Glyphosate-resistant Alfalfa Research Articles

Development of glyphosate-resistant alfalfa (Medicago sativa L.) upon transformation with the GR79Ms gene encoding 5-enolpyruvylshikimate-3-phosphate synthase.

The glyphosate-resistant gene, GR79Ms, was successfully introduced into the genome of alfalfa. The transgenic events may serve as novel germplasm resources in alfalfa breeding. Weed competition can reduce the alfalfa yield, generating new alfalfa germplasm with herbicide resistance is essential. To obtain transgenic alfalfa lines with glyphosate resistance, a new synthetic glyphosate-resistant gene GR79Ms encoding 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) was introduced into alfalfa germplasm by Agrobacterium tumefaciens-mediated transformation. In total, 67 transformants were obtained. PCR and Southern blot analyses confirmed that GR79Ms was successfully inserted into the genome of alfalfa. Reverse transcription-PCR and western blot analyses further demonstrated the expression of GR79Ms and its product, GR79Ms EPSPS. Moreover, two homozygous transgenic lines were developed in the T2 generation by means of molecular-assisted selection. Herbicide tolerance spray tests showed that the transgenic plants T0-GR1, T0-GR2, T0-GR3 and two homozygous lines were able to tolerate fourfold higher commercial usage of glyphosate than non-transgenic plants.

Effect of Synthetic Auxin Herbicides on Seed Development and Viability in Genetically Engineered Glyphosate-Resistant Alfalfa

Feral populations of cultivated crops have the potential to function as bridges and reservoirs that contribute to the unwanted movement of novel genetically engineered (GE) traits. Recognizing that feral alfalfa has the potential to lower genetic purity in alfalfa seed production fields when it is growing in the vicinity of foraging pollinators in alfalfa seed fields, industry has established production standards to control feral plants. However, with the commercialization of GE glyphosate-resistant (GR) alfalfa and the need to support the coexistence of both GE and conventional production, effective methods to control transgenic feral alfalfa need to be developed. Therefore, a study was conducted in 2012, 2013, and 2014 to determine the effect of several synthetic auxin herbicides on seed development in GR alfalfa. GR alfalfa, var. Genuity (R44BD16), was treated with dicamba, 2,4-D, triclopyr, and aminopyralid when alfalfa plants contained green seed pods. Two weeks after herbicide application, plants were harvested, air dried, and seed yield, seed germination, and seedling emergence from the soil were determined. In 2013, dicamba, triclopyr, and 2,4-D decreased alfalfa seed yield per plant compared wih nontreated plants, whereas in 2014, all four herbicides decreased alfalfa seed yield per plant 24 to 49% (by weight) compared with nontreated plants. The same trend was evident in 2012, but seed yield was variable and was not significantly different among treatments. Seed germination averaged 43, 50, and 72% in 2012, 2013, and 2014, respectively, and was not affected by the four herbicides applied at early pod-fill stage. However, seeds harvested from plants treated with dicamba, 2,4-D, and triclopyr often produced deformed and abnormal seedlings, and when planted in soil, frequently failed to emerge. The combined effects of dicamba, 2,4-D, and triclopyr in reducing seed yield, seedling emergence, and seedling growth could contribute to managing feral alfalfa populations.

