Exercise 4 - Minnesota: Simulate a Cover Crop and Windbreak

Skill Building: This exercise will start with a template management and add a spring barley cover crop to protect the sugar beet the first 40 days of growth. WEPS (version 1) can only grow one crop at a time for this exercise so you need to know how to build a work around for these special management rotations. We will also explore the effects of a windbreak.

Scenario: The farm is located in Clay County, Minnesota. The CLIGEN station is FARGO AIRPORT and the WINDGEN station is FARGO/HECTOR FIELD. The critical dominant soil in the field is 426 Foldahl loamy fine sand (13.4% of the field and the most erodible by wind) (Figure 1.48). The field is 80 acres (1320 ft north and south and 2640 ft east and west). There is a single row of elm trees on the north side of the field and the leaves are on the trees during the period of erosive winds in the spring. The trees are 25 feet tall and about 20 feet in width. The porosity of the windbreak is about 50% during the critical wind period.

The crop rotation is Spring Wheat-Sugarbeets-Soybean. There is a problem with the sugarbeets just after planting. One out of three years that sugarbeets are planted, they have to be replanted because of abrasion of the young beet plants by blowing soil. We will call the rotation: S Wheat-Beet-SB CMZ 1.man. Crop yields are: Spring Wheat, 45 bu/ac; Sugarbeets, 20 tons/ac; and Soybean, 40 bu/ac.

Getting Started

Step 1: Make sure you have the Student project loaded. Enter all the above information but without the wind break on the main WEPS interface. Click the Template Folder button next to the 'Man' button and load the S Wheat-Beet-SB CMZ 1.man file. Open MCREW and click the Yield Calibrate button and make sure that the yields match the inventory values above. Also change all tillage directions to 90 degrees (parallel to the long dimension). See Figure 1.48 .

Step 2: Make a Calibration Run and call it: S Wheat-Beet-SB CMZ 1 cal. Use the Biomass Adjustment Factors in the Project. Notice all of them are more than 0.05 above 1.0 (Figure 1. 49).

Notice that the soil loss is 9.6 ton/ac, and that the yields are close to the ones mentioned in the inventory (Figure 1.50 ). The producer has had trouble with the sugarbeet being damaged by blowing soil just after planting. Look on the detailed report and find the soil loss for the period just after the beets are planted.

What is the sum of soil lost just after planting the beets?

Answer: The first three periods sum to 1.0 ton/ac (Figure 1.51).

The NRCS National Agronomy Manual, Crop Tolerance to Blowing Table lists crop sensitivity to wind erosion after planting. Sugarbeets have a 0.0 - 0.5 ton/ac tolerance (See Table 1.? in the Exercises Introduction).

Is this run OK?

Answer: No, it is too high because of the sensitivity of sugarbeets to abrasion.

Run the Model with a Windbreak

See if the windbreak will help the abrasion problem on sugarbeets just after planting.

Step 1:

There is a one row windbreak on the north side of the field. In the 'Wind Barrier Interface' panel on the left side of the main interface, click the radio button next to the 'N' to place a barrier on the north side (N) of the field (see Figure 1.52). Notice that a yellow bar appears on the north side of the simulation area.

The trees for this windbreak are given as 25 ft tall, 20 ft wide, and have porosity fraction in April of 0.50 (50%). Click on the down arrow next to the barrier in the 'Wind Barrier Information' panel. Select the record labeled 'Tree, leaf on (1 row)'. Notice that the barrier bar turns red indicating that the barrier type has been selected (Figure 1.52). Then click the 'Edit Selected Barrier' button . Enter the information from the onsite inventory and click 'Save'. Notice the barrier type has '<mod>' added in front of the name indicating the barrier properties have been modified by the user.

Step 2: Make the run by clicking the 'Run' button on the main tool bar and call the run S Wheat-Beet-SB, windbreak CMZ 1.

Note: On the run summary, the windbreak reduced soil loss 5.4 ton/acre (9.6 - 4.2 tons/acre) (Figures 1.53 and 1.50). If we multiply the 5.4 ton/acre by 80 acres in the field we see that the barrier saved about 432 tons of soil per year on a field basis. Also notice that the net soil loss (4.1 ton/ac) is lower than the gross loss (4.2 ton/ac). This is because soil was deposited in front of the windbreak before leaving the field during storms with a southerly wind.

What was the savings for the sugarbeets after planting on the detailed report?

