Once soybean harvest is complete many swine 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 (N) 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-inch depth. This usually occurs in late October in Southern Minnesota.
A research study was established at Waseca in October of 2010 to evaluate the application of the nitrification inhibitor Instinct to fall manure applications. Two research questions were addressed: 1) will the addition of Instinct to fall applied swine manure slow nitrification of N or "stabilize the N" in the manure? 2) Are early October applications of manure at the greatest risk for N loss compared to early November applications when the soil temperatures are cooler?
Treatments included two manure application timings (October and November) with three rates of Instinct (0, 35, and 70 oz./ac), plus 120 lb N/ac as anhydrous ammonia with N-Serve in November, and a zero-N control. Swine finishing manure was applied at a rate to give 120 lb of available N/ac assuming 80% first-year availability if sweep injected. The manure was applied to a Webster clay loam soil on October 5, and November 5, 2010. Four-inch soil temperatures in October and November of 2010 averaged 4.9 and 1.4 degrees greater than normal, respectively. Six-inch soil temperatures averaged 55.3 and 38.0 degrees 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 (Figure 1). These data clearly showed nitrification of N in the manure was reduced with Instinct.
Corn grain yields were 11 bu/ac greater when swine manure was applied on November 5th compared with October 5th, when averaged across Instinct rate (Table 1). This is not surprising considering the warmer than normal fall temperatures we observed in 2010. Adding Instinct to fall applied swine manure increased corn grain yields 10-12 bu/ac compared to without Instinct, when averaged across manure application timings. No significant interactions between application timing and Instinct rate were found. Corn grain yields were not statistically different among the November manure application with Instinct treatments and the November anhydrous ammonia with N-Serve treatment. The addition of 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.