Glyphosate-Resistant Alfalfa Response to Harvest Frequency and Weed Management

Glyphosate-resistant (GR) alfalfa offers growers new options for weed control in alfalfa. One potential benefit of using GR alfalfa is increased longevity of an alfalfa stand under frequent harvests. It was hypothesized that GR alfalfa would have a greater longevity because of removal of weed interference with minimal crop injury. To study GR alfalfa yield, weed invasion, alfalfa stand persistence, and relative forage quality (RFQ), a field experiment with three weed control methods (no herbicide, glyphosate, and hexazinone) under two harvest frequencies (high and moderate) was established in August 2003 at the Michigan State University Agronomy Farm in East Lansing, MI. Forage yield of established alfalfa was not adversely affected by herbicide treatments. There were no differences in weed biomass between alfalfa treated with glyphosate and that treated with hexazinone, except in 2007. Average GR alfalfa stand density decreased approximately 90% (from 236 to 27 plant m−2), and yield decreased approximately 30% (from 11.04 to 7.87 Mg ha−1) during the 7-yr period (2004 to 2010) of the experiment. Stand density of GR alfalfa showed natural thinning during the 7-yr period regardless of harvest intensity or herbicide treatment. In most production years (4 out of 5 yr), relative forage quality of GR alfalfa was higher under a high-intensity harvesting system (4 to 5 harvests yr−1) than it was with a moderate intensity harvesting system (3 to 4 harvests yr−1). Relative forage quality was not affected by weed removal with herbicides in most years. Weed removal and harvest intensity in established GR alfalfa had no effect on stand persistence.

Effect of Seeding Rate and Weed Control on Glyphosate-Resistant Alfalfa Establishment

The introduction of glyphosate-resistant (GR) alfalfa offers a new weed management system for alfalfa establishment; however, alfalfa seeding rates are based on conventional cultivars. Determining optimum seeding rates allows forage producers to maximize yield, quality, and profitability with GR alfalfa. Field experiments were established in 2005 and 2006 to determine the effect of seeding rate and weed control on GR alfalfa yield, forage quality, and persistence up to 3 yr after establishment. Seeding rates of 4.5, 9.0, and 18 kg ha−1were evaluated. Weed control methods during the seeding year included no herbicide, glyphosate applied once before the first harvest, and glyphosate applied once before the first harvest and then 7 to 10 d following subsequent harvests. Alfalfa yield was greater at higher seeding rates and when weeds were removed with glyphosate. Season forage yields were the greatest with the 18 kg ha−1seeding rate and where no herbicide was applied. Weed biomass often was lower at the higher seeding rates and was 91 to 98% lower in the glyphosate treatments compared to the nontreated. Forage quality was not affected by seeding rate but varied by herbicide treatment depending on establishment year. Plant density increased with seeding rate and treatment effects persisted for three growing seasons. Herbicide treatment did not affect stand density as greatly as seeding rate and did not influence stand longevity.

Industry exhales as USDA okays glyphosate-resistant alfalfa

Industry exhales as USDA okays glyphosate-resistant alfalfa

Critical Period for Weed Control in Alfalfa

The critical period for weed control for many crops has been well investigated and the results have been used to develop better weed management recommendations. However, research is limited on the critical period for weed control for alfalfa, a perennial crop with multiple harvests. With the advent of new, more effective POST herbicides for alfalfa, an understanding of the critical period may further enhance forage yield. The objective of this study was to determine the critical period for weed control in spring-seeded alfalfa. Alfalfa was seeded conventionally at two locations in Pennsylvania in the spring of 2004 and 2005; glyphosate and glyphosate-resistant alfalfa were used for weed management. A surrogate weed, Japanese millet, was seeded at three planting rates to simulate varied levels of weed density. In the analysis, the critical period for weed control was identified for the 2004 seedings, but not for 2005. In 2004, the critical period varied by location, but began at the 0.5 trifoliate alfalfa growth stage and ended at the 7 trifoliate growth stage. When identified, the critical period was similar for both first-harvest yield and first-year cumulative yield. Higher weed density caused the critical period to begin earlier than under lower weed densities, but weed density had no effect on when the critical period ended. Under moderate to heavy weed populations, the critical period for weed control can increase herbicide use efficiency and maximize alfalfa returns, although under low weed severity, the critical period may not exist.

