Illinois Fertilizer
Conference Proceedings
January 24-26, 2000
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S.A. Ebelhar, K.L. Barber, and A.H. Anderson1
More and more farmers in southern Illinois are finding the need to plant wheat after corn rather than wheat after soybean. New replacements for atrazine, such as Broadstrike, allow the planting of wheat after corn with little concern for herbicide carryover. Wheat after corn may be extremely important environmentally because it would trap excess nitrates left over from corn after a drought.
Utilizing a corn-wheat-soybean rotation allows three grain crops to be grown in two years, and may increase the profitability of farms in the southern part of the state. Wheat after corn is growing in popularity in the southern part of the state because it allows better flexibility on crop acres.
Management of wheat after corn is very different than after soybean because of the tremendous difference in residue left after harvest of corn versus soybean. Corn stalks and cobs are very low in nitrogen and have very high C:N ratios. As this material breaks down, there is a tendency for N to be tied up, which lessens its availability to wheat. N losses from leaching and denitrification under this heavier residue may be quite different than under soybean residue. Therefore, it is time to take a close look at nitrogen management for wheat after corn, and to revisit the impact of tillage, seeding rates, and timing of fertilizer N.
Tillage, as it affects residue left at the surface, would have a large effect on the response of wheat to N. No-till would leave the residue intact and incorporate little of it into the soil. This would slow the breakdown of the residue at the surface and affect the water relations in the soil. This would have an impact on the N availability to the wheat crop, especially early (one to six weeks after planting) when good growth is critical for winter survival.
Nitrogen management is crucial with respect to planting densities and final stand. Alley et al. (1) report that tillering numbers are highly dependent on stand and nitrogen management early in the life cycle of wheat. Stand density is dependent on planting rate and tillage. When tiller numbers are low (<50 tillers/sq foot), Alley et al. recommend N fertilizer application at Feekes 3.0 stage of wheat growth (about mid-February). When tiller numbers are high (>100), N fertilizer should be applied closer to when the plant needs it the most (after Feekes 5.0). The earlier N application promotes tillering but is subject to larger losses of N through leaching or denitrification. This theory has not been tested in the south-central corn belt, nor has the impact of tillage system been evaluated.
The objectives of our study are divided into two segments as indicated below:
To evaluate N management practices for wheat after corn compared to wheat after soybean.
To determine effects of seeding rate, tillage and N management treatments on grain yield and test weights of wheat after corn compared to wheat after soybean.
Field studies that began in the fall of 1996 were continued through 1999. Two studies were conducted at the Dixon Springs Agricultural Center (DSAC) and the Brownstown Agronomy Research Center (BARC). The two studies are described below, with study site information contained in Table 1.
A split-split plot design with three replications was utilized with previous crop (corn versus soybean) as whole plots, tillage (no-till versus tilled) as subplots, and N rates and timings as sub-subplots. The N rates and timings are indicated below. The tillage treatment was accomplished with the use of a field cultivator and cultipacker at Brownstown and a rotovator plus cultipacker at Dixon Springs.
| Treatment | Fall N (lb/A) | Spring N (lb/A) | Total N (lb/A) |
|---|---|---|---|
| A | 0 | 40 | 40 |
| B | 0 | 80 | 80 |
| C | 0 | 120 | 120 |
| D | 20 | 40 | 60 |
| E | 20 | 80 | 100 |
| F | 20 | 120 | 140 |
| G | 40 | 40 | 80 |
| H | 40 | 80 | 120 |
| I | 40 | 120 | 160 |
| J | 60 | 40 | 100 |
| K | 60 | 80 | 140 |
| L (Check) | 0 | 0 | 0 |
Corn and soybean were grown prior to wheat planting to initiate the study. Broadstrike + Dual was applied to both crops so as to eliminate the confounding associated with herbicide carryover when different herbicides are used for corn than for soybean. The nitrogen source for this study was ammonium nitrate broadcast. Using this source should eliminate the problem of N loss through volatilization. Spring applications of N occurred at green-up (see Table 1 for details).
Wheat was drilled at a seeding rate of 30 seed/square foot (approximately 90 lbs/acre). A high-yielding adapted variety was chosen at each location.
A split-split plot design with three replications was utilized with previous crop (corn versus soybean) as whole plots, tillage (no-till versus tilled) as subplots, and seeding rate X N management treatments as sub-subplots. The seeding rates were 20 and 40 seeds per square foot (approximately 60 and 120 lb/acre, respectively). Nitrogen management treatments are indicated below.
| Treatment | Fall N (lb/a) | Spring N GS 3.0 | Spring N GS 5.0 | Total N |
|---|---|---|---|---|
| A | 0 | 120 | 0 | 120 |
| B | 20 | 100 | 0 | 120 |
| C | 40 | 80 | 0 | 120 |
| D | 60 | 60 | 0 | 120 |
| E | 40 | 40 | 40 | 120 |
| F | 40 | 0 | 80 | 120 |
Corn and soybean was grown prior to wheat planting to initiate the study. Broadstrike + Dual was applied to both crops so as to eliminate the confounding associated with herbicide carryover when different herbicides are used for corn than for soybean. The nitrogen source for this study was ammonium nitrate broadcast. Using this source should eliminate the problem of N loss through volatilization. Spring applications occurred at Feekes GS 3.0 and 5.0 on dates indicated in Table 1.
