Illinois
Fertilizer Conference Proceedings
January 25-27, 1999
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S.A. Ebelhar and K.L. Barber1
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 Broadstike, 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. In a drought, corn does not utilize all of the applied N fertilizer. Wheat would be able to take advantage of the leftover N.
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 those areas because it allows better flexibility on crop acres.
Management of wheat after corn is very different than wheat 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 that we take a close look at nitrogen management for wheat after corn, and 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 into the soil. This would slow the breakdown of the residue at the surface and affect the water relation 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). If tiller numbers are high (>100), N fertilizer should be applied closer to when the plant needs it 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 research are divided between two studies:
Study 1
1) To evaluate N management practices for wheat after corn compared to wheat after soybean.
2) To determine effects of N rates, timing of N fertilization, and tillage on grain yields and test weights of wheat after corn and soybean.
Study 2
3) 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.
4) To determine the effects of N management and previous crop on tillering of wheat grown at two different plant densities.
Field studies began in the fall of 1996 and were continued in the fall of 1997. Two studies were conducted at the Dixon Springs Agricultural Center (DSAC) and two at the Brownstown Agronomy Research Center (BARC). The two studies are described below; study site information appears in Table 1.
Study 1: N rates X Timing
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 subsubplots. 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 and 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 highyielding adapted variety was chosen at each location.
Study 2: Seeding rates X N management
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 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 and 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 the dates indicated in Table 1.
For both of the studies described above, whole wheat plants were sampled for nitrogen concentration at flowering. Grain yields, moisture and test weights were taken at physiological maturity.
Weather
Weather conditions were very different between the two years of this study. Average monthly high temperatures and rainfall were quite a bit different in the 1997-98 growing season than in the 1996-97 season (Figures 1, 2 , 3 and 4). These differences and their effects on the results of this study will be discussed in more detail below.
Study l: 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 and Figure 5). These yield advantages corresponded to higher stand densities and head counts. Stand counts were slightly lower for NT after corn or soybeans compared to CT. This would be related to differences in residue management. Overall, there appears to be every indication that wheat can be produced with either NT or CT and after either corn or soybeans.
Differences in nitrogen concentrations at flowering were inconsistent with lower N concentrations for CT and a previous crop of corn or for NT and a previous crop of soybean. 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 inches 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 in January and in April through June may have increased mineralization of soil N, which may have led to a net increase in available N rather than the decrease 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 and Figure 6). This is in sharp contrast to last season, in which no-tillage 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 or tillage. However, plant stands were very low at this location, which also resulted in much lower head counts than at DSAC. The very low rainfall from October to March probably prevented us from obtaining a good stand at this location.
There were no differences between previous crops or tillage effects on whole plant N concentrations. Again, this is in sharp contrast to last year's study, in which significantly higher N levels were obtained with a previous crop of corn. Plant levels were lower than at Dixon Springs, an indication of some N losses occurring at BARC. This loss potential is also supported by above-average temperatures and high rainfall from April through June (Figures 1 and 3).
Study l: Effects of Nitrogen Rates and Timing
Both fall and spring N rates had large impact on yields at DSAC (Table 4 and Figure 7), although effects were mostly negative. In general, the highest wheat yields occurred at total (fall+ spring) N rates <80 lb N/acre, regardless of how the rate was split between fall and spring. This was in direct contrast to last year, when the spring N rate was the most influential factor; the highest yields occurred at the 120 spring N rate and was reduced slightly as some of the N was applied in the fall. This indicates that, unlike in 1996-97 where 120 lb of N was needed in the spring regardless of how much was applied in the fall, high N rates in 1997-98 led to a lowering of yields due to lodging and/or excessive vegetative growth. It appears that nearly all of the N applied was plant-available and that mineralization probably contributed to the highest yield occurring at the 40 lb N/acre spring N rate.
Neither fall nor spring N rates affected plant stands. However, head counts increased (compared to check plots) as some N was applied either in the fall or spring, but increasing fall N rates slightly reduced head counts. Whole plant N levels all increased with increasing fall, spring, or total N rates (Figure 9). In 1997-98, fall N and spring N rates were additive, compared to 1996-97, when 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. Test weights were significantly reduced as spring N treatments increased, indicating that there was a problem with excessive vegetative growth and lodging, reducing the grain-filling potential.
