Data from a total of six studies went into determining the impact on nitrate and corn yield. Current thoughts of the nitrogen science team are that the price variability in nitrogen in recent years has limited the cost difference between fall and spring application, therefore, the same fertilizer nitrogen cost is used for all timing comparisons. There will be a possible economic gain due to increased yields with a change in application timing.
(Randall and Sawyer, 2008)
Interpretation section – “Spring application of N is superior to fall application in most cases.” The
advantages are limited, however, to warm and wet conditions. Authors suggest losses of fall applied N may be as much as 50% under perfect denitrification conditions. Reductions of N loss due to leaching are estimated to be around 15% with as little as no reduction and as much as 25%, depending on application timing and weather conditions. Applying in spring could cost between $5 and $10 per acre more. However, this could be a wash if more is applied in the fall to offset expected losses. Authors suggest an estimated 12.9 million acres out of 50.6 million acres in the Corn Belt could benefit. This paper was not used in the practice table but was used to guide estimates of fall nitrogen application.
(Randall and Mulla, 2001)
This paper reports an average of 20% load reduction at Waseca, Minnesota (1987-‐1993) when comparing fall vs. spring nitrogen application over a 4-‐year period. The addition of nitrapyrin reduced nitrate-‐N concentrations by 15%. The split application (pre-‐plant along with sidedress in a 40%-‐60% split) also
reduced annual nitrate-‐N concentrations from tile lines by 20% over the same 4-‐year period. This study also included information about nitrate-‐N concentrations from different cropping systems, which was the same as information in (Randall et al., 1997). Data from this paper was not included in the practice table.
(Randall, 2008)
This paper has nitrate concentration numbers for both fall and spring applications, however, all fall applications used N-‐Serve, meaning there is no real control treatment to compare against. A point of interest is the fall 135 kg N/ha (120 lb N/acre) treatment with N-‐Serve and the spring 135 kg N/ha (120 lb N/acre) treatment have weighted nitrate-‐N concentrations of 13.2 and 13.7 mg/L, respectively. Corn yields for the fall 120 lb N/acre treatment with N-‐Serve were 0.9 Mg/ha (14 bu/acre) higher than the
corresponding spring application. Data for yield and nitrate was added to the table for timing, inhibitor, and sidedress.
(Vetsch and Randall, 2004)
This paper has limited data for use in this project. Fall corn yields for grain and silage were 10.9 and 16.8 Mg/ha, respectively, while spring yields for corn were 11.7 and 17.6 Mg/ha for grain and silage,
respectively. Anhydrous ammonia at 123 kg N/ha was applied to both spring and fall treatments. Data was not included in the practice table.
(Randall and Vetsch, 2005c)
This 6-‐year study from Waseca, Minn., has information about nitrogen application timing as well as the use of a nitrification inhibitor with a 134 kg N/ha application rate. All data has been added to the table as site years. The main effects are:
• 6-‐year 11% average increase in yield when moving from fall to spring application with 1 year having
a 71% increase. The average over the other 5 years is actually slightly negative.
• 6-‐year average of 8% increase in yield with the addition of N-‐Serve. One year had a 41% increase
with a 1.6% increase excluding that year. Data was included in the practice table.
(Randall et al., 2003a)
This was a 7-‐year study at Waseca, Minn., (1987-‐1993) with 150 kg N/ha application rate. This study looked at timing, nitrapyrin, and sidedress. Site years have been added to the table. Main effects are:
• 7-‐year 5.4% average increase in corn yield when moving from fall to spring.
• 7-‐year 10.2% average increase in corn yield when moving from fall to pre-‐plant + sidedress (40-‐60
split).
• 7-‐year 5.9% average increase in corn yield when using nitrapyrin in the fall.
Data was included in the practice table.
(Randall et al., 2003b)
This was the drainage component of the research at Waseca, Minn., from 1987 to 1994. Nitrogen
application rate was 150 kg N/ha. Site years have been added to table and include both corn and soybean. One note is that there was no drainage in the soybean plots in 1988 or 1989 and no drainage in the corn plots in 1989. Main effects are:
• 7-‐year 6.8% average nitrate-‐N decrease when considering the entire rotation and moving from fall
to spring nitrogen application over the study years. The range was an increase of 80% in the soybean year of 1992 and a reduction of 22.9% in the corn year of 1990.
• 7-‐year 4.8% average nitrate-‐N decrease when considering the entire rotation and moving from fall
application to a pre-‐plant/sidedress split (40-‐60). The range was an increase of 60% in the soybean year of 1992 and a reduction of 26.3% in the corn year of 1991.
Data was included in the practice table.
(Randall and Vetsch, 2005a)
This research was carried out at a site in Waseca, Minn., between 1994 and 2000. The study investigated nitrogen loss from plots with anhydrous applied at 135 kg N/ha in the corn year of a corn-‐soybean rotation. Information on a full rotation was collected between 1995 and 1999 with 1994 having a corn crop only and 2000 having a soybean crop only. Results show nitrate-‐N concentrations for spring-‐applied nitrogen are lower than the corresponding fall-‐applied treatments in the corn year. However, the soybean plots have nearly the same nitrate-‐N concentrations for both treatments. All site year data has been added to the practice table. This paper also had information on nitrification inhibitors, which was added to the practice table.
(Clover, 2003)
This thesis explored nitrate-‐N concentrations from three years of a corn-‐soybean production in central Illinois. The treatments involved a fall and spring application as well as using a nitrification inhibitor. In addition to the spring application the study investigated a sidedress application. Both fall and spring treatments included a 76 kg N/ha, 156 kg N/ha, and a 234 kg N/ha rate. The inhibitor and sidedress treatments were applied at the 156 kg N/ha rate. Nitrate-‐N concentrations were lower coming out of the
spring-‐applied and fall-‐application (depending on the year). The timing, sidedress, and inhibitor numbers have all been added to the practice table.