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The loss and degradation of habitat have been identified as major waterfowl management problems in North America (United States Fish and Wildl. Serv. and Canadian Wildl. Serv. 1986). Marshes deteriorating from cattail invasion are used infrequently by waterfowl, in part, because of low invertebrate and benthic productivity (Murkin et al. 1982). Generally, breeding bird populations respond positively to the creation of marshes with interspersed emergent vegetation and water (Weller and Spatcher 1965, Nelson and Dietz 1966, Beule 1979). Therefore, federal and state wildlife agencies (e.g., U.S. Fish and Wildlife Serv., Wisconsin Department of Nat. Res.) fragment dense cattails stands by using herbicides, mechanical destruction, burning, grazing, water level manipulation, and combinations of these techniques (Beule 1979, Murkin and Ward 1980, Kantrud 1986, Schultz 1987, Solberg 1989).

Other investigators have reported on the effect of modified cattail marshes on blackbird nesting (Beule 1979, Schultz 1987, Murkin et al. 1989). To our knowledge, only Linz et al. (1992) have reported on the response of roosting blackbirds to fragmented cattail marshes. Dispersing or reducing congregations of blackbirds by altering their roosting habitat (i.e., cattails) may in turn disperse or reduce damage to sunflower.

In 1989, Linz et al. (1992) began studying the use of RodeoR, an aquatic herbicide (registered trademark of Monsanto Company, St. Louis, MO; Monsanto 1990) for fragmenting cattail marshes. RodeoR is a post-emergent, nonselective herbicide, containing the active ingredient isopropylamine salt of glyphosate (N-(phosphonomethyl) glycine). The U.S. Department of Agriculture does not endorse RodeoR or any other product used in this study. Assessments of the treated marshes in 1990 indicated RodeoR killed the cattails, and blackbirds no longer used these marshes as roost sites. These promising results lead to an increased research effort to develop this method of eliminating blackbird roosts.

In 1990 and 1991, we continued to investigate the effects of cattail fragmentation on bird populations by comparing bird counts from 16 RodeoR treated cattail marshes and 8 untreated cattail marshes. Our objectives were (1) to determine the feasibility of fragmenting dense cattail marshes to discourage the establishment of blackbird roosts, (2) to assess the response of breeding and migrating bird populations to altered cattail marshes, and (3) to assess the usefulness of this technique for reducing or dispersing sunflower damage. Here, we describe our research objectives and methodology and discuss preliminary observations.

STUDY AREA AND METHODS

Our study marshes are in Benson, Nelson, Grand Forks, and Walsh Counties, which are located in the prairie pothole region of North Dakota. In 1990, we selected 12 semipermanent cattail marshes (2.4 - 10.3 ha; classification of Stewart and Kantrud 1971), that contained water, had abundant cattail, and historically or potentially could harbor blackbirds. These marshes were randomly assigned 1 of 3 spray coverages (treatments): 0%, 70%, or 90% areal coverage with RodeoR. In 1991, we selected 12 additional cattail marshes (5.2 - 32.0 ha) and randomly assigned 1 of 3 treatments (0%, 50%, or 70%) to each marsh. Application

The herbicide solution was applied at 46.8 L/ha, containing 5.8 L/ha RodeoR (2.8 kg/ha glyphosate), 0.2 L/ha surfactant (Valent X-77 Spreader, Trademark of Valent U.S.A. Corporation, Greeley, CO), 0.4-0.6 L/ha drift retardant (Chem-trol, Trademark of Loveland Industries, Greeley, CO), and sufficient water to bring the solution to final volume. A fixed-winged spray aircraft began on the marsh edge and alternated between treating 15 m strips and skipping either 15, 6, or 2 m strips for 50%, 70%, or 90% treatments, respectively. Marshes were sprayed in mid- to late July. Measurable precipitation did not contact the treated plants for at least 6 hr after treatment.

Determining Herbicide Efficacy
Prior to treatment, each marsh was divided into 2 strata of equal width. We divided each stratum into 15 m strips and 1 strip randomly selected for assessment of cattail density. A transect was established in the center of the selected strips and 10-0.25 m2 quadrats placed along the 2 transects. The quadrat interval was obtained by dividing the total length of both transects by 10. The location of the first quadrat was a random distance in meters between the marsh edge and quadrat interval. The remaining plots were located at uniform intervals along the transect. We will count the number of live (green) cattail stems and measured depth of water within the quadrats for at least 2 years posttreatment. Additionally, aerial photos are taken of all study marshes and computer analyzed for vegetation and water coverage.

