Research Made Possible By The Alabama Turfgrass Research Foundation.

Poa annua Control with TranXit GTA

R. Harold. Walker, Ph.D. and Jason Belcher, Research Associate

Poa annua var. annua is a common winter annual grass that infests turfgrasses throughout the U.S.   In the Southeast, P. annua typically becomes established in the fall, just as warm-season turfs enter dormancy.  It remains as a problematic weed through the following spring at which point it typically dies out leaving unsightly brown patches.   A  perennial variety, Poa annua var. reptans, often co-exists with the more common  var. annua.  The perennial variety is most frequently found infesting golf putting greens (Wehtje and Walker 2002).

TranXit GTA (Tranxit for remainder) is a product of Griffin LLC and was registered for use in bermudagrass turf in fall 2001.  It  has three potential uses:  P. annua control in non-overseeded bermudagrass turf; 2) P. annua control in bermudagrass turf and hybrid bermudagrass  putting greens before overseeding; 3) removal of perennial ryegrass and/or rough bluegrass overseed from bermudagrass turf.  Use sites include professionally-managed turfs such as golf courses, sod and seed farms and commercial and industrial lawns.

The sulfonylurea herbicides are a rapidly-expanding group with good potential for use in the turfgrass industry.  Individual herbicides vary as to whether they are predominately soil or foliar active. The active ingredient in Tranxit is rimsulfuron, a member of this group, that is registered for use on field crops and turf.  Based upon experience in field crops,  rimsulfuron is considered to be largely postemergence active (Green and Green 1993) and rapidly degraded in soil (Ahrens 1998).  However, observations of the duration of P. annua control in bermudagrass turf by the authors suggested that the control of this species may reflect preemergence, in addition to postemergence, activity.  And since many sulfonylureas exhibit both soil and foliar activity, it is a reasonable hypothesis that this may be the case with activity of Tranxit in controlling P. annua in turf.  Determining whether preemergence (i.e. soil) activity does occur,  and evaluating its importance relative to postemergence activity was our first objective and was researched in a greenhouse environment.

Previous research at Auburn University identified 2 ounces/acre of Tranxit as the standard rate.  However, there is interest in using only 1 ounce/acre in order to reduce costs.  Also, previous research (Green and Green, 1993) has shown that methylated seed oils were the most  effective adjuvants for use with this herbicide.  Therefore, research was initiated to determine P. annua control as influenced by Tranxit rates and various admixtures including adjuvants, and other herbicides.  Two separate field experiments were conducted for this objective.

METHODS

 Greenhouse Studies.   Experiments were conducted in a greenhouse equipped with evaporative cooling such that daily maximum temperature did not exceed 78 F.  Plants received only natural light.  Control was visually evaluated 4 weeks after treatment.  All experiments were conducted twice. 

Soil versus foliar activity on established plants.  P. annua seedlings (approximately 5/cup) were grown in 20-oz styrofoam cups until seed heads were evident (about 1 month) prior to treatment.  Experimental variables  included application type, growth media, and variety of P. annua.   Three application types were included: soil only, foliar only and soil + foliar.  To achieve the soil-only application, the amount of the spray solution that an individual cup would receive during a standard application with the sprayer as described below was calculated based upon surface area of the filled cup.  This amount of the spray solution was diluted into 10 ml of water (about 2 teaspoons) and added directly to the soil so as to avoid any foliar contact.  Both the foliar-only and the soil + foliar treatments were applied with a moving belt-type sprayer calibrated to deliver 20 gpa.  A nonionic surfactant was included in these applications at 0.25% v/v.   To achieve the  foliar-only application the soil surface was covered with activated charcoal to intercept and prevent any herbicide from entering the growth media.  Charcoal was removed after the application.  Two growth media were included: a fine sandy loam soil (a typical agricultural soil for our area, pH = 5.8) and a sand-peat mixture [90:10 (v:v) mixture of medium sand and sedge peat moss].  Both the annual and perennial varieties were included.  Tranxit was applied at a single rate of 3 oz product/acre.  Individual treatments consisted of all possible combinations of the aforementioned variables.

Soil activity against germinating seeds.  P. annua seeds (approximately 20) replaced established plants as the basic experimental unit in these studies.   Two studies were conducted.  Experimental variables in the first study included the two growth media and the two P. annua varieties.  An additional variable included in this study was whether seed were covered or not.  For this, seeds were either placed on media surface and watered with a gentle mist so that seeds became embedded into the surface, or the seeds were covered by 0.3 inch of the appropriate growth media.  Tranxit was applied at a single rate of 3 oz product/acre. 

The second study was to determine the influence of Tranxit rate on preemergence activity.  Procedures were identical to that of the first study,  however, Tranxit was applied at 3 rates; i.e. 1, 2 and 3 oz product/acre.