For both of the studies indicated above, whole wheat plants were sampled for nitrogen concentration at flowering. Grain yields, moisture, and test weights were taken at physiological maturity.
Weather conditions were very different in the three years of this study. Average monthly high temperatures and rainfall were quite a bit different in the 1997?98 and 1998?99 growing seasons compared to the 1996?97 season (Figure 1, Figure 2 , Figure 3, and Figure 4). These differences and their effects on the results of this study will be discussed more below.
Effects of Previous Crop and Tillage. At DSAC, both no-tilling (NT) and tilling (CT) after soybeans produced higher wheat yields than after corn (Table 2). There was a definite advantage to conventional tillage after soybeans, much higher than in previous years. These yield advantages corresponded to higher head counts. There was no difference between tillage systems or previous crops with stand counts. Overall, there appears to be every indication that wheat can be produced with either NT or CT and after either corn or soybeans, with a slight advantage for CT after soybeans.
Differences in nitrogen concentrations at flowering were small, with slightly higher N concentrations for CT and a previous crop of soybeans. There is no indication of nitrogen carryover from the previous corn crop. Soil nitrate levels at wheat planting indicated about 6-8 ppm NO3--N in the top 12" of soil under both tillage systems and both crop residues. In general, plant nitrogen levels were adequate, an indication that nitrogen was not limiting at this study site. Rainfall data (Figure 4) indicates normal precipitation for February and March at DSAC, and there appeared to be little or no N loss on these plots. Above-average temperatures (Figure 2) in October through February and April through June may have increased mineralization of soil N, which may have led to a net increase in available N rather than a decrease in available N normally associated with wheat production.
At BARC, no-tillage performed better than tillage with a previous crop of soybeans producing slightly higher yields than a previous crop of corn (Table 3). This is very similar to 1997-98 but in sharp contrast to 1996-97, where NT produced significantly lower yields, especially after soybeans (Ebelhar and Barber, 1998). Head counts were not significantly different between previous crops of corn and soybeans or between tillage systems. Plant stands also showed very little difference between previous crops, but NT was slightly better than CT. The very low rainfall in October-December probably gave a slight advantage to NT in obtaining a good stand at this location (Figure 3).
There was slightly higher whole plant N concentrations following soybeans, but there were no differences between tillage effects. Again, this would be in sharp contrast to 1996-97, where there were significantly higher N levels obtained with a previous crop of corn. Plant levels were higher than at Dixon Springs, an indication of either less N losses or more N mineralization occurring at BARC. This loss potential is supported by higher rainfall in March and April at DSAC (Figure 3 and Figure 4).
Effects of Nitrogen Rates and Timing. Both fall and spring N rates had large impact on yields at DSAC (Table 4), with effects being all positive, as compared to 1997-98 where effects were mostly negative. In general, the highest wheat yields occurred at total (fall + spring) N rates of 120 lb N/acre with at least 40 lb N/acre in the fall. This was in contrast to previous years, where spring N rate was the most influential factor.
Neither fall nor spring N rates affected plant stands, head counts, or test weights. Whole plant N levels all increased with increasing fall, spring, or total N rates. In 1998-99, as in 1997-98, fall N and spring N rates were additive, as compared to 1996-97, where fall N was used less efficiently than spring-applied N and higher fall rates could not account for an equal reduction in spring N rate.
There was no wheat yield responses to N at BARC (Table 5). Increasing fall N rates slightly increased whole plant N and stand counts but slightly lowered head counts. Indications are that the higher temperatures and lower rainfall at BARC were ideal for N mineralization to occur with very little N loss such that additional N fertilizer was not needed.
Previous Crop, Tillage, and N Treatment Interactions. There were few significant interaction effects of previous crop, tillage, and N treatments at DSAC (data not shown). There was a significant tillage by N treatment effect on whole plant N. In this case, CT responded better to increasing N rates than NT.
There were no significant interaction effects of previous crop, tillage, and N treatments at BARC.
Effects of Previous Crop and Tillage. Study 2 compared different split applications of N at the same total N rate but for two different seeding rates of wheat. In this study, a previous crop of soybean and conventional tillage at DSAC had the highest yield, just as in Study 1 (Table 6). There was also an increased stand for wheat after soybeans compared to wheat after corn. Other than a slightly lower test weight with NT, tillage had no effect on grain yields, stands, whole plant N, or head counts. Whole plant N levels were quite a bit higher than in Study 1, perhaps reflecting differences (such as drainage, slope, aspect, etc.) in sites between these two studies.