Answer: Soil loss went down from 1.0 ton/ac to 0.7 ton/acre (Figure 1.54). This is helpful, but let's see what a spring barley cover crop can do. An example of a barley cover crop after spraying and planting to winter wheat is shown in Figure 1.55

Add the Spring Barley Cover crop

The producer told us that the sugarbeets blow out 1 out of 3 years. We need to consider an alternative that can keep them from being killed by blowing soil. Adding a cover crop (spring barley) that grows simultaneous with a crop (sugarbeets) requires special crop files and special operations. These will require the NRCS user to contact the National Wind Erosion Specialist to develop these files.

WEPS can only grow one crop at a time. In this case we will model the barley growth for the first 40 days, and then switch to the sugarbeets that are growing under the cover of the spring barley.

Step 1: On the main interface window, click the 'Man' button to open the Management Editor, and make sure you have the original management file loaded: S Wheat-Beet-SB CMZ 1.man. Click 'File', 'Save as...' and rename the file S Wheat-Beet, cover crop-SB, CMZ1.man.

On the 'May 1, 02' line right click the Operation Name column and add two operations. The first operation will be the 'Seeder-Broadcast' and the second will be the 'Harrow, spike tooth, cover seed' operation. The seeder operation requires a crop name in the next column. Select Barley, spring covercrop. This file is a standard barley file that will grow about 300 lbs/ac biomass during the 40 days it will be in the field. Both the seeder and harrow need the same date 'Apr 30, 02'.

Now, right click the operation cell on 'May 01, 02' and change the Planter, small veg seed to Planter, into growing cover. This is a special planter that shows tilled ground on the day the planter was used, but does not call in a growing crop of sugarbeets yet. You have planted them in the field on that day but for now we want to continue to grow only the spring barley cover crop until the sugarbeets produce a dominant amount of vegetation.

On the next line (Oct 09, 02) in the operation cell, right click and add an operation, Sprayer, kill cover in growing crop. Make the date 'Jun 10, 02', 40 days after planting the cover crop. In the next column over, click the pull down arrow and find and select, Sugarbeet, growing after cover crop kill. This is a modified Sugarbeet file that will show some biomass growth in the roots and leaves on the date the beets were planted.

Note: This type of logic will be necessary when a growing cover crop is planted and then killed after both crops grow for a time. We can use the same logic if a cover such as rye is seeded and started to grow in a growing crop.

Click the Yield Calibrate button and make sure the barley cover crop line is unchecked because this is a forage crop and we want WEPS to control the yield growth. The Biomass Adjustment Factors must be the same as the first run. They are Wheat, 1.3; Sugarbeets, 1.075; and Soybeans, 1.150 (Figure 1.56). We will not calibrate the cover crop since it will be killed before planting the sugarbeets.

Now click the 'Return' button to return the main interface.

Now click the 'Return' button to return the main interface.

Step 2: Make sure you uncheck the radio button for the windbreak by selecting barrier type 'none' by using the drill down arrow . Click 'Calibrate Run' button to make a Calibration WEPS Run, call it S Wheat-Beets, cover crop-SB CMZ 1 cal.

Note: The biomass adjustment factor for the beets changed from 1.075 to 1.236. This means that WEPS needed to make additional adjustment to produce the 20 ton/ac yield. We know that this is possible in the field, however there may be times we do not know what effect growing the barley may have on the crop. If moisture is short, there may be a yield reduction. Click "Use in the Current Project" to save the new factors.

Without the windbreak on this sandy site, the cover crop (40 days) does little to reduce the overall erosion (Figure 1.57). We started with 9.6 tons/ac and only reduced it to 7.0 tons/acre. This is still too high and we would suggest additional runs with less tillage to find a system where the rate is closer to the desired 5 ton/acre rate.

Note: The spring barley cover has reduced the wind erosion from 1.0 to 0.5 for the sugarbeet crop (Figure 1.58). This can save about $170/ac in reseeding cost and loss of yield potential.

Now add back the windbreak and see the effects a windbreak and cover crop.

With the windbreaks and the cover crop on these sandy soils, WEPS indicates there continues to be a problem (Figure 1.59). However, net soil loss of 4.4 tons/acre/year is below the T value of 5.0 tons/acre/year. To further reduce soil loss and provide protection for emerging crops an alternative could be a rye or winter wheat cover crop after harvest of the sugarbeets.