PDF version
for printing

Impact
Statement

© 2007 Plant Management Network.
Accepted for publication 26 October 2006. Published 1 February 2007.

Evaluating Glyphosate Treatments on Roundup Ready Alfalfa for Crop Injury and Feed Quality

Lawrence E. Steckel and Robert M. Hayes, Department of Plant Sciences, University of Tennessee, Jackson 38301; Robert F. Montgomery, Monsanto Company, Union City, TN 38261; and Thomas C. Mueller, Department of Plant Sciences, University of Tennessee, Knoxville 37996

Corresponding author: Thomas C. Mueller. tmueller@utk.edu

Steckel, L. E., Hayes, R. M., Montgomery, R. F., and Mueller, T. C. 2007. Evaluating glyphosate treatments on roundup ready alfalfa for crop injury and feed quality. Online. Forage and Grazinglands doi:10.1094/FG-2007-0201-01-RS.

Abstract

Weed control is one of the factors that impact alfalfa producers, with negative effects on quality often in the year of establishment. Glyphosate is a broad-spectrum herbicide that controls many troublesome annual and perennial weeds, and new cultivars that are tolerant of glyphosate application have been developed. The crop response of glyphosate on these new varieties has not been reported. This research examined alfalfa tolerance under field conditions, and high rates were used to challenge the plants to determine the level of safety. Postemergent glyphosate treatments ranging from 0.75 to 3 lb a.e./acre sprayed before each of four alfalfa harvests had no meaningful crop injury in the establishment year or in the subsequent two years. The high glyphosate dose of 9 lb a.e./acre over a 3-year period caused no reduction in alfalfa yield or nutritive value at any cutting in any of the three years.

Alfalfa Weed Control

Companion crops and weeds compete with alfalfa during establishment and affect forage yield, feed composition and quality, and ultimately net income (3). Two major periods when weeds are the greatest concern in alfalfa are during seedling establishment and as the stand thins near the end of the production cycle. Weed control options during seedling establishment include 2,4-DB (4), imazethapyr (7), and others (5). A small grain companion crop is sometimes used to facilitate alfalfa establishment (7). Forage quality is usually highest in weed-free stands (7,11,15). Once alfalfa is established, other herbicides may be used, such as paraquat applied to dormant alfalfa and between cuttings (6,9), imazethapyr or a postemergent grass herbicide (2,13), or residual herbicides such as pronamide and metribuzin, depending on the production system used (8,12). Glyphosate is labeled for application to dormant alfalfa stands that are not glyphosate tolerant. But if the glyphosate application was delayed until after spring growth had resumed, injury was observed (14). Glyphosate-tolerant (Roundup Ready, Monsanto Company, St. Louis, MO) alfalfa was developed to eliminate this injury. The purpose of this study was to validate the crop safety of sequential applications of glyphosate over the top of Roundup Ready alfalfa under field conditions.

Fig. 1. Roundup WeatherMAX application to Roundup Ready alfalfa was accomplished using a back-pack sprayer. This picture from the field location in Jackson, TN was taken on March 12, 2004.		Fig. 2. Roundup Ready alfalfa in western Tennessee in late April of 2003, approximately 2 months after stand establishment.

Establishing and Conducting Field Research on Roundup Ready Alfalfa

Field research was initiated in March 2003 on Lexington silt loam (Ultic Hapludalf) with a pH of 6.3 and an organic matter content of 1.1% on the West Tennessee Research and Education Center in Jackson, TN. The subsoil pH was 6.1, and the study was not limed prior to study establishment. Roundup Ready alfalfa (RR variety BD-185) was broadcast at a rate of 20 lb of seed per acre on March 10, 2003 onto tilled soil and incorporated to 0.25 inch deep with a roller. Rainfall soon after seeding encouraged rapid alfalfa germination and early growth. Fertility over the three years of the study consisted of topical applications of 0-20-20 according to soil test, usually 250 to 400 lb/acre/year. Boron at a rate of 2 lb/acre was also applied in March of each year. Insect pests, mainly alfalfa weevil and potato leaf hopper, were treated as needed with Warrior T (lambda-cyhalothrin) insecticide applied at 3.8 fl oz/acre. The same field plots were maintained through 2003, 2004, and 2005. The Roundup WeatherMAX supplemental label states that up to 44 fl oz may be applied (label insert). However, most farmers will probably use 22 fl oz as the normal application rate, based upon discussions with Monsanto personnel. If injury was probable, this research used rates that would elicit injury under field conditions.