Effects at BARC were somewhat similar to Study 1 at BARC (Table 7). However, there were no effects on grain yields. Head counts were slightly higher following soybeans than following corn, and test weights and whole plant N were slightly lower with NT compared to CT.
Effects of Seeding Rate and N Treatments. Increasing the seeding rate from 20 to 40 seeds per square foot significantly increased stand densities and head counts, but had no effect on yields (Table 8). Increasing fall N rates increased yields, especially at the 20 seed/sq It seeding rate. The 6016 fall N + 60 lb spring N rate had the highest yield at the 20 seed/sq ft rate, and the 40 lb fall rate had the highest yield at the 40 seed/sq ft rate. Differences in head counts, stand densities, and whole plant N levels were small or nonsignificant. Spring?splitting the N rates between Feekes 3.0 and Feekes 5.0 had no effect on grain yield or head counts, but the 80 Ib/acre N rate at Feekes 5.0 increased whole plant N concentrations while lowering test weights slightly.
At BARC, increasing the seeding rate significantly increased head counts but not stand densities and had no effect on grain yield (Table 9). Again, spring-splitting the N rates between Feekes 3.0 and Feekes 5.0 had no effect on grain yield or head counts but did increase whole plant N concentrations while decreasing test weights.
Previous Cron, Tillage, Seeding Rate, and N Treatment Interactions. There were very few significant interactions between previous crop, tillage, seeding rate, and N treatment effects on grain yields and head counts at DSAC and BARC (data not shown). The largest effect was slightly lower yields and test weights for split N or late N applications to wheat after soybeans compared to wheat after corn. This may be due to higher N availability in these plots, causing slight lodging problems.
Indications are that more N may be available to wheat following corn than following soybeans when corn does not utilize all of the applied fertilizer N and/or when there is greater mineralization of N from corn residue the following spring. Fall N rate was much less important than total (or spring) N rate, except in 1998-99 at DSAC. High fall N rates should not substitute for spring N rate because of the possibility of inefficient fall N utilization by the wheat crop. When conditions exist for very little N loss, high N rates may be detrimental. We need to be able to better predict what the proper N rate should be, taking into account N losses and net N gains to the system.
Planting wheat no-till at DSAC and BARC was as good or better than CT, but there may still be a problem with the Cisne soil at BARC having less slope for water runoff and poorer internal drainage, making NT less desirable in wet weather. In most cases, there is still a slight advantage for wheat following soybeans over wheat following corn.
Increasing the seeding rate from 20 to 40 seed/sq ft had little effect on yields in 1998-99 and 1997-98 as compared to 1996-97, where it increased wheat grain yield. In general, increasing seeding rates increased head counts and stand densities at both locations. Nitrogen treatments responded differently at the two different seeding rates, but there was still no advantage to spring-splitting N rates between Feekes 3.0 and Feekes 5.0.
Table 1. Experimental Conditions and Details, 1998-99
Table 2. Previous crop and tillage effects on wheat at Dixon Springs (Study 1), 1999
Table 3. Previous crop and tillage effects on wheat at Brownstown (Study 1), 1999
Table 4. Nitrogen rate and application date effects on wheat at Dixon Springs (Study 1), 1999
Table 5. Nitrogen rate and application date effects on wheat at Brownstown (Study 1), 1999
Table 6. Previous crop and tillage effects on wheat at Dixon Springs (Study 2), 1999
Table 7. Previous crop and tillage effects on wheat at Brownstown (Study 2), 1999
Figure 1. Average Monthly High Temperatures for BARC, 1996-99
Figure 2. Average Monthly High Temperatures for DSAC, 1996-99
Figure 3. Average Monthly Rainfall for BARC, 1996-99
Figure 4. Average Monthly Rainfall for DSAC, 1996-99
1 S.A. Ebelhar is Agronomist, Dept. of Crop Sciences, Univ, of Illinois; K.L. Barber is Agronomist, Golden Harvest; and A.H. Anderson is Visiting Senior Research Specialist, Dept, of Crop Sciences, Univ. of Illinois, Urbana, IL.
Alley, M.M., P. Scharf, D.E. Brann, W.E. Baethgen and J.L. Hammons. Nitrogen management for winter wheat: principles and recommendations. Virginia Polytechnic Institute and State University Cooperative Extension Bulletin.
Ebelhar, S.A. and K.L. Barber. 1998. Nitrogen management of wheat following corn and soybeans. In R. G. Hoeft (ed.) Illinois Fertilizer Conference Proceedings, pp 73-96.
