Densities and Herbicide Rates for No-Till Drilled Soybeans

by S. A. Ebelhar, E. A. Adee, K. L. Barber, C. A. Bradley, S. E. Hart, R. A. Hines, L. E. Paul,
W. L. Pedersen, G. A. Raines, G. K. Roskamp and L. M. Wax

• Introduction
• Procedures
• Results
• Summary
• Acknowledgement
• Table 1

Introduction:

A field study was conducted at six locations in Illinois to evaluate the following objectives: 1) Compare adapted soybean varieties at three planting rates for days to canopy closure and evaluate weed control when no-till drilled into corn stubble; and 2) Evaluate the effects of 0.5x, 0.75x and 1x of recommended herbicide rates for a total Pre-emergence (PRE) and a total Post-emergence (POST) program, and a weedy check, on weed control and soybean yields. The locations included DeKalb and Monmouth in northern Illinois, Urbana (1995 and 1996 only) and Perry (1994 and 1995 only) in central Illinois, and Brownstown and Dixon Springs in southern Illinois.

Procedures:

At each location 3 adapted soybean varieties were combined with 3 planting densities designed to provide final plant stands of 140,000, 180,000 and 220,000 plants/acre. Varieties used at the northern and central locations included Pioneer brands 9273 (early) and 9342 (late) along with Asgrow 3237 (mid-season). Varieties at the two southern locations included Pioneer brands 9394 (early) and 9451 (mid) along with Asgrow 4715 (late).

Variety by planting rate combinations were randomized in strips and replicated 4 times. Across these 9 strips, 7 herbicide treatments were applied (in a strip-plot design or checkerboard pattern). The herbicide treatments included a check, 0.5 PRE, 0.75 PRE, 1.0 PRE, 0.5 Post, 0.75 Post and 1.0 Post. The 1.0 PRE consisted of 2 pints Dual II (2.5 pints were used in northern IL and Urbana) + 7 oz Canopy. The entire plot area had been treated with 1 qt Roundup + 2,4-D + NIS and Ammonium Sulfate approximately 10 days before planting. The 1.0 POST treatment consisted of 0.25 oz Classic + 0.25 oz Pinnacle + 8 oz Assure II + 0.25% v/v NIS + 2.5% v/v UAN.

Weed seeds were sown just prior to planting at 4 out of the 5 locations in 1995 (Perry was excluded) and all 5 locations participating in 1996. Weeds sown included common lambsquarters (Colq), redroot pigweed (Rrpw), giant foxtail (Gift) and velvetleaf (Vele). Data collected in 1995 and 1996 include final soybean plant stands, grain yields, and days to canopy closure as well as weed counts and ratings (for each of the weeds above plus any significant residual weeds). In 1994, grain yields, plant stands and weed control ratings were taken.

Results:

Plant Stands. Final plant stand varied by location and years, but on average were within a reasonable percentage (20%) of intended stand (Table 1 and Figure 1). Locations where stands were much lower than anticipated were usually affected by heavy rainfall in May and June especially immediately after planting. There were occasionally location/years which showed varietal differences but they were small and tended to not affect final yields. Four out of five locations had stand losses associated with check herbicide treatments (Figure 2). These effects are probably due to increased competition from weeds in 1995. Only one time and at one location did we observe a stand loss associated with the high POST rates. Overall, planting rate had a much larger effect on final stand than did variety or herbicide treatments.

Canopy Closure. In 1995 and 1996, canopy closure was measured as the number of days from planting until a canopy was formed over the middles of the rows. Five out of six locations showed planting rate effects on days to canopy closure averaged over 1994-95 (Figure 3 and Table 1). Planting rate differences varied due to differences in final stand but in general those locations with greater than 180,000 plants/acre canopied 3-7 days sooner than the lowest stands at the same location. Growing conditions and varieties also factored into canopy development (data not shown). Check treatments canopied soonest at each location. In general, PRE herbicide treatments canopied 2-4 days earlier than POST. High rates of POST delayed canopy closure only at locations with poor stands due to weather problems.

Weed Control. Gift, pigweeds (a mixed bag including waterhemp), Colq, and Ilmg were the predominant weed species present at most of the locations in most years. An average of all weed control ratings across years was determined for each location (Figure 4) and an average was determined across locations (Table 1). For the most part, planting rate and variety effects on weed control were insignificant.

In general across locations and years, POST treatments did better than PRE for Gift control, PRE equaled POST for pigweed and Ilmg control except for Dixon Springs in 96 where PRE did worse for Ilmg control, and the POST did worse than PRE for Colq control. At nearly every location, PRE and POST herbicide applications, even reduced rates, provided significantly better weed control than where no herbicides were applied. For most of the weeds in these plots, the 0.5 POST treatment provided as good of weed control as any other treatment. Many times however, the 0.5 PRE treatment faired worse than the 1.0 PRE treatment.

Grain Yields. All six locations increased yields with increasing seeding rates, however on average yield increases were only one to three bushels per acre (Table 1). Final plant stands were highly correlated with yield at each location (Figure 5). Optimal yields occurred at about 160,000 plants/ acre final stand. Variety differences were even less on average than planting rate differences.

The check treatments had the lowest yields at every location (Figure 6). The checks yielded about 10 bu/acre less than the herbicide treated plots (Table 1). Averaged across all locations, PRE and POST yielded about the same.

Summary:

• Drilling adapted soybean varieties at a seeding rate to deliver final stands above 160,000 should provide for maximum yields and the quickest canopy closure.
• In our study, density effects on canopy closure helped in weed control, but differences in canopy closure of 3-7 days sooner for the high plant densities, were small compared to the effects of PRE and POST herbicides on weed control.
• At nearly every location, PRE and POST herbicide applications, even reduced rates, provided significantly better weed control than where no herbicides were applied. This accounted for a 10 bushel increase in soybean yields when averaged across the six locations.
• For most of the weeds in these plots, the 0.5 POST treatment provided as good of weed control as any other treatment. Many times however, the 0.5 PRE treatment faired worse than the 1.0 PRE treatment.
• There is an indication that if the farmer has a good knowledge of weeds in his fields, he may be able to use lower herbicide rates with no-till drilled soybean.

Acknowledgment:

The authors wish to acknowledge the Illinois Soybean Operating Board for financial support for this project. They also wish to extend thanks to the workers at all of the research locations who worked in cooperation on this project.

Table 1. Effect of planting rate and herbicide treatments on soybean final plant stands, days to canopy closure, average percent weed control, and grain yields averaged across locations and years.