The experiment was conducted using a randomized complete block design with three replications and an incomplete factorial arrangement of treatments. A complete factorial of five glyphosate dosages at four possible application timings result in one hundred possible treatments (5Χ5Χ4). Conducting a study of this magnitude was not possible due to limited seed supplies at the time of research establishment. Treatments varied from those not expected to cause injury (22 fl oz applied two or four times) through various combinations to severely challenge the tolerance mechanism (high rates applied multiple times). The supplemental label for Roundup WeatherMax indicates a rate of 44 fl oz/acre from emergence to four trifoliate leaves and the same rate from 5 trifoliate leaves to within 5 days before first cutting in establishing stands. After the first cutting, in-crop application per cutting to within 5 days of harvest 44 fl oz/acre may be applied — all this to an annual maximum application of 132 fl oz/acre/year. Means for the various evaluations were separated by LSD at the 5% significance level.

All herbicide applications were made using a backpack sprayer operated at 40 lb/inch² with 80015VS flat fan nozzles. A four-nozzle boom was held 18 to 22 inches above the alfalfa canopy to deliver 10 gal/acre. Four postemergent applications were made each year, with applications on 22 April, 10 June, 16 July, and 15 August in 2003; on 25 March, 18 May, 21 June, and 20 July in 2004; and on 4 April, 23 May, 23 June, and 19 July in 2005. All glyphosate was applied using Roundup WeatherMAX, a potassium salt of glyphosate formulated with surfactants to aid in herbicide movement into plants. No additional adjuvants, such as ammonium sulfate, nor any other crop protection chemicals, such as insecticides, were included in herbicide applications.

Data collected included visual estimations of crop injury as measured by chlorosis or necrosis, fresh weight biomass accumulation, dry matter accumulation, dry matter percent, and annual total dry matter per acre. For brevity, not all data is presented. Means for the various evaluations were separated by LSD at the 5% significance level. Due to significant differences in alfalfa yield each year, the data was not pooled (analysis not shown).

To determine the effect of glyphosate on feed quality, samples were collected from each plot. Alfalfa was cut from the middle 15 ft in each plot using a 30-inch wide sickle-bar mower (Kinco Mfg. Co., St. Paul, MN) four times each year. Samples were dried 72 h at 140°F in a constant-temperature, forced-draft oven. Feed quality parameters measured included dry matter, crude protein (CP), available protein, unavailable protein, adjusted crude protein, soluble protein in percent of CP, degradable protein in percent of CP, fiberbound protein, crude fat, acid detergent fiber (ADF), neutral detergent fiber (NDF), non-fiber-carbohydrates, starch, sugar, lignin, calcium, phosphorus, magnesium, potassium, sulfur, and ash were estimated by near infrared analysis (NIR) (1). In vitro total digestibility 48 h and NDF digestibility 48 h in percent of NDF were determined by wet chemistry (1). Relative feed value (RFV) was calculated from the analysis data.

The field site was established in an area with minimum weed populations, as it has been cropped for several years in a weed-free system. The purpose of the study was not to evaluate weed control from postemergent glyphosate applications, since the efficacy of this treatment has been previously verified. The purpose of the study was to validate the crop safety of sequential applications of glyphosate over the top of Roundup Ready alfalfa under field conditions.

An untreated control plot was included for comparison purposes. Glyphosate application rates included 0.75, 1.125, 1.5, 2.25, and 3.0 lb a.e./acre. This rate range represents a normal application rate, 0.75 lb a.e./acre (1X or 22 fl oz), and then higher rates and ratios of 1.5, 2, 3, and 4 times the normal application rate. Additionally, these application rates were applied in a variety of different combinations (Table 1). If there were alfalfa injury risks, this series of treatments would have provided the scenario to demonstrate this crop injury.

