Profitability of Variable-Rate N Application to Wheat Following Sugarbeet

  D.W. Franzen
Extension Soil Specialist,
North Dakota State University,
Fargo, ND

 Introduction

There have been two different approaches to site-specific farming in a sugarbeet rotation. One approach investigates the profitability of site-specific techniques on sugarbeet yield and quality while the other approach recognizes that although sugarbeet is regarded as a high-value crop, the cropping system that it is grown in is important as well. This study builds a base of information for N application to the 1999 spring wheat crop on information gathered in 1997 following spring wheat harvest and  information gathered in 1998 when the field was in sugarbeet. The ability to use information gathered on one crop to enhance profitability in the entire rotation should be one of the goals of site-specific fertilizer management in a sugarbeet rotation.

Methods

In 1997, following spring wheat harvest, a 40 acre field five miles west of Gardner, in Cass County, Gardner township, section 20, was sampled to 4 foot in depth in a 110 foot grid. The NO3-N map is shown in Figure 1. The patterns of nitrate are highest in the northeast and extending in a band to the south-central boundary of the field. An area of relative high NO3-N levels was also found in the northwest. Sugarbeet was grown in the field in 1998. The soil tests from the field following the 1998 harvest showed no spatial variation except in the northwest corner where the sugarbeet tare piles are discarded and along the south fence where sugarbeet yields were poor due to excess water early in the season (Figure 2).

Normally, a soil test from an field such as this would be fertilized uniformly because most of the soil test levels were low and relatively uniform. However, the satellite imagery from the 1998 sugarbeet crop revealed similar patterns of greener canopy cover in areas of the field that displayed higher N content the previous year (Figure 3). A previous crop credit similar to residual soil NO3-N levels in 1997 was used to modify the recommendations of 14 treatment blocks, each approximately 2.8 acres in size, and divided in half, north and south, so that within each block was a treatment of urea-N applied based on a variable-rate recommendation and one which used a composite soil test figure from the 1998 soil test (Figure 4). N rate reduction varied from 0-75 lb N/acre depending on the vigor rating and residual soil N in each of the four zones defined by imaging that would have been sampled in a commercial field. Each treatment was approximately 330 feet long and 180 feet wide. Harvest was made using a yield monitor.

Figure 1. NO3-N levels following 1997 spring wheat.

Figure 2. NO3-N levels, 1998, following sugarbeet. No spatial relationship.

Figure 3. Satellite imagery, 1998 sugarbeet crop, showing more vigorous areas similar to areas of high NO3-N levels following 1997 wheat crop.

 

Figure 4. 1999 N application map, by treatment. V=variable-rate, C= conventional rate.

 

Results and Discussion

Yields are shown in Figure 5. The yield in the south part of the field are from 15 to 20 bu/acre higher than the north part of the field. Whether this was due to a variety change, or a glitch in the combine sensor, or difference between days of harvest is not known. Fortunately, the higher yielding area encompass a three block area of the field with equal representation between treatments, so the effect of whatever the non-nutrient induced differences were had no effect on the outcome of the experiment. There were no significant differences between areas that were treated variably and those treated conventionally (Table 1.). This is what the expected consequences of the experiment were. Conventional N application directed by the low NO3-N levels following sugarbeets would naturally result in N levels that would not be limiting. A worst case scenario would have resulted in the reduced recommendations based on beet top vigor would have resulted in lower yields in the variable-rate, but this did not happen. Therefore it supports the reduction of N levels due to beet top vigor differences and resulted in a savings of fertilizer N, as well as a reduction in season ending soil NO3-N levels as shown in Table 1.

Figure 5. Spring wheat yields, Hunter, 1999.

Table 1. Mean yields and ending NO3-N levels by treatment.

  Treatment                            Yield                       Ending NO3-N

                                               Bu/acre                       lb/acre                                
Conventional                           44.0                            41.1                       
Variable-rate                            45.6                            32.0

Significance/LSD 5%             None                              9.0          

Table 2. Urea use for a forty acre field based on rates used by the conventional and variable-rate treatments.

Treatment                             Urea use

                                                 lbs                          
Conventional                            6087 
Variable-rate                             4944  

As a consequence of the variable-rate application, urea use was decreased from 6087 lbs had a conventional treatment been applied to the entire field to 4944 lbs if the variable-rate had been applied to the entire forty acres. Whether or not the savings in fertilizer would have paid for the four zone directed soil samples that needed to be taken from the field would depend on the cost of fertilizer and sampling/analysis costs. This would vary depending on the year and was not figured into the paper. The reader may decide what was the value of the variable-rate application depending on the costs, the environmental benefits and the value to other crops in the rotation. 

Conclusions

This study supports previous findings by Moraghan (1998), Franzen et al.(1999) and Reitmeier et al. (1999), that sugarbeet top consideration is an important component of N management following sugarbeets in a rotation. It supports the use of sugarbeet top vigor as a previous crop credit, resulting in a possible reduction of N to following crops with little risk of yield reductions. It further advances the environmental responsibility of N applications using a combination of site-specific techniques, remote imagery and soil testing as tools to reduce the effects of nutrients on ground and surface waters.

References

Franzen, D.W., L. Reitmeier, J.F. Giles, and A.C. Cattanach. 1999. Aerial photography and satellite imagery to detect deep soil N levels in potato and sugarbeet. p. 281-290. In Precision Agriculture, Proceedings of the 4th International Conference, 19-22 July, 1998, St. Paul MN. P. Robert et al., ed. ASA, CSSA, SSSA, Madison, WI.

Moraghan, J. 1999. A successful application of a new precision farming technique by a sugarbeet grower. p. 113-124. In 1998 Sugarbeet Research and Extension Reports. Vol. 29.

Reitmeier, L., D.W. Franzen, J.F. Giles, A.C. Cattanach, and N.R. Cattanach. 1999. Nitrogen management in a wheat/potato/sugarbeet crop rotation. p. 125-134. In 1998 Sugarbeet Research and Extension Reports. Vol. 29.

Acknowledgements

The author would like to thank Rick Bergum and Ola Anderson of Arthur Companies for their assistance in providing a site and managing crop planting, fertilizer application and harvest. Also thanks to Darren Johnson of Precision Partners for help in producing a fertilizer application map for the complex application on this farm. Thanks also to the Sugarbeet Research and Education Board of Minnesota and North Dakota for providing funding for this project.