Wheat yield responses to N were less variable at BARC (Table 5 and Figure 8). Increasing spring N rates increased yields, but increasing fall N rates had little or no effect. The highest yield came when N was applied at an 80 lb/acre spring rate. Fall N was less critical, indicating that some of the fall N was lost from the system before plant uptake. Stand counts were unaffected by fall, spring, or total N rate, but there was a slight increase in head counts with increasing fall N rates. Increasing fall and spring N rates both increased whole plant N levels (Figure 10).
Study 1: 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 yield. In this case, it appears that NT had a slightly higher yield at the higher N rates but lower yields at the lower N rates.
Similarly, there were few significant interaction effects at BARC.
Study 2: 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, previous crop and tillage had little effect on yields, head counts, and test weights at DSAC (Table 6 and Figure 5). The slightly higher whole plant N levels may be associated with higher N availability following corn as opposed to following soybeans. Whole plant N levels were quite a bit lower than in Study 1, perhaps reflecting site differences (drainage, slope, aspect, and so on,) between these two studies.
Effects at BARC for Study 2 were similar to those from Study 1 (Table 7 and Figure 6). Grain yields were slightly higher with NT, especially following soybeans. There were no significant differences between previous crop effects on yield, head counts, or stand densities.
Study 2: 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 reduced yields (Table 8). There was no significant seeding rate by N treatment interaction effect on grain yields (Figure 11), as was seen in 1996-97. Increasing fall N rates increased yields, especially at the 20 seed/sq ft seeding rate. The 60 lb 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 non-significant. Spring splitting the N rates between Feekes 3.0 and Feekes 5.0 had no effect on grain yield, head counts, test weight or whole plant N concentrations.
At BARC, increasing the seeding rate significantly increased head counts and stand densities while decreasing whole plant N level but had no effect on grain yield (Table 9). There was a significant interaction between seeding rate and N treatment; the highest yields occurred with a 40 lb N/acre fall N rate and a seeding rate of 20 seed/sq ft, and with a 0 lb N/acre rate and a seeding rate of 40 seed/sq ft (Figure 12), similar to results at DSAC the prior year. Apparently, high seeding rates combined with high fall N rates can cause problems under certain conditions, and these conditions are yet to be determined. Again, spring splitting the N rates between Feekes 3.0 and Feekes 5.0 had no effect on grain yield, head counts, or whole plant N concentrations.
Study 2: Previous Crop, 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).
Indications are that more N may be available to wheat following corn than to wheat 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. 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 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.
Increasing the seeding rate from 20 to 40 seed/sq ft had little effect in 1997-98 as compared to 1996-97, when the change increased wheat grain yield, 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, 1997-98.
Table 2. Previous crop and tillage effects on wheat at Dixon Springs (Study 1), 1998.
Table 3. Previous crop and tillage effects on wheat at Brownstown (Study 1), 1998.
Table 4. Nitrogen rate and application date effects on wheat at Dixon SPrings (Study 1), 1998.
Table 5. Nitrogen rate and application date effects on wheat at Brownstown (Study 1), 1998.
Table 6. Previous crop and tillage effects on wheat at Dixon Springs (Study 2), 1998.
Table 7. Previous crop and tillage effects on wheat at Brownstown (Study 2), 1998.
Figure 1. Average Monthly High Temperatures for BARC, 1996-98.
Figure 2. Average Monthly High Temperatures for DSAC, 1996-98.
Figure 3. Average Monthly Rainfall for BARC, 1996-98.
Figure 4. Average Monthly Rainfall for DSAC, 1996-98.
Figure 5. Previous Crop and Tillage Effects on Yield, DSAC, 1998.
Figure 6. Previous Crop and Tillage Effects on Yield, BARC, 1998.
Figure 7. N Effects on Wheat Yields at Dixon Springs, 1998, Study 1.
Figure 8. N Effects on Wheat Yields at Brownstown, 1998, Study 1.
Figure 9. N Effects on Whole Plant %N at Dixon Springs, 1998, Study 1.
Figure 10. N Effects on Whole Plant %N at Brownstown, 1998, Study 1.
Figure 11. Planting Rate and N Effects on Yield, DSAC, 1998.
Figure 12. Planting Rate and N Effects on Yield, BARC, 1998.
1 S.A. Ebelhar is Agronomist, Dixon Springs Agricultural Center, UI; K.L. Barber was Senior Res. Specialist, Brownstown Agronomy Res. Center, UI, and is now Res. Agronomist, Golden Harvest-Thorp Seed Co., Clinton, 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.
