Illinois Fertilizer
Conference Proceedings
January 26-28, 1998
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S.A. Ebelhar and K.L. Barber 1
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. In a drought, corn does not utilize all of the applied N fertilizer. Wheat would be able to take advantage of the residual 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 rapidly in popularity in the southern part of the state because it allows far better flexibility on crop acres. This is important, especially to farmers in the farm programs where bases of corn and wheat are very tight.
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 it 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 in tact and incorporate little of into the soil. This would slow the breakdown of the residue at the surface and effect 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) then Alley et al. recommend N fertilizer application at Feekes 3.0 stage of wheat growth (about mid-February), but if tiller numbers are high (>100) then 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 and are indicated below:
Two studies were each 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 vs soybean) as whole plots, tillage (no-till vs 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 |
20 |
40 |
E |
20 |
60 |
80 |
F |
20 |
100 |
120 |
G |
40 |
0 |
40 |
H |
40 |
40 |
80 |
I |
40 |
80 |
120 |
J |
60 |
20 |
80 |
K |
60 |
60 |
120 |
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 will occur at green-up (around March 1).
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 3 replications was utilized with previous crop (corn vs soybean) as whole plots, tillage (no-till vs tilled) as subplots, and seeding rate X N management treatments as sub-subplots. The seeding rates were 20 and 40 seeds per square foot (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 dry matter accumulation and nitrogen concentration at flowering. Grain yields, moisture and test weights were taken at physiological maturity.
Effects of Previous Crop and Tillage
At DSAC, no-tilling (NT) after corn or tilling (CT) after soybeans produced slightly higher wheat yields than other systems (Table 2 and Figure 2). At DSAC these yield advantages corresponded to higher stand densities and head counts. If one assumes that the difference between plant stands and head counts is associated with tillering, then NT after corn and CT after soybeans produced the greater number of tillers. Stand counts were slightly lower for CT after corn than NT, and about the same for both tillage systems after soybeans. This could be related to differences in residue management. Perhaps burying all the corn residue left the subsurface unfavorable for wheat establishment. 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 plant growth (whole plant weights) and nitrogen concentrations at flowering indicate larger plants but lower N concentrations when the previous crop was soybean compared to corn. Higher N levels following corn could be an indication of nitrogen carryover from the previous corn crop. However, in general, plant nitrogen levels were low, an indication that nitrogen may have been limiting at this study site. Rainfall (Figure 1) data indicates a very large amount of precipitation for March at DSAC. This may have contributed to greater N losses and/or poor root growth (associated with poor photosynthesis from cloudy days) from which to take up N from the soil. Therefore, it appears that the higher N levels after corn may be from the breakdown of corn residue and release of N in late spring.
At BARC, tillage performed better than no-tillage with a previous crop of corn producing higher yields than a previous crop of soybeans (Table 3 and Figure 2). Head counts were significantly higher for a previous crop of corn than soybeans even though stand counts were slightly lower. This would be associated with much better tillering following corn. Greater tillering is often associated with better N supply to the plants, which could be due to higher nitrate levels in the ground after the corn crop. In our study, a modest amount of nitrogen was applied to the corn but yields were poor in 1996 due to low rainfall with every possibility that some of the nitrogen was not utilized by the corn.
Whole plant weights were not significantly affected by previous crop or tillage, but significantly higher N levels were obtained with a previous crop of corn. Again, this is an indication of excess N carryover following the corn crop. However, as indicated above, the additional N may be from the mineralization of corn residue in the spring. Soil nitrate levels will be monitored in future years of this study to get a better handle on nitrogen release and carryover potentials.
Effects of Nitrogen Rates and Timing
Nitrogen rate had a larger impact on yields at DSAC than did time of N application (Table 4 and Figure 3). In general, the highest wheat yields occurred at the 120 lb N/acre total N rate regardless of how the rate was split between fall and spring. The highest absolute yield occurred at the 120 spring N rate, and was reduced slightly as some of the N was applied in the fall. Higher plant stands were achieved where at least 20 lb of N was applied in the fall. Head counts increased as total N rates increased but increasing fall N rates had no effect. Whole plant N levels and plant heights all increased with increasing total N rates, whereas plant N levels and plant heights decreased with increasing fall N rates. Again, there is an indication that when fall N rates were increased, it was at the expense of spring N rates at equal total N rates. This indicates that while some fall N may be needed, it is used less efficiently than spring applied N and higher fall rates should not account for an equal reduction in spring N rate. Test weights and whole plant weights were only slightly affected by N treatments.
Wheat yield responses to N were highly variable at BARC (Table 5 and Figure 4). Increasing total N rates increased yields but increasing fall N rates had variable and non-significant results. The highest yield came when N was applied at a 20 lb/acre fall rate followed by a 100 lb/acre spring rate. There was a reduction in head counts and yields with the 40 and 60 lb N/acre fall rate followed by an 80 and 60 lb N/acre spring rate, respectively. This indicates that there may be a problem with higher fall N rates at this site. Increasing total N rates also increased plant heights and whole plant N levels, whereas, increasing fall N rates had little effect.
