Delayed Glyphosate Application Doesn’t Reduce Plant Density of Glyphosate-Tolerant Alfalfa

Marvin H. Hall and Nick S. Hebrock, Department of Crop and Soil Science, The Pennsylvania State University, University Park 16802; Keith A. Diedrick, 1680 Madison Avenue, Ohio State Cooperative Extension, OARDC, Wooster 44691; and Justin M. Dillon, Department of Crop and Soil Science, The Pennsylvania State University, University Park 16802

Abstract

Commercial seed of glyphosate-tolerant alfalfa contains small percentages of seeds, called nulls, that do not posses the herbicide trait. Differential competitiveness of glyphosate-tolerant seedlings compared with null seedlings may be detrimental to alfalfa stand density if delayed glyphosate application allows the null and tolerant seedlings to compete with each other. Our objective was to determine if delayed glyphosate application affects plant density of glyphosate-tolerant alfalfa stands. Commercially available glyphosate-tolerant alfalfa was seeded at 17 and 29 kg/ha, plots were treated with glyphosate one month (3- to 6-trifoliate leaf stage) or one-year after planting and plant density was monitored throughout the study. Regardless of seedling rate an average of 6.5 and 5.5% of the alfalfa plants died when glyphosate was applied one month and one year after planting, respectively, but plant density remained unchanged during the same time periods when glyphosate was not applied. Planting alfalfa at 17 kg/ha resulted in lower seedling density, but also lower plant mortality during the seeding year than planting at 29 kg/ha. We conclude that delaying glyphosate application to glyphosate-tolerant alfalfa until one year after planting does not alter the percent of glyphosate-tolerant and non-tolerant plants.

Glyphosate Resistance and Reduced Alfalfa Seeding Rate

Glyphosate [N-(phosphonomethyl) glycine]-tolerant alfalfa (Medicago sativa L.) offers a weed control option to producers that does not involve weed identification or fear of damaging the alfalfa (14). Commercial seed of tolerant alfalfa contains some nulls (seeds that do not have the herbicide trait). The tolerant and null plants may also be different in other ways (e.g., seedling vigor, disease and insect tolerance) than the resistance to glyphosate (10). A glyphosate application when glyphosate-tolerant alfalfa seedlings are at the 3- to 6-trifoliate leaf stage is recommended to eliminate nulls from the alfalfa stand (1,3). Early elimination of nulls presumably removes intraspecific competition that the null seedlings might impose on the glyphosate-tolerant seedlings. Producers, however, may wish to delay glyphosate application if weeds are not present at the 3- to 6-trifoliate leaf stage. Delayed glyphosate application (not removing the null seedlings) would allow prolonged intraspecific competition between the null and tolerant seedlings. This intraspecific competition could result in a greater percentage of null seedlings in the stand when glyphosate is applied. Consequently, delayed glyphosate application to a glyphosate-tolerant alfalfa stand could cause greater alfalfa mortality than if the glyphosate had been applied at the 3- to 6-trifoliate leaf stage.

The additional expense for glyphosate-tolerant alfalfa seed has renewed interest in reduced alfalfa seeding rates (7,8). Reducing the seeding rates of non-glyphosate-tolerant alfalfa to 9 kg/ha had no affect on stand density (5,6,9,11,12,13) or yield (5,7,9). However, reduced seeding rates of glyphosate-tolerant alfalfa combined with delayed glyphosate application could reduce stand density enough to shorten the time before stand density falls below an economic threshold.

There is a dearth of literature on the response of alfalfa stand density when glyphosate application to glyphosate-tolerant alfalfa is delayed. Knowledge of this response is important as producers explore different management options such as reduced seeding rate and/or delayed glyphosate application. The objective of this research was to determine if delayed glyphosate application affects plant density of glyphosate-tolerant alfalfa stands.

Description of Field Study

In April of 2006, uncoated, rhizobium-inoculated, and glyphosate-tolerant ‘DeKalb RR05-060104’ alfalfa was planted in rows at 17 and 29 kg/ha of pure live seeds (PLS) into 1 × 5-m, tilled plots located on a Murrill silt loam (fine-loamy, mixed, mesic Typic Hapludults) soil at the Russell E. Larson Agricultural Research Center near Rock Springs, PA, and on a Wooster-Riddles silt loam (fine-loamy, mixed, mesic Typic Fragiudalfs and fine-loamy, mixed, mesic Typic Hapludalfs) soil at the Ohio Agricultural Research and Development Center near Wooster, OH. Land areas were selected at each location that had a history of limited or no weed problems. The seed source was thoroughly mixed prior to packaging the seeds for planting. The high seeding rate was 12 kg/ha greater than recommended for the two locations, but was included to ensure competition between the glyphosate-tolerant and null alfalfa seedlings (4). Soil pH, available P, and exchangeable K were maintained above the recommended levels of 6.5, and 70 and 336 kg/ha, respectively, throughout the experiment (2). Three harvests were made at first flower during the seeding year.

The experimental design was a split-plot arrangement of a RCB design with four replicates. Whole plots were three glyphosate application times (none, and applied one month or one year after planting) and subplots were seeding rates. One month after planting (T1, 3- to 6-trifoliate leaf stage of alfalfa development), alfalfa plant density was determined by counting live plants in three randomly selected 0.1-m² areas within each subplot. Glyphosate (Roundup WeahterMax, Monsanato Company, St. Louis, MO) was then applied at the rate of 0.84 kg/ha to one of the whole plots (1). Fourteen days later (T2) alfalfa plant density was determined again using the same methodology.

