1.

ALS-Tolerant Johnsongrass in Kentucky

J. D. Green, W. W. Witt, Plant and Soil Science

David Herbst, Adair County Extension

Fields in Question

Poor johnsongrass control was observed in two corn fields; one located in Marion county and another in Adair county.  Both fields were in continuous corn production for at least five years. The field in Adair county was frequently used for corn silage production.  Herbicides containing nicosulfuron were applied each year for johnsongrass control.   

Greenhouse study:  Johnsongrass seeds collected from problem fields in Marion county (2004) and Adair county (2005) along with seed from a population known to be susceptible to Accent were planted in the greenhouse.  Seedling johnsongrass (8 to 12 inches tall) from Adair County and Marion County was not controlled when Accent was applied at 1x, 5x, or 10x of the normal application rate of 0.67 oz/A (Figure 1).  Fresh weight or dry weight (data not shown) of treated johnsongrass plants were not reduced even at the 10x rate with plants grown from seed collected from these two sites.  Whereas, plant biomass was reduced by 70% or more with plants from a susceptible population of johnsongrass, even when treated with the lowest rate of Accent (0.67 oz/A). 

·    Two johnsongrass populations are tolerating normal rates of ALS-inhibiting herbicides.

·    ALS-inhibiting herbicides include Accent, Beacon, Option and Lightning.

·    Relying on the same mode of action increases the risk of resistance.

Figure 1.  Johnsongrass response to Accent (nicosulfuron) herbicide in greenhouse studies. Less fresh weight means better control with Accent.

Field study:  Johnsongrass control at the Adair field site was poor with all ALS-type herbicides (Figure 2).  Little or no control of johnsongrass was observed with Accent (nicosulfuron) and Option (foramsulfuron) at both the 1x and 2x application rates and Beacon (primisulfuron) at the 1x rate.  Beacon at the 2x rate and other ALS-herbicides such as Steadfast (nicosulfuron + rimsulfuron) and Lightning (imazethapyr + imazapyr) provided 35 to 45% visual control. 

The three non ALS-herbicides (Select, Assure II and Roundup) provided good control (80% or more visual control of treated johnsongrass).  The exception was 68% control with Assure II when applied at the 1x rate.  Best results (90% control) were achieved with the glyphosate (i.e. Roundup) treatment.

Figure 2. Field study in Adair County to evaluate Johnsongrass response to herbicides sprayed in the field. The 1x rates are listed beneath each herbicide. 100% = all dead plants; 0% = no dead plants.

Johnsongrass Summary

Our results indicate that at least two johnsongrass populations in Kentucky were not effectively controlled by nicosulfuron, or other ALS-inhibitor herbicides.  Other herbicide products used for johnsongrass control or suppression which have ALS modes of activity include Beacon, Equip, Exceed, Spirit, Steadfast, Option, and Lightning.   Therefore, caution should be used to avoid year to year dependence on these herbicide products for johnsongrass control in corn.

Since the effect on succeeding generations have not yet been conducted to confirm resistance, these findings are not fully conclusive that johnsongrass populations are resistant to nicosulfuron (Accent) or other ALS-herbicides.  However, we do know that these two populations were not controlled by nicosulfuron in greenhouse evaluations nor in the field in 2006.  Clearly, these are strong indications of resistance.

Management of this problem in corn will require planting corn hybrids which have tolerance to glyphosate (eg. Roundup, Glyphomax, Touchdown, etc.). Management of this problem in soybeans can include Assure II, Fusion, Select, or glyphosate (i.e. Roundup Ready soybeans).  Weed management programs with non-ALS herbicides will be required for two to three years to reduce johnsongrass populations in these fields. Crop producers should always alternate herbicide chemistry to help prevent or minimize future herbicide resistant problems.

At this time we do not know the full extent at which poor johnsongrass control is occurring throughout Kentucky following the use of ALS-type herbicides.  We are interested in knowing about other fields in which johnsongrass was not effectively controlled by Accent, Beacon, Equip, Exceed, Spirit, Steadfast, Option, Lightning, or other ALS-inhibiting herbicides.  The development of widespread resistance to these herbicides by johnsongrass (or other weeds) would have an enormous impact on corn production.  Please notify us through your local county Extension office if you are aware of any fields in which lack of johnsongrass control could be attributed to continuous use of the above mentioned herbicides.

More information about the findings from this study are printed in WS 07-2 and are available at the Grain Crops Extension website:  http://www.uky.edu/Ag/GrainCrops/.

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2.