Table 1. Effect of multiple Roundup WeatherMAX applications on alfalfa chlorosis in a field study at Jackson, TN. Herbicide rates expressed as fluid ounces of product per acre, with 22 fl oz corresponding to a normal, 1X application. No alfalfa chlorosis was observed at any other evaluation times.

Glyphosate treatments prior to the first, second, and third cuttings in all years caused no visible alfalfa injury. Chlorosis (yellowing) was observed in alfalfa treated prior to the 4th cutting in 2003 and 2004 where 3.0 lb a.e./acre had been applied (Table 1). Alfalfa chlorosis disappeared within seven days and was not observed in 2005 (Table 1). This transient chlorosis was observed only in alfalfa that received sequential applications of glyphosate at rates exceeding label recommendations, and the chlorosis did not reduce alfalfa yield. Visual observation of Roundup Ready alfalfa growth following weed control treatments using Roundup WeatherMAX indicates good tolerance of glyphosate by alfalfa. The total amount of forage harvested also was not affected by glyphosate application (Tables 2, 3, and 4).

Table 2. Effect of multiple Roundup WeatherMAX applications on alfalfa dry weight in a field study in 2003 at Jackson, TN. Herbicide rates expressed as fluid ounces of product per acre, with 22 fl oz corresponding to a normal, 1X application.

Table 3. Effect of multiple Roundup WeatherMAX applications on alfalfa dry weight in a field study in 2004 at Jackson, TN. Herbicide rates expressed as fluid ounces of product per acre, with 22 fl oz corresponding to a normal, 1X application.

Table 4. Effect of multiple Roundup WeatherMAX applications on alfalfa dry weight in a field study in 2005 at Jackson, TN. Herbicide rates expressed as fluid ounces of product per acre, with 22 fl oz corresponding to a normal, 1X application.

At glyphosate dosages of 2.67 times maximum label rates for four harvests, there was no decrease in forage quality (Table 5). For additional analysis there was no significant difference in any of the treatments within harvest times (data not shown). Average forage quality measurements over all years included CP of 21.6%, RFV of 144, ADF of 30.3%, NDF of 38%, available protein (20.1%), unavailable protein (1.5%), adjusted crude protein (21.2%), soluble protein (40.2% of CP), degradable protein (62.1% of CP), fiberbound protein (4.4%), crude fat (2.8%), non-fiber-carbohydrates (32.1%), starch (2.4%), sugar (9.3%), lignin (7.1%), calcium (1.3%), phosphorus (0.3%), magnesium (0.2%), potassium (2.5%), sulfur (0.2%), and ash (9.8%)

Table 5. Effect of multiple Roundup WeatherMAX applications on alfalfa quality in a field study in 2005 at Jackson, TN. Herbicide rates expressed as fluid ounces of product per acre, with 22 fl oz corresponding to a normal, 1X application. All data shown from fourth cutting, and all other data similar

Although weed control was not measured, it is well documented that glyphosate has good activity on both grass and broadleaf weeds (10). Other features of this new technology include flexibility of applications with no temperature restrictions, a 5-day pre-harvest interval, and essentially no risk of crop injury from direct glyphosate application or glyphosate drift onto Roundup Ready alfalfa from adjacent areas. This technology provides an easy method to kill volunteer wheat or other companion crops and tillage can be greatly reduced. Another possible benefit of Roundup Ready alfalfa is the removal of poisonous plants that can be detrimental to livestock.

A concern about glyphosate application for weed control in Roundup Ready alfalfa is the potential for development of herbicide-resistant weed populations. If glyphosate were used solely for weed control in alfalfa production systems, weeds tolerant to glyphosate could develop. However, alfalfa is routinely harvested multiple times each year, which provides an additional method of weed control. The growth and canopy development of a healthy alfalfa stand are significant barriers to summer annual weeds. This further reduces the threat of developing resistant weeds in this production system. Also, all other herbicides currently labeled for alfalfa are still available for use in controlling weeds in Roundup Ready alfalfa.