In April of the year after planting (T3), alfalfa plant density was determined by digging and counting live plants (crowns) in three, randomly selected, 0.1-m² areas within each plot. Glyphosate was then applied at the recommended rate for established alfalfa of 1.68 kg/ha to the whole plots that had not been treated with glyphosate previously (1). Fourteen days later (T4) alfalfa plant density was determined by digging and counting live plants in all plots. Plant mortality was considered the difference in plant density over time. Changes in mortality rates between T1-T2 and T3-T4 would indicate survival differences between the glyphosate-tolerant and the null seedlings.

Preliminary reviews of the data were completed by plotting them with the PROC PLOT procedure of SAS (SAS Institute Inc., Cary, NC) followed by mixed-model analysis to assess the effect of treatments on alfalfa plant density. Glyphosate application time, seeding rate and time of plant density determination were fixed variables while location and replicates were considered random variables.

Alfalfa Plant Density

Precipitation at the two locations differed during alfalfa germination, emergence, and initial growth but then was similar later in the planting year. The Ohio location had 38 mm more precipitation while the Pennsylvania location had 53 mm less precipitation than normal in May after planting. Total growing season precipitation was 43 mm above normal for both locations. In May and June of the planting year, average daily temperature averaged 0.6 and 0.7°C cooler in Ohio and Pennsylvania, respectively.

Weeds were not present in any plots at either location. There were no interactions for location × seeding rate or location × time of glyphosate application. There were seeding rates × time of glyphosate application, seeding rates × time of plant density determination, and time of glyphosate application × time of plant density determination interactions (Table 1). However, none of these interactions involved a change in treatment ranking but rather changes in magnitude of difference in plant density between different seeding rates and time of glyphosate application.

Table 1. Statistical summary of glyphosate-tolerant alfalfa when seeded
at two rates and treated with glyphosate at one month and 12 months
after planting. Data are the means of two locations.

Treatment	Pr > F
Seeding rate (SR)	0.0001
Glyphosate application timing (GT)	0.9392
SR × GT	0.0492
Time of plant density determination (TD)	0.0001
SR × TD	0.0300
GT × TD	0.0047
SR × GT × TD	0.9051

One month after planting, plant densities averaged 27 and 46 plants/0.1 m² for the 17 and 29 kg/ha seeding rates, respectively. These densities account for only 36% of the PLS that were planted regardless of seeding rate. Previous research has shown from 20 to 70% of the seeds planted are actually present as plants one month after planting (5,6,11,12). This wide variation in number of alfalfa seeds that actually become seedlings is unrelated to seeding rate but rather a result of seedbed conditions affecting planting depth and seed-to-soil contact (11).

Plant densities of the 17 and 29 kg/ha seeding rates were different for the duration of this study (Table 1). However, when no glyphosate was applied the higher seeding rate had greater plant mortality (44%) than the lower seeding rate (35%) (Fig. 1). These findings support the previous findings of Hall (5) and Nelson et al. (11,12,13) showing that higher seeding rates experienced a greater proportion of seedling death compared to lower seeding rates within the first 12 months after planting. Glyphosate application one or 12 months after planting had no affect on plant density at T4 (Fig. 1). The plant density at T4 for all treatments is unlikely to provide space for weeds species to invade the stand since weed encroachment generally does not occur until plant density in established alfalfa stands falls below 10 plants 0.1/m² (6).

Fig. 1. Plant density of glyphosate-tolerant alfalfa when seeded at 17 and 29 kg/ha and not treated or treated with glyphosate at one month (T1) or 12 months (T3) after planting. Time T2 and T4 are 2 weeks after T1 and T3, respectively. Data are the means of four replicates at each of two locations. A star above a cluster of means indicates difference at the 0.05 probability level.

Plant density decreased after each glyphosate application except at the low seeding rate 12 months after planting (Fig. 1). Regardless of seeding rate, an average of 6.5 and 5.5% of the alfalfa plants died when glyphosate was applied 1 month and 12 months after planting, respectively, but plant density remained unchanged during the same time periods when glyphosate was not applied (Fig. 1). Similar plant mortality regardless of when glyphosate was applied indicates that there is no difference in intraspecific competitiveness between glyphosate-tolerant and non-glyphosate-tolerant alfalfa seedlings. Had there been a change in alfalfa mortality after the T1 or the T3 glyphosate applications it would have indicated that the glyphosate-tolerant and null seedlings had different intraspecific competitiveness. Even in a more intraspecific competitive environment of 29 kg/ha seeding rate, the similarities in mortality rate after the T1 and T3 glyphosate applications further supports that the competitiveness of glyphosate-tolerant and null alfalfa seedlings are not different. The greatest amount of plant death occurred between T2 and T3 regardless of treatment.

Conclusions

Higher alfalfa seeding rates had greater plant mortality during the first year after planting than the lower seeding rate. The quantity of non-glyphosate tolerant nulls in the commercial glyphosate-tolerant alfalfa seed used in this study was about 6% and had no negative affect on stand density. We found no difference in plant mortality between glyphosate-tolerant and non-glyphosate-tolerant alfalfa plants within the first 12 months after planting. Consequently, there is no practical reason for producers to apply glyphosate at the 3- to 6-trifoliate leaf stage of glyphosate-tolerant alfalfa development unless weeds are present. The option to delay glyphosate application until weeds are present can reduce alfalfa establishment costs by minimizing the number of herbicide applications while retaining the opportunity to use a broad-spectrum herbicide later in the life of the stand to control weeds.

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