S. A. Ebelhar, C. D. Hart, F. Fernandez and J. D. Hernandez 1
With just over one million acres in the state, mostly in southern Illinois, wheat remains the third largest grain crop in Illinois. The state average wheat yield is approximately 60 bu/acre. However, recent work at Southern Illinois Univ. and the Univ. of Illinois suggests that the use of intensive wheat management could increase wheat yields by 20-40%. Little has been done in recent years to determine if intensive wheat management systems require higher soil/fertilizer levels of phosphorus (P) and potassium (K). Wheat has a very high phosphorus need and certainly one would expect a 100 bu wheat crop would require higher P levels than 60 bu wheat.
With an apparent higher need for fertilizer P, intensive wheat areas that are fertilized with DAP would receive higher amounts of fall applied N. There is uncertainty that this higher fall N is used as efficiently as spring applied N. In southern Illinois, double-cropping with soybeans makes the wheat-soybean system economical. Farmers who fertilize the wheat crop for both wheat and double-crop soybeans would apply even higher amounts of DAP and potash in the fall, increasing the fall N further. Work needs to be done to help these farmers optimize their N, P and K management of wheat and double-crop soybeans.
The objectives of this study are to 1) determine optimum nitrogen, phosphorus and potassium fertilization of high yield intensive wheat with double-crop soybeans, 2) evaluate fertilizer application to wheat for both wheat and double-crop soybeans compared to applying the fertilizer to wheat and soybeans separately, and 3) determine if nitrogen from fall applied DAP is available for wheat uptake similar to spring applied nitrogen. The goal is to optimize fertilizer use in intensive wheat and double-crop soybean rotations in southern Illinois.
While the project began with wheat planting in the fall of 2007, there was an opportunity to place some of the double-crop soybean fertilizer treatments out in 2007. Two locations, the University of Illinois (UI) Dixon Springs Agricultural Center (DSAC) and the UI Brownstown Agronomy Research Center (BARC), each had wheat plots with N rates and timing of N application variables (see Table 1) for which this study could be superimposed. P and K rates were applied to previous wheat management treatments in a factorial arrangement with four replications per location. The P and K rates fell into 3 categories – low, medium and high rates (Table 2).
Fertilizer P was applied as either triple-super phosphate (TSP, 0-46-0) or as diammonium phosphate (DAP, 18-46-0). Soybeans were drilled in 7.5" rows shortly after wheat harvest. P and K fertilizer treatments were surface applied broadcast at time of soybean planting.
A late spring freeze at DSAC had a significant impact on wheat yields which were about half the yields obtained at BARC (Table 3). Wheat yields at both locations topped out at about the 90 lb/acre N rate and were significantly higher at both locations when N was applied in March rather than February. At DSAC the performance of the wheat had a significant effect on double-crop soybean yields. The higher wheat yields with increasing N rates led to reduced soybean yields. This may be related to higher residue levels associated with higher wheat yields reducing soybean emergence and/or slowing early growth. Wheat yields at DSAC were 13 bu/acre higher with the March N application compared to February, but soybean yields were 3 bu/acre less. This is another indication that higher wheat yields may be creating conditions which reduce soybean yields. Soybean yields at BARC were much lower than DSAC due to later planting and dry conditions, and there were no effects of previous wheat N rate or time of N application.
Fertilizer P and K rates had no effect on soybean yields at DSAC, but lowered the yields at BARC by about 1.5 bu/acre (Table 4). There was no significant difference between using either TSP or DAP fertilizer P sources at either location. No interactions were observed between P sources and P rates.
It is too early to make any definitive conclusions, but it does appear that high wheat yields may be associated with lower double-crop soybean yields, but whether there are management options to reduce this effect are yet to be determined. This project should continue for several more years.
Table 1. Site information for each location, 2007.
Table 2. P and K rates for double-crop soybeans after wheat at each location, 2007.
Table 4. Effects of P and K rates and P source on double-crop soybean yields, 2007.
1S. A. Ebelhar is an agronomist and C. D. Hart is a research specialist, Dept. of Crop Sciences, University of Illinois, Dixon Springs. F. Fernandez is an assistant professor, Dept. of Crop Sciences, University of Illinois, Urbana.
J. D. Hernandez is an assistant professor, Southern Illinois University, Carbondale.