The full economic benefits of Roundup Ready alfalfa are difficult to estimate at this time. As the first perennial crop containing the Roundup Ready gene, pricing and marketing factors for forage from Roundup Ready alfalfa are not fully established. In some situations, much of the actual benefit of Roundup Ready alfalfa will be seen during seedling establishment and at the first cutting, when glyphosate will improve weed control by reducing competition from early spring and winter annual weeds. The technology may have value in later years when a stand is getting too thin to suppress weeds. It may be possible in some situations that glyphosate treatment could prolong an alfalfa stand for one or more additional years. Alfalfa stands naturally thin over time, and plant density, disease, cutting schedule and other factors all play a role in alfalfa stand persistence.

Competition from spring and winter annual weeds often decreases the success of producer efforts to establish stands in March. Glyphosate should be applied to Roundup Ready alfalfa at label rates prior to the 4th trifoliate leaf stage to eliminate alfalfa plants that do not contain the gene for resistance to Roundup and insure stand purity in the established alfalfa while also reducing weed competition that threatens spring plantings.

Acknowledgments

The technical assistance of Patricia Brawley and Ernest Merriweather is gratefully acknowledged. Technical suggestions from Paulette Pierson are appreciated. This work was funded by Monsanto Company and the Tennessee Agricultural Experiment Station.

Literature Cited

1. Anonymous. 2004. Forage lab analytical procedures. Online. Dairy One Cooperative Inc., Ithaca, NY.

2. Arregui, M. C., Sanchez, D., and Scotta, R. 2001. Weed control in established alfalfa (Medicago sativa) with postemergence herbicides. Weed Technol. 15:424-428.

3. Becker, R. L., Sheaffer, C. C., Miller, D. W., and Swanson, D. R. 1998. Forage quality and economic implications of systems to manage giant foxtail and oat during alfalfa establishment. J. Prod. Agric. 11:300-308.

4. Cudney, D. W., Orloff, S. B., and Adams, C. J. 1993. Improving weed control with 2,4-DB amine in seedling alfalfa (Medicago sativa). Weed Technol. 7:465-470.

5. Darwent, A. L., Cole, D., and Malik, N. 1997. Imazethapyr alone or with other herbicides for weed control during alfalfa (Medicago sativa) establishment. Weed Technol. 11:346-353.

6. Foy, C. L., and Witt, H. L. 1993. Effects of paraquat on weed control and yield of alfalfa (Medicago sativa) in Virginia. Weed Technol. 7:485-506.

7. Hoy, M. D., Noore, K. J., George, J. R., and Brummer, E. C. 2002. Alfalfa yield and quality as influenced by establishment method. Agron. Journ. 94:65-71.

8. Malik, N., Bowes, G. G., and Waddington, J. 1993. Residual herbicides for weed control in established alfalfa (Medicago sativa) grown for seed. Weed Technol. 7:483-490.

9. Smith, A. E. 1991. Paraquat for managing weeds in alfalfa (Medicago sativa). Weed Technol. 5:181-184.

10. Tingle, C. H., and Chandler, J. M. 2004. The effect of herbicides and crop rotation on weed control in glyphosate-resistant crops. Weed Technol. 18:940-946.

11. Wilson, R. G. 1981. Weed control in established dryland alfalfa (Medicago sativa). Weed Sci. 29:615-618.

12. Wilson, R. G. 1989. New herbicides for weed control in established alfalfa (Medicago sativa). Weed Technol. 3:523-526.

13. Wilson, R. G. 1994. Effect of imazethapyr on legumes and the effect of legumes on weeds. Weed Technol. 8:536-540.

14. Wilson, R. G. 1997. Downy brome (Bromus tectorum) control in established alfalfa (Medicago sativa). Weed Technol. 11:277-282.

15. Zaman, M. S., Moyer, J. R., Boswall, A. L., and Mir, Z. 2003. Nutritional quality and yield of seedling alfalfa established with a barley companion crop and weeds. Anim. Feed Sci. Technol. 103:163-169.