Field Studies.  Test sites were located at the Auburn University Turfgrass Research Unit on Tifway bermudagrass managed as a golf course fairway which was heavily infested with a native population of Poa annua var. annua.    Plot size was 4 by 12 feet and there were four replicates of each treatment.  All treatments were applied with a CO₂ backpack sprayer delivering 30 gallons of spray/acre.  Sprays were allowed to stay on the foliage for 6 hours and then washed into the root zone with irrigation.  P. annua control and Tifway green-up were evaluated over time.

Experiment I.  Treatments are shown in Table 4.  Initial treatment was applied February 5, 2002 to P. annua that was in early bloom.  The second treatment, where appropriate, was applied March 8, 2002 when P. annua was in full bloom.

Experiment II.  Treatments are shown in Table 5.  Test site was located adjacent to the previous study.  However, treatments were applied March 5, 2002 to P. annua that was in full bloom.  No repeat applications were included in this study.

  

RESULTS

Greenhouse Studies.

Soil versus foliar activity on established plants.   Postemergence applications were significantly influenced only by the main effects of application type and the variety of P. annua,  and by the interaction of these two variables.  With var. annua, both the soil + foliar and soil-only applications provided the maximum control;  i.e. >84% (Table 1).  With var. reptans, application effectiveness, in descending order were as follows: soil + foliar, soil only and foliar only.  Control was not significantly different between the two media.  Control, as averaged over all other variables, was 68 and 64% for the sand-peat mixture and soil, respectively (data not shown).

Soil activity against germinating seeds.  In the first study, the activity of soil-applied Tranxit against germinating seeds was significantly influenced only by the main effects of variety and application (i.e. seed planting depth).  For both varieties, control was reduced when seeds were covered.  However, even in the most challenging situation of covered seeds of var. reptans, control was consistently >86%. 

In the second study,  P. annua control was significantly influenced by the main effects of rimsulfuron rate, P. annua variety, growth media, and seed coverage.  In addition, nearly all 2-way interactions of these variables were also significant.  For both varieties, greater control was obtained when the seeds were not covered (Table 3).  Control was generally Tranxit-rate dependent.  However, this rate response was more pronounced with the perennial than with annual variety.   With covered seed of the perennial variety, control was 89, 83 and 72% for the three rates evaluated in descending order.  In contrast, with the annual variety and seeds not covered, control was 94% across all application rates. 

Field Studies.

Experiment I.  Destiny adjuvant (methylated seed oil) was required to achieve good results over time with a single application of Tranxit at 1 oz/acre.  Repeating the 1 ounce rate or applying a single dose at 2 ounces provided complete control.   Finale provided similar results as the Tranxit + Destiny treatments.  Reward provided unacceptable control of P. annua.  All treatments except Reward produced good and equivalent results on Tifway green-up (Table 4).

Experiment II.  There was only a small response to Tranxit rates ranging from 1 to 3 oz/acre for P. annua control and Tifway green-up.  There was however, significant improvement in control, and a trend towards better green-up with the 4 ounce rate.  Unfortunately, this high rate would probably not be economically feasible.  However, P. annua control and Tifway green-up equivalent to 4 oz/acre of Tranxit was achieved with the admixture that contained 1 oz/acre Tranxit + 1% Destiny + 3 gallons/acre of 28% urea ammonium nitrate.  The addition of either Plex, ammonium sulfate or Prograss to Tranxit (1 oz) + 1% Destiny neither improved control nor Tifway green-up.  Antagonism was observed with admixtures of Tranxit + Sencor (Table 5).

 SUMMARY

Greenhouse Studies.

The overall effectiveness of preemergence applications in controlling germinating seeds, combined with the requirement for postemergence applications to include soil exposure in order for them to be effective,  indicates that root entry is equally important as foliar entry.  In some situations, root entry may be more important.  Tranxit did provide preemergence control of P. annua in this greenhouse environment.

Field Studies.

The 1 oz/acre rate of Tranxit was effective for control of large P. annua when combined with methylated seed oil + urea ammonium nitrate adjuvants.  When using the lower 1 oz/acre rate, re-treatment may be necessary for complete control of large P. annua.  Caution should be used when combining Tranxit with other herbicides since antagonism was documented when tank-mixed with Sencor.

 

LITERATURE CITED

Ahrens, W. H., ed. 1998 Herbicide Handbook. Supplement to 7^th ed. Champaign, IL. Weed             Science Society of America, pp. 60-62.

Green, J. M. and J. H. Green.  1993.  Surfactant structure and concentration strongly affect             rimsulfuron activity.  Weed Technol.  7:633-640.

Wehtje, G. and R. H. Walker.  2002.  Response of two annual bluegrass varieties to

preemergence and postemergence rimsulfuron.  Weed Technol. 16:612-616.