© 2007 Plant Management Network.
Comparing Roundup Ready and Conventional Systems of Alfalfa Establishment
Craig C. Sheaffer, Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul 55108; Daniel J. Undersander, Department of Agronomy, University of Wisconsin, Madison 53706; and Roger L. Becker, Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul 55108
Corresponding author: Craig C. Sheaffer. firstname.lastname@example.org
Sheaffer, C.C., Undersander, D.J., and Becker, R.L. 2007. Comparing Roundup Ready and conventional systems of alfalfa establishment. Online. Forage and Grazinglands doi:10.1094/FG-2007-0724-01-RS.
Roundup Ready (RR) technology provides a new approach for weed control during alfalfa (Medicago sativa L.) establishment. We determined the effect of RR and conventional establishment systems on alfalfa yield, weed yield, and forage quality when alfalfa was established using solo-seeding or oat mulch methods. A RR system was a RR alfalfa in combination with glyphosate (Roundup) and a conventional system was a non-RR variety with imazamox (Raptor). Non-RR and RR alfalfas were also seeded with an oat companion crop. Alfalfa yields, plant populations, and forage quality were similar for the RR and conventional systems within solo-seeding and oat establishment methods in the seeding year. Total seeding-year alfalfa yield was greater when solo-seeded using an herbicide than when seeded with an oat companion crop harvested at boot. Alfalfa yield for the oat mulch and oat companion crop treatments were not consistently different over locations.
Establishment of dense, vigorous stands of alfalfa is essential for long-term profitability, but establishment can be challenging because seedling alfalfa is vulnerable to competition from weeds and wind and water erosion (12). Weed contamination can also reduce the quality of alfalfa hay (1,6). Establishment strategies have been designed to reduce the risk of establishment. The two most widely used strategies are companion crops and herbicides. Companion cropping with a small grain like oat (Avena sativa L.) is a traditional strategy in the Midwest (11). Companion crops reduce stand loss due to wind and water erosion and may suppress growth of some weeds. For spring seedings in the Midwest, companion crops also provide greater assurance of economic return in the seeding year. Even with reductions in seeding rates and removal before maturity, small-grain companion crops can provide competition with new alfalfa seedlings and reduce alfalfa yield potential (3,4,8,9). Oat seeded with alfalfa but removed with herbicide early in development is termed oat mulch seeding (1). Oat mulch systems minimize wind or water erosion at sites where protection of alfalfa seedlings is needed, yet minimize the competitive stress of companion crops with alfalfa seedlings.
Establishment of alfalfa with herbicides, also called "direct-" or "solo-seeding," has been promoted since the 1970s (9,12). Solo-seeding with herbicides removes competition from weeds and companion crops, enhances seeding-year alfalfa yield potential and often results in superior alfalfa stands compared to companion crop seeding (9). Postemergence herbicides such as (imazethapyr) Pursuit and imazamox are used for weed control during establishment of alfalfa varieties in conventional, non RR systems. Development of RR alfalfa allows for the use of the postemergence application of glyphosate for control of weeds during establishment (7,13), but this strategy has not been thoroughly evaluated in the upper Midwest.
Our objective was to determine the effect of RR and conventional establishment systems on alfalfa yield, weed yield, and forage quality when alfalfa was established. We defined a RR system as a RR alfalfa in combination with glyphosate herbicide and a conventional system as a non-RR variety with imazamox herbicide using solo-seeding, oat mulch, or oat companion crops.
Research was conducted from 2004 to 2006 at Rosemount, MN on a Waukegan silt loam (fine-silty over sandy, mixed Typic Hapludoll) and at Arlington, WI on a Plano silt loam (fine-silty, mixed, mesic, Typic Argiudoll). Soil fertility was adjusted to levels recommended for alfalfa growth. Trials at each location were seeded in 2004 and again in 2005. The experimental design at both locations was a randomized complete block with four replicates. Treatments were applied in a split-plot arrangement. Whole-plot treatments were establishment methods. At Rosemount, four whole-plot alfalfa establishment methods were: (i) solo-seeded with herbicide, (ii) solo-seeded without herbicide, (iii) oat mulch, and (iv) oat harvested at boot stage. At Arlington, three establishment methods were: (i) solo seeded with herbicide application, (ii) oat mulch, and (iii) oat harvested at boot stage. Sub-plot treatments within whole plots were RR or conventional establishment systems. Establishment systems were two experimental RR entries (RR-1 and RR-2) or two conventional alfalfa varieties (54V46 and Rebound 5.0) with corresponding glyphosate or imazamox herbicides. For the solo-seeded herbicide and oat mulch establishment methods where herbicides were applied, RR alfalfas received only glyphosate while conventional alfalfas received only imazamox.
