 38.0 degrees F. for October and November, respectively.

In November of 2010, about one month after manure application, nearly equal amounts of nitrate-N and ammonium-N were observed in the October manure application without Instinct treatment; whereas significantly more ammonium-N and less nitrate-N were found when Instinct was used. These data clearly showed nitrification of N in the manure was reduced with Instinct.

Corn grain yields were 11 bu./acre greater when swine manure was applied on Nov. 5 compared with Oct. 5, when averaged across Instinct rate. The results were not surprising considering the warmer-than-normal fall temperatures in 2010, researchers noted. Adding Instinct to fall applied swine manure increased corn grain yields 10-12 bu./acre compared to manure without Instinct, when averaged across manure application timings.

No significant interactions between application timing and Instinct rate were found. Corn yields were not statistically different among the November manure application with Instinct treatments and the November anhydrous ammonia with N-Serve treatment. Adding Instinct to swine manure also reduced grain moisture about 1.3 percentage points at harvest.

Based on these limited data, the addition of the nitrification inhibitor, Instinct, to fall-applied swine manure had both agronomic and environmental benefits. Delaying manure applications until soil temperatures have cooled is also a best management practice.
Read the full research report online at blog.lib.umn.edu/efans/.