Principal Investigators: Peter Scharf, Bill Johnson, and Jim Wait Agronomy Department, University of Missouri

Objective: Evaluate yield loss due to carryover of pH-sensitive herbicides at different soil pH levels in a no-till soybean-corn rotation.

Methods:

• Experiments conducted at Bradford Farm east of Columbia
• No-till soybean-corn rotation
• Five treatments were applied in December 1998 to set up a range of soil pH values:
  • 2 tons lime/ac
  • 1 ton lime/ac
  • untreated
  • 1 ton iron sulfate/ac
  • 2 tons iron sulfate/ac
• Salt pH values to a one-inch depth were measured for all plots in March 1999 and ranged from 5.5 to 7.1
• The experimental area was divided with corn grown on half the plot and soybean grown on the other half in 1999.
• Three pH-sensitive herbicide treatments were applied to each crop, as shown in Table 1, in April or May 1999.
• Roundup Ready crops were grown and weeds were controlled with postemergence Roundup applications so that there was no variation in weed control between plots.
• In 2000, the crops were rotated such that corn was grown on the soybean plots and vice versa to evaluate carryover of corn herbicides to soybean and soybean herbicide to corn. No herbicide treatments were applied in 2000 except for Roundup. Yield and moisture were measured at harvest.

Table 1. Herbicide Treatments applied in 1999.

Corn		Soybean
Herbicide	Rate (lb/ac)	Herbicide	Rate (lb/ac)
Atrazine	1.25	Pursuit	0.063
Atrazine	2.5	Scepter	0.125
Peak	0.018	Classic	0.04
Untreated		Untreated

Results for 2000:

• 2000 corn yields were not affected by pH or herbicide treatments
• 2000 soybean yields were affected by both pH and herbicide treatments
  • Yields were slightly lower for plots that had been treated with Peak or the high rate of atrazine in 1999 (Table 2)
  • Yields were slightly lower for limed plots than for untreated or acid-treated plots (Table 3); this effect seemed to occur mainly in plots receiving atrazine (Figure 1)
  • Plots receiving either rate of atrazine in 1999 yielded higher at medium pH levels than at low or high pH (Figure 1)
  • Plots receiving Peak in 1999 yielded lower than control plots across all pH levels (Figure 1)
  • No visible symptoms of herbicide damage were associated with these yield losses

Table 2. Effects of corn herbicide carryover on soybean yield averaged over soil pH.

Table 3. Effect of lime or acid treatments on soybean yield averaged over herbicide treatments.

Figure 1. 2000 no-till soybean yields as a function of salt pH of the top inch of soil in March 1999 for each of the four corn herbicide treatments applied in 1999. Statistically significant trends were seen when atrazine had been applied in 1999, but not for the Peak or untreated plots.

Conclusions:
The 1999-2000 season was favorable for herbicide carryover due to drought from July 1999 to May 2000. Herbicide breakdown is slower under dry soil conditions for many herbicides (though faster for some herbicides).

Soybean yield reductions were seen in 2000 as a result of corn herbicides applied in 1999. Peak lowered yield by 3 bu/acre regardless of soil pH or liming. Atrazine lowered yield mainly at high pH values (salt pH above 6.5, which roughly equals water pH above 7.0). Avoiding over-liming appears to be important for soybean profitability when atrazine is used in a no-till soybean-corn rotation. Small, frequent lime applications may be appropriate.

Corn yields in 2000 were not affected by soybean herbicides applied in 1999.

Preliminary Observations for 2001:

• We are continuing these experiments in 2001-2002.
• In spring 2001, additional lime and iron sulfate were applied to increase the range of soil pH values in the experiment.
• Herbicide treatments listed in Table 1 have been applied to the same plots again in 2001.
• Peak herbicide treatment was observed to cause early-season yellowing and stunting of corn in 2001.
• Yield effects of treatments on rotational crops will be measured in 2002.

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