Alfalfa was seeded at 15 lb/acre (pure live seed) and oat was seeded at 56 lb/acre on 30 April 2004 and 2 May 2005 at Rosemount and on 13 April 2004 and 17 April 2005 at Arlington, WI. Plot size of individual alfalfa entries was 3 by 20 ft. For the solo-seeded and oat mulch treatments, glyphosate was applied to RR alfalfas when weeds or oat were 4 to 6 inches tall, while imazamox was applied to conventional alfalfas when weeds or oat were 3 to 4 inches tall. Imazamox was applied at 4 oz /acre (0.032 lb ai/acre) with 1 qt/acre of crop oil concentrate and 2 qt/acre 28% urea ammonium nitrate. Glyphosate was applied at 16 oz/acre as WeatherMAX (0.56 lb ae/acre) + 5.9 pt/A N-Pak ammonium sulfate (AMS) liquid (2.5 lb AMS/acre). Drift of herbicide between plots receiving either glyphosate or imazamox treatments was eliminated by planting a 6-ft border around herbicide treatments and by use of hooded spray equipment.
Alfalfa in the solo-seeded and oat mulch treatments was harvested at late bud to early flowering on 14 July, 25 August, and 11 October at Rosemount and on 14 July and 20 August at Arlington. A 3- by 17-ft strip was harvested and a 1-lb subsample was dried at 130°F for 72 h, weighed to determine dry matter yield and saved for forage quality analysis (2). An additional 1-lb subsample was collected from each plot and separated into alfalfa, weed, and when appropriate oat composition. Each component was dried, weighed, and used to determine the yield contribution of each fraction. Plant populations from 1-ft² quadrats in each plot were determined in July of each establishment year (2004 and 2005) and in May of 2005 and 2006. All plots were harvested three times in the year following seeding when alfalfa reached first flower on about 1 June, 1 July, and 30 August using the same procedures as in the seeding year.
The dried subsample from the seeding year was ground and analyzed for crude protein (CP), neutral detergent fiber (NDF), in vitro digestibility (IVD), and NDF digestibility (NDFD) using near infrared reflectance spectroscopy (NIRS) and calibration approaches as previously described (2,5,10). Forage quality data was also averaged for the season based on a weighed mean approach that adjusts average seasonal forage quality based on the yield at an individual harvest.
To compare year and treatment effects, data were subjected to analysis of variance using SAS (SAS Institute Inc., Cary, NC) and means were compared using LSD (0.05). Forage yield and quality data was averaged for 2004 and 2005 due to similar treatment responses. Results from Arlington, WI and Rosemount, MN were analyzed separately because of differences in treatments and seeding-year harvest number. In addition, because the two alfalfas were not grown over both sub-plot establishment system treatments, the data from the two alfalfas were averaged and used in the statistical analysis.
Forage and Weed Yield
Alfalfa yields were similar for the RR and conventional establishment systems within the establishment methods for the first harvest in the seeding year and for total harvests in the seeding and the year following seeding (Tables 1 and 2). At Rosemount, first cutting and total season alfalfa yields were similar for the solo-seeded herbicide and oat mulch treatments. At Arlington, first cutting and total season alfalfa yields were similar for the solo-seeded herbicide and oat mulch treatments, but yields for the oat mulch and oat boot treatments were similar. At Arlington, the solo-seeded herbicide treatment had the greatest first cut yield while the oat-boot treatment had the lowest yields. Alfalfa yields for the oat mulch treatment were intermediate. The solo-seeded control treatment that was only at Rosemount resulted in total season alfalfa yields that were less than the oat mulch treatment and greater than the oat boot treatment.
Table 1. Effect of establishment method and Roundup ready (RR) or conventional (Conv.) systems on alfalfa and weed yields in the establishment year (Year 1) and year after establishment (Year 2) at Rosemount, MN. Data is averaged for trials seeded in 2004 and 2005 (Year 1) and in the year following seeding (2005 and 2006, Year 2).
x Oat-boot. Oat and alfalfa intercrop was cut when the oat was at the boot. Average oat yield was 2.08 and 0.80 at the first and second harvest, respectively.
y LSD for comparison of two establishment method means; RR and Conv. system means within methods were not different (P = 0.05).
Table 2. Effect of establishment method and Round-up ready (RR) or conventional (Conv.) establishment systems on alfalfa yields in the establishment year (Year 1) and year following establishment (Year 2) at Arlington, WI. Data is averaged for trials seeded in 2004 and 2005 (Year 1) and in the year following seeding (2005 and 2006, Year 2).
x Oat yield was 1.93 at the first harvest.
y LSD for comparison of two establishment method means; RR and Conv. system means within methods were not different (P = 0.05).
In the year following seeding, alfalfa yields across establishment methods and establishment systems did not differ at Rosemount. However, at Arlington the oat-boot establishment method had lower yields than the solo-seeded herbicide and oat-mulch treatments.
Significant weed yields only occurred for the solo-seeded without herbicide treatment at Rosemount. The only other measurable weed yields occurred for the first harvest of the conventional oat mulch treatment at Rosemount. There were no measurable weed yields at Arlington. The oat yields for the oat-boot treatment averaged 2.1 and 0.8 ton/acre for the first and second harvests, respectively, at Rosemount, and 1.93 at the first harvest at Arlington. The results for establishment methods at Rosemount confirm those of previous research that show the benefits of weed control in the seeding year on increased alfalfa yields and show that companion crops reduce alfalfa yields (1,9). Becker et al. (1) had also reported similar alfalfa yields when alfalfa was established with the oat mulch and solo-seeded herbicide treatment.
