Illinois Fertilizer
Conference Proceedings
January 22-24, 2001
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S. A. Ebelhar, K. L. Barber, and A. H. Anderson1
Utilizing a corn-wheat-soybean rotation allows three grain crops to be grown in two years and could increase the profitability of farms and allow for better flexibility on crop acres. Wheat after corn may be important environmentally because wheat would trap excess soil nitrate from corn when it does not utilize all of the applied N fertilizer, such as after a drought.
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 also be different than under soybean residue.
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. 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.
The objectives of our study are indicated below:
To evaluate N management practices for wheat after corn compared to wheat after soybean.
To determine effects of N rates, timing of N fertilization, and tillage on grain yields and test weights of wheat after corn and 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.
To determine the effects of N management and previous crop on tillering of wheat grown at two different plant densities.
Field studies were conducted from the fall of 1996 through 1999 at two locations—the Dixon Springs Agricultural Center (DS) and the Brownstown Agronomy Research Center (BR). The soil type at Dixon Springs was a Grantsburg silt loam and at Brownstown was a Cisne silt loam. Specifics on planting dates, varieties, nitrogen application dates, etc. are described in previous proceedings (see the 1998, 1999 and 2000 Illinois Fertilizer Conference Proceedings).
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. Total N rates ranged from zero to 160 lb/acre, and timings included fall N rates of 0, 20, 40 and 60 lb N/acre in combination with spring N rates of 0, 40, 80, and 120 lb N/acre. The tillage treatment was accomplished with the use of a field cultivator and cultipacker at Brownstown and a rotovator plus cultipacker at Dixon Springs.
The nitrogen source for this study and the one below was ammonium nitrate broadcast. Wheat was drilled at a seeding rate of 30 seed/square foot (approximately 90 lbs/acre).
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 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 included fall rates as above plus spring split applications between Feekes GS 3.0 and 5.0, for a total N applied of 120 lb N/acre.
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 among 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.
On average, tilling (CT) after soybeans produced higher wheat yields than after corn or no-tilling (NT) (Table 1). About half the time, there was a slight advantage for wheat after soybeans, and the other half of the time, there was no difference. When summed over the six site-years of study (two locations for three years), the yield advantage for wheat after soybeans was about 1.5 bu/acre. Tillage was significant only at Brownstown (BR), but NT outperformed CT two out of three years. Overall, there was only a 2 bu/acre yield advantage for CT, primarily due to the large tillage difference at BR in 1997. This difference in 1997 at BR was primarily due to excessive rainfall, which caused standing water on the NT areas and resulted in poorer stands and/or poor growth.
Head counts were not significantly different between previous crops of corn and soybeans or between tillage systems (data not shown).
There were few significant effects of previous crop and tillage on wheat test weights (Table 2). In 1999 at DS, there was a slight increase in test weights after soybean, and in 1997 at DS, there was a slight advantage for CT over NT, but when averaged over the six site-years of study, there were no significant differences among tillage system or among previous crop for test weights.
Previous crop and tillage also had little effect on wheat stand counts (Table 3). As with test weights, there were a few significant differences for a particular year and location, but overall there were no significant differences among tillage system or among previous crop for stands.
Overall, there were slightly higher whole plant N concentrations following corn, but there were no differences among tillage systems (Table 4). Most of the advantage following corn came after low-yielding corn crops, which did not adequately utilize fertilizer N, causing some of the N to carry over to the wheat crop. In areas where corn yields were better, less N carried over, which then gave the advantage to soybeans as a previous crop or there was no advantage to previous crop. Nitrogen losses from denitrification and N gains from mineralization further add to the confusion. In 1997, some the fertilizer N was lost, especially at BR, as indicated by warm and wet conditions, especially in March (Figure 1 and Figure 3). This may also explain the lower yields with NT, although the N levels at flowering don't indicate that N is limiting. In 1998 at DS and 1999 at BR, higher N levels appear to indicate higher N mineralization.
