Changes in yield potential associated with technological advances such as insecticide seed treatments and foliar fungicides should increase the need for additional fertilizer nitrogen (N). However, increased efficiencies of N use are expected as plant roots are protected from insect damage. Improving photosynthetic capacities may extend and enhance grain filling but would also be effective in prolonging root efficacy to take up soil nutrients, including N.
As N prices continue to climb, rate reductions associated with increased use efficiency become more of an economical consideration. These gains in efficiency could be combined with potential increases in yield to more than offset the costs of insecticide seed and/or foliar fungicide treatments.
A field study was conducted at two locations in southern Illinois from 2008 through 2010 to determine if changes in corn yield potential associated with insecticide seed treatments and foliar fungicides could modify the optimum economic fertilizer N requirement. Corn was treated with either Poncho 250 or 1250 insecticide seed treatment in a factorial without and with Headline foliar fungicide in whole plots. Subplots consisted of N rates of 0, 60, 120, 180, and 240 lb/acre.
On average across the 6 site-years, increasing N rates significantly increased leaf diseases, especially without fungicide application (Figure 1). Insecticide seed treatments had no effect on leaf diseases, and there were few significant interactions between those treatments and either N rates or fungicides.
Stalk rot was maximum at N rates of 60 to 120 lb/acre (Figure 2) and was slightly reduced by fungicide application (significant in 3 of 6 site-years), with a significant interaction between N rates and fungicide in 2 of 6 site-years. Insecticide seed treatment had no effect, nor was there a significant interaction between it and N rate .
It appears that high demand for N during grain fill tends to rob leaves and stalks of tissue N. This seems to create conditions of increased stalk rot at lower N rates, especially without the use of fungicides, but increased leaf diseases at higher N rates, again without fungicides.
Increasing N rates significantly increased corn grain yields at both locations for all 3 years (Figure 3). Insecticide seed treatment had no effect on yield at either location in any year except at Belleville in 2010, where there was a decrease of 5 bu/acre with the 1250 treatment, presumably due to its lower stand density. Application of a foliar fungicide significantly increased grain yield in 2 of 6 site-years, although there was a trend toward higher yields for every site-year, and on average there was an increase of 6 bu/acre associated with the fungicide application. There was not a significant interaction between N rates and fungicide treatment.
There was a significant quadratic response to increasing N rates at both locations all three years, which allowed us to calculate the economically optimum nitrogen rate (EONR) assuming a N price of 50 cents and a corn price of $6. At the EONR, the fungicide treatment increased corn yields by 10 bu/acre compared to the no-fungicide treatment, and the 1250 insecticide seed treatment decreased yields by 8 bu/acre compared to the 250 treatment.
Dixon Springs Agricultural Center
Carl A. Bradley
Associate Professor of Plant Biology