Alfalfa populations were similar for RR and conventional systems within establishment methods. Alfalfa populations were also similar for the establishment methods at each location even though alfalfa yields were suppressed by the oat-boot treatment. Alfalfa populations exceeded 30 plants/ft² in the fall of the seeding year and were similar among treatments the following spring.
In the oat mulch treatment, glyphosate killed the oat faster than imazamox. However, by first harvest, oat control was complete with both herbicides. The rapidity of the oat mulch reduction with glyphosate may be an issue on slopes where water erosion could be severe. Further research is needed to delineate the glyphosate herbicide rate response curve for interseeded oat.
Common lambsquarters (Chenopodium album L.) and Powell amaranth (Amaranthus powellii S. Wats) were the predominant weeds at Rosemount. Velvetleaf (Abutilon theophrasti Medicus), common ragweed (Ambrosia artemisiifolia L.), and giant foxtail Setaria faberi Herrm.) were also present to a lesser extent. At Arlington, chickweed (Stellaria media (L.) Cyrillo), lambsquarters, and quackgrass (Elytrigia repens (L.) Nevski) were present. Excellent control of predominate weeds occurred with either imazamox or glyphosate at both locations. In solo-seeded herbicide treatments, glyphosate provided 100% control of common lambsquarters while imazamox control was only 85%, but these visual differences did not result in readily apparent differences in alfalfa yield or quality throughout the season. More common ragweed survived in imazamox treatments than glyphosate treatments in both solo seeded and oat mulch treatments, but population densities were reduced and survivors remained severely stunted and noncompetitive.
For the first cut and average seasonal forage quality, the concentration of CP, NDF, IVD, and NDFD was similar for the RR and conventional systems within the four establishment methods (Tables 3 and 4). Our results concur with those of McCann et al. (7) who had reported that the protein and fiber composition of an experimental glyphosate-tolerant alfalfa was similar to that of conventional varieties.
Table 3. Effect of establishment method and Roundup ready (RR) or conventional (Conv.) systems on forage crude protein (CP), neutral detergent fiber (NDF), in-vitro digestibility (IVD), and neutral detergent fiber digestibility (NDFD) at the first cutting and averaged for the seeding year at Rosemount, MN. Data is averaged for 2004 and 2005.
x LSD for comparison of two establishment method means; RR and Conv. system means within establishment methods were not different (P = 0.05).
Table 4. Effect of establishment method and Roundup Ready (RR) or conventional (Conv.) systems on forage crude protein (CP), neutral detergent fiber (NDF), in-vitro digestibility (IVD), and neutral detergent fiber digestibility (NDFD) at the first cutting and averaged for the seeding year at Arlington, WI. Data is averaged for 2004 and 2005.
x LSD for comparison of two establishment method means; RR and Conv. system means within establishment methods are not different (P > 0.05).
Total forage quality was affected by establishment method. At the first harvest, the solo-seeded herbicide and oat mulch treatments that had the greatest yields of alfalfa also had the highest CP and IVD and lowest NDF concentration. The oat-boot treatment that was mostly oat forage at the first harvest had lower CP and IVD concentration and higher NDF concentration compared to the other treatments. At Rosemount, the solo-seeded control, which had the greatest contribution of weeds to forage yield, had intermediate levels of CP, NDF, and IVD. NDFD concentration did not consistently differ for the four establishment methods. The effects of establishment method on average seasonal forage quality were similar to those for the first harvest although the differences among establishment method treatments were less because of the ameliorating affect of additional harvests of pure alfalfa. Differences in forage quality among establishment approaches are consistent with those previously reported (1,4,9)
A RR system that includes use of experimental glyphosate resistant alfalfas and glyphosate herbicide provides comparable seeding-year forage yield and forage quality as a conventional system. For both systems, solo-seeding alfalfa using herbicide produced the highest first cut yields, but total yields for the seeding year were not different for the solo seeded herbicide and oat mulch treatment. If soil erosion is an issue, using an oat mulch system will produce first year alfalfa yields similar to solo seeding with herbicide use. Solo-seeding without herbicide produced intermediate first year alfalfa yields. Seeding alfalfa with oat and harvesting at boot resulted in the highest total forage yield, but the lowest forage quality and the lowest first year alfalfa yield. In the year following seeding, alfalfa yield and population were similar for the RR and conventional establishment systems. Establishment method had no effect on alfalfa yields in the year after seeding at Rosemount, but at Arlington the oat-boot treatment had alfalfa lower yields than the other treatments. Roundup Ready technology provides another tool for efficient establishment of alfalfa similar to conventional herbicide technology. Future research is needed to determine if RR technologies offer additional benefits over conventional herbicides through unique weed management potential for alfalfa beyond the seeding year.
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