Fall N rate appeared to have little impact on wheat grain yields regardless of planting rates (Table 5). Fall plus spring N rates had large impact on yields at both locations nearly every year. In 1997, wheat yields increased with increasing N rates up to 120 lb/acre at DS and up to 80 lb/acre at BR (Figure 5). In 1998, yields were very different (Figure 6). At DS, yields increased as N rates increased up to 60 lb/acre and then decreased with increasing N rates. At BR, yields increased up to 80 lb N/acre and then leveled out. The differences between years is due to weather. In 1997, high rainfall and temperatures led to high N losses, especially at DS, which led to the positive effect of N rates on yield. In 1998, mineralization combined with little N loss created a situation of excess N availability, which caused lodging problems and yield losses. In 1999, yields increased with increasing N rates to 60 and 120 lb/acre for BR and DS, respectively (Figure 7). Weather effects on N response played a more important role in determining wheat yields than previous crop or tillage.
Neither fall nor spring N rates affected plant stands, head counts, or test weights regardless of planting rate. Whole plant N levels all increased with increasing fall, spring, or total N rates as expected (data not shown).
The effect of spring splitting N rates between Feekes Growth Stage (GS) 3.0 and GS 5.0 are shown in Table 6. At no time were there any significant differences among all of the spring N applied at GS 3.0, at GS 5.0, or split with 50 percent at GS 3.0 and 50 percent at GS 5.0. For test weights, two out of five site-years responded to split N treatments. In these two cases, test weights decreased with delaying all or a portion of the N to GS 5.0.
There were few significant interaction effects of previous crop, tillage and N treatments at DS and BR (data not shown). In general, the responses to fertilizer N were independent of previous crop or tillage. When averaged over the six site-years, there was a 6.3 bu/acre yield advantage for wheat after soybean over wheat after corn at the economic optimum N rate for each previous crop (Figure 8). The economic optimum N rate for wheat after soybean was 91 lb N/acre (based on $0.25/lb of fertilizer N and $2.50/bu wheat price). The optimum N rate for wheat after corn was 62 lb/acre, reflecting a lower demand for N because of lower yields and more N availability after poor-yielding corn crops in some years. The higher N needs of wheat after soybean are more than offset by the higher yield. Optimum N rates based on tillage were basically the same.
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 5). Increasing fall N rates failed to increase yields even at the 20 seed/sq ft seeding rate. Differences in head counts were highly significant, with the 40 seed/sq ft seeding rate having more heads than the 20 seed/sq ft seeding rate, but differences were small. Test weights were unaffected by seeding rates.
There were very few significant interactions between previous crop, tillage, seeding rate, and N treatment effects on grain yields and head counts at DS and BR (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.
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.
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. 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 the proper N rate, taking into account N losses and net N gains to the system. Tillage had little effect on N recovery from previous crops of either corn or soybean.
Planting wheat no-till at DS and BR was as good as or better than CT, but there may still be a problem with the Cisne soil at BR 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/square foot had little effect on yields and test weights. In general, increasing seeding rates increased head counts and stand densities at both locations.
There was still no advantage to spring splitting N rates between Feekes 3.0 and Feekes 5.0.
Table 5. Effect of fall N rate and seeding rate on wheat (averaged across locations and years).
Table 6. Split-spring N treatment effects on wheat yields at DS and BR.
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.
Figure 5. Nitrogen Effects on Wheat Yields, DSAC and BARC, 1997.
Figure 6. Nitrogen Effects on Wheat Yields, DSAC and BARC, 1998.
Figure 7. Nitrogen Effects on Wheat Yields, DSAC and BARC, 1999.
| Footnotes and References |
1S. A. Ebelhar is Agronomist, Dept. of Crop Sciences, Univ. of Illinois; K. L. Barber is Agronomist, Golden Harvest; and A. H. Anderson is Research Specialist, Dept. of Crop Sciences, University of Illinois.
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.) 1998 Illinois Fertilizer Conference Proceedings, pp 73–96.