Comparison of Herbicide Programs for Weed Control in Glyphosate-Resistant Alfalfa

The objective of this experiment was to determine the effectiveness and crop safety of glyphosate vs. alternative herbicides for weed control in glyphosate-resistant alfalfa. Glyphosate-resistant alfalfa was established at two sites in Pennsylvania in 2004 and in 2005, and herbicides were applied either PRE or POST for weed control. Data were collected on herbicide performance, alfalfa and weed yield, and forage quality. Alfalfa forage response to weed control was variable and depended on weed severity. A single or split application of glyphosate provided similar or better weed control than conventionally based herbicide programs. The most differences from weed control occurred during the first harvest and dissipated in subsequent harvests. Cumulative alfalfa yield for the establishment year of the spring seeding was 26% lower in the untreated check relative to the mean of the herbicide-treated plots in 2004; but no differences were detected in 2005. Forage quality was highest where weed content of the forage was lowest. Effective management of weeds with herbicides during alfalfa establishment can improve forage yield and quality, and weed control is particularly important when summer annual weed populations are severe and emerge with the crop.

Influence of Seeding Rate and Herbicide Treatments on Weed Control, Yield, and Quality of Spring‐Seeded Glyphosate‐Resistant Alfalfa

Field trials were conducted in 2005 and 2006 in central and southwest Missouri to investigate the effect of seeding rate and herbicide programs on weed control, forage quality, and yield within the year of establishment in spring seeded glyphosate‐resistant (GR) alfalfa (Medicago sativa L.). In all experiments, EPTC (4.4 kg ha−1), 2,4‐DB plus sethoxydim (1.1 plus 0.21 kg ha−1), imazamox (0.043 kg ha−1) and glyphosate (0.86 kg ae ha−1) were applied to GR alfalfa that was seeded at 4.5, 8.9, 13.5, and 18 kg ha−1 Across all locations, the glyphosate application provided good control of weeds regardless of alfalfa seeding rate. When applied at the same timing, imazamox provided similar levels of annual broadleaf weed control as glyphosate, while 2, 4‐DB plus sethoxydim provided similar levels of annual grass and broadleaf weed control as glyphosate. Neither glyphosate nor the conventional herbicide treatments reduced alfalfa stem density between the first and last annual harvests, but 2, 4‐DB plus sethoxydim resulted in the most visual injury to seedling alfalfa stands and lowered initial yields at all locations. In addition, alfalfa annual yields were similar or higher in the glyphosate treatment compared to the other herbicide programs. Harvested forage in the herbicide treatments generally had greater crude protein (CP) content than the untreated control forage. Application of glyphosate resulted in similar or higher forage CP content compared to conventional herbicide treatments. In vitro true digestibility of the harvested GR alfalfa ranged from 750 to 860 g kg−1 DM at all locations with few differences across herbicide treatments.

Forage Quality of Glyphosate-Resistant Alfalfa as Influenced by Establishment Systems

Field studies were conducted in 2004 and 2005 in Michigan to determine the effect of seeding establishment method and weed control on forage quality of glyphosate-resistant alfalfa in the establishment year. Seeding methods included alfalfa only (clear-seeding) and alfalfa with a companion crop of oat (companion-seeding). Herbicide treatments included an untreated control and glyphosate treatment for both establishment systems, and either imazamox in the clear-seeding system or imazamox + clethodim in the companion-seeding system. The greatest differences among treatments in forage quality were observed at the first harvest in both establishment years. Results suggest high quality, productive alfalfa stands can be established utilizing glyphosate-resistant alfalfa in a clear seeding system.

Establishment Systems for Glyphosate-Resistant Alfalfa

Glyphosate-resistant alfalfa offers new weed control options for alfalfa establishment. Field studies were conducted in 2004 and 2005 to determine the effect of establishment method and weed control method on forage production and alfalfa stand establishment. Seeding methods included clear seeding and companion seeding with oats. Herbicide treatments included glyphosate, imazamox, imazamox + clethodim, and no herbicide. Temporary stunting from the glyphosate treatments was observed (< 7%); however, injury did not reduce forage yield or stand density in 2004. No glyphosate injury was observed in 2005. Weed control with glyphosate was more consistent than with imazamox or imazamox + clethodim. In 2004, total seasonal forage yield, which consisted of alfalfa, weeds, and oats (in some treatments), was the highest where no herbicide was applied in the oat companion crop and was reduced where herbicides were applied in both establishment systems. In 2005, seeding method or weed control method did not affect tot...