Planting More Corn?  Use Soil Testing to Choose Your Fields Wisely

Greg Schwab, Plant and Soil Sciences

Many Kentucky farmers are planning to plant more corn this year, due to higher prices.  Looking at their economic situation, it is easy to see that some of this new acreage will likely come from fields currently in forage production.  How much additional fertilizer will these fields require to grow corn? The answer depends on how well the forages were managed. Keep in mind, nitrogen requirements for corn following legume forages is 25-50 lbs N/acre less than when corn follows other grain crops, so part of the additional fertilizer expense will be offset by lower N requirements. 

Soil samples where forages were the intended crop provide some insight into the expected fertilizer costs for switching to corn. Approximately 22,500 soil samples where forages were the intended crop were submitted to the UK Soil Testing Labs over the past three years.  Recommendations from those soil test results were recalculated assuming corn would be the new intended crop. Fertilizer prices of $0.22/lb P₂O₅, $0.22/lb K₂O, $1.20/lb Zn, and $12/ton lime were used to calculate fertilizer cost (nitrogen cost is not factored into this analysis). 

Results show that the cost ranged from $0 to $146/acre, with an average of $22 to $45/acre depending on the region. The Bluegrass had the lowest average cost while the Western Coalfields had the highest average cost.  In all of the regions, there were a high percentage of samples requiring more than $50/acre of fertilizer. 

By soil testing early this spring, corn producers can identify fields that have a relatively low need for additional fertilization, thus minimizing input costs.  In fact, if fields are chosen correctly, corn following forages could be significantly cheaper than corn following grain crops because of the lower N requirements.  Alternatively, if corn is going to be grown on some of these very poor testing soils, then farmers must realize that a significant investment in fertilizer is needed to avoid yield limiting soil pH or nutrient deficiencies.   

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·    If switching from forages to corn, then check soil fertility levels.

·    Fertilizer requirements could cost $50/acre.

3.

Better Soil as Important as Fertilizer N When Growing Corn After Grass/Legume Forages

John H. Grove, Plant and Soil Sciences

High corn prices are causing cattle producers to consider growing more of their feed grain needs in their pasture and hay fields. In Kentucky’s environment, plant available water (soil moisture) and nitrogen are the two most corn yield-limiting factors and both need to be considered to maximize corn’s profit potential. Pasture and hay production are often located in fields not thought to contain “good corn soils”, but even these will vary in their corn yield potential.

UK’s fertilizer N rate recommendations consider soil drainage, tillage, fertilizer N timing, nitrification inhibitor use, and previous crop. If forage is a previous crop, then recommended N rates can be reduced by 25 to 50 lb N/acre. Figure 1 illustrates some of our data averaged from 2004 and 2005 production years. No-till corn was grown after corn, wheat/doublecrop soybean, and grass/clover hay on a Maury silt loam. Ammonium nitrate was applied at different rates just after planting.

Yield response to fertilizer N was determined for corn in each crop rotation. Maximum fertilizer N rates were determined at the point where no further yield response to fertilizer N occurred. On a Maury silt loam in 2004 and 2005, fertilizer N rates for corn after a grain crop were within range of the UK fertilizer N rates recommended. Maximum fertilizer N rates for corn after grass/clover hay fell well below that range. The “yield penalty” to corn after corn (the rotation effect) was 10 bu/acre when compared to first-year corn after grass/clover hay. This penalty was independent of the yield response to fertilizer N rate.

·    Corn after forages: reduce N rates by 25 to 50 lbs/acre

·    Poor soils with low yield potential: N rates above 40 lb/acre, less likely to make a profit.

The data in Figure 1, for only two years, was combined with earlier data, from the mid-1990’s, to give 5 years of N response information for corn after corn or corn after grass/clover hay (Table 1). The corn after grass/clover hay data were separated by soil. The better soil area, which provides more plant available water to the crop, was a mixture of deep Maury and Huntington series, while the poorer soil area was a mixture of McAfee and shallow Maury soils.

Table 1 gives the average corn yield response to fertilizer N, the average increment in corn yield with each additional 40 lb N/acre, and the increment ratio (bu/lb N). If fertilizer N is worth $0.45/lb, and corn is $3.75/bu, then the increment ratio must be greater than 0.12 bu/lb N for that increment of 40 lb N/acre to be profitable. When corn was grown after corn, fertilizer N was usually profitable (4 out of 5 years) up to 120 lb N/acre. The profitability of 160 or 200 lb N/acre was a coin toss (50%). Corn after grass/clover hay was usually responsive (3 out of 5 years) to 40 lb N/acre, r