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 previous crop by N treatment effect on yield and a significant tillage by N treatment effect on head counts (Table 6). Check yields were much lower for wheat yields after soybeans compared to yields after corn indicating more N was available following corn than soybeans. There was an increase in yields as total N rates increased for each tillage system and previous crop, but in only one case (soybean, CT) did yields respond to fall N rates, and for the most part, this response was negative. In most cases at DSAC, increasing total N rate increased head counts but fall N rate had no effect.
As at DSAC, there was a significant previous crop by N treatment effect on wheat yields at BARC (Table 7). Again, check yields were much lower for a previous crop of soybeans compared to corn. Differences associated with fall or total N rates were small and or highly variable. There were no significant interaction effects on head counts.
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 no effect on yield, head counts, plant heights and test weights at DSAC (Table 8). Whole plants were slightly larger following soybeans but N levels were lower. There were higher plant stands following soybean than following corn and for NT as compared to CT. The higher N levels are associated with higher N availability following corn as discussed earlier.
Effects at BARC were similar to DSAC in that whole plants were slightly larger following soybeans but N levels were higher following corn (Table 9). There were no significant differences between previous crop and tillage effects on yield, head counts, stands densities, and test weights. The heights of the NT plants were slightly shorter than the CT plants.
Effects of Seeding Rate and N Treatments
Increasing the seeding rate from 20 to 40 seed per square foot significantly increased grain yields, head counts, test weights, and stand densities at DSAC (Table 10). Whole plant weights were slightly reduced as seeding rate increased. There was a highly significant seeding rate by N treatment interaction effect on grain yields (Figure 5). The 60 lb fall N + 60 lb spring N rate had the highest yield at 20 seed per square foot rate but the lowest yield at the 40 seed per square foot rate. Differences in head counts, stand densities and whole plant N levels do not account for this difference in yield response. The highest yield associated with the 40 seeding rate was from the 0 fall + 120 spring N rate. Again, other plant measurements do not account for this difference, but apparently the yield benefit is due to the higher N rate in the spring.
At BARC, increasing the seeding rate significantly increased yield, head counts, plant heights, and stand densities while decreasing whole plant weight and N level (Table 11). There was not a significant interaction between seeding rate and N treatment, but it is interesting to note that the N treatments producing the highest yields at the 20 and 40 seeding rates were the same N treatments that gave the highest yields at DSAC (Figure 5).
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 (Table 12). There is a highly significant tillage by seeding rate effect on head counts. There were higher head counts for the CT than the NT treatment at the 20 seed square foot rate. These differences were non-existent at the 40 seeding rate.
The only significant interaction at BARC was a seeding rate by previous crop effect on grain yields. At the 20 seeding rate wheat after soybeans had a higher yield than wheat after corn, but at the 40 seeding rate, there were no differences (Table 13).
With only one year's data available, definite conclusions are not available. Early indications are that more N may be available to wheat following corn than following soybeans. This could be due to greater carryover of unused corn fertilizer N and/or 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.
Planting wheat no-till at DSAC was as good or better than CT while at BARC, CT had a slight advantage. This may be a function of soil type with the Cisne soil at BARC having less slope for water runoff and poorer internal drainage making NT less desirable. If the BARC was in a long-term NT situation, internal drainage may be improved.
Increasing the seeding rate from 20 to 40 seed/square foot increased wheat grain yield, head counts and stand densities at both locations. Nitrogen treatments responded differently at the two different seeding rates.
Table 1. Experimental Conditions and Details, 1996-97.
Table 2. Previous crop and tillage effects on wheat at Dixon Springs (Study 1), 1997.
Table 3. Previous crop and tillage effects on wheat at Brownstown (Study 1), 1997.
Table 4. Nitrogen rate and application date effects on wheat at Dixon Springs (Study 1), 1997.
Table 5. Nitrogen rate and application date effects on wheat at Brownstown (Study 1), 1997.
Table 8. Previous crop and tillage effects on wheat at Dixon Springs (Study 2), 1997.
Table 9. Previous crop and tillage effects on wheat at Brownstown (Study 2), 1997.
Figure 1. Rainfall during 1996-97
Figure 2. Previous crop and tillage effects on wheat yields, 1997
Figure 3. Nitrogen Effects on Wheat Yields at Dixon Springs, 1997
Figure 4. Nitrogen Effects on Wheat Yields at Brownstown, 1997
Figure 5. Planting Rate and N Effects on Yields at DSAC and BARC, 1997
1 S.A. Ebelhar is an Agronomist, Dixon Springs Agricultural Center, University of Illinois; K.L. Barber is a Senior Research Specialist, Brownstown Agronomy Research Center, University of Illinois.