Bird Censuses
During August, we counted blackbirds as they exited the test marshes at sunrise and as they entered to roost in the evening (Meanley 1965). In 1990, all birds using the 12 test marshes were counted between sunrise and 1030 hr, 1 time during the following the dates: June, July, and August. The observer(s) started counting blackbirds, waterfowl, and wading birds from the highest point near the marsh (e.g., hilltop). After completing this count, the observer(s) walked around the perimeter of the marsh recording all other species seen or heard in the marsh and within 25 m of the marsh.

In 1991, we used a fixed-radius point count method of estimating birds using test marshes (Hutto et al. 1986). Eight count points were established around the perimeter of the marsh within 5 m of the marsh vegetation (usually cattails). The count point intervals were determined by dividing the circumference of the marsh by 8. The location of the first point was a random distance between a designated corner of the marsh and count point interval. The remaining points were systematically placed around the marsh. At each count point, 2 observers waited 1 minute and then recorded all birds seen or heard during a 5 minute count period. Birds were counted 1 time during June, July, and August. We recorded new birds discovered while moving between count points for completeness but did not include these birds in our analysis. Marshes were censused in random order. The censuses were not conducted in rain or if winds exceeded 24 km/hr.

Damage Assessments
Sunflower fields surrounding each study marsh treated in 1990 were randomly selected for damage assessments. We divided each field into 4 strata, 1 row was randomly selected from each stratum, and 24-1.5 m plots were proportionally distributed among the 4 rows based on their length. Plot interval was determined by dividing the total length of the row by the number of plots assigned to that row. The location of the first plot in each row was a randomly selected distance between the field edge and the plot interval. The diameter of each head and undeveloped center was measured to the nearest cm with a measuring tape (Hothem et al. 1988). The area of seed (cm2) missing from the head was estimated using a gridded plastic template (Dolbeer 1975).

RESULTS AND DISCUSSION

Efficacy of Spray Applications
Under the environmental conditions of northeastern North Dakota, RodeoR herbicide applied at 5.8 - 7.0 L/ha in August effectively controls cattails for at least 2 years. RodeoR applied at 4.7 L/ha significantly reduced cattail density after 1 year. Comes and Kelly (1989) found glyphosate applied at 3.4 kg/ha (equivalent to 7.0 L/ha RodeoR) in mid-September was optimum for controlling cattails in a seasonal flowing drainage ditch in central Washington. In August 1985 and July 1986, Solberg (1989) aerially applied RodeoR at 7.0 to 8.8 L/ha on cattail marshes in South Dakota and achieved nearly 100% control of cattails. Cattails regenerate quickly on mud-flats and shallow water (<30 cm) marshes (Solberg 1989, Merendino and Smith 1991); whereas, wetlands with at least 30 cm of water will remain free of cattail for several years (K. Higgins, pers. commun., U.S. Fish and Wildlife, Brookings, South Dakota). In 1990 and 1991, we observed small, dense patches of immature cattails (<8 cm tall) growing where spikes of mature seeds had fallen into shallow water. In addition, dense stands of taller (<30 cm) seedling cattails grew in areas of the marshes where mud flats were created by water evaporation. In one marsh these patches of cattails reached 120 to 150 cm tall in 1991 but did not flower.

Cattails killed in 1989, were still present in September 1991, but the majority had fallen into the water. Mason and Bryant (1975) reported dead cattail (Typha angustifolia) shoots collapse after 2 years and take an additional 2 years to decompose completely. Burning the dead cattails in the fall or spring following treatment may be an effective way of rapidly creating openings in treated marshes. Additionally, reducing the amount of litter in the marsh may lessen any adverse effects on water quality caused by the decomposition of large amounts of vegetation.

Bird Populations
Our data indicate fragmenting solid stands of cattails with herbicide reduces their use by fall-migrating blackbirds. We speculate that dispersing blackbirds may dissipate and reduce sunflower damage. However, studies are needed to quantify sunflower damage patterns before formulating specific management recommendations.

Populations of marsh wrens and rails appear to decrease with the eradication of cattails, probably because these birds require dense emergent vegetation for foraging and nesting. We expect these populations of birds will begin to rebound as cattails repopulate the marsh. Preliminary analyses indicate the number of ducks and shorebirds did not differ between treatment and untreated (controls) marshes one year after treatment. This is not surprising since the dead cattails were still standing after one year. Additionally, ducks and shorebirds are probably correlated with water levels and cattail densities. Generally, marshes with dense stands of tall emergents are used less by waterfowl than marshes with interspersions of open water and emergent vegetation (Kantrud 1986). Normal water levels coupled with broken stands of emergent vegetation should increase the number of adult and young waterfowl using the test marshes (Kaminski and Prince 1981, Murkin et al. 1982). Marshes with high water levels or those that are dry probably would not harbor many shorebirds.

Economics of Using Rodeo®
If managing cattail marshes proves effective in dispersing blackbirds, individual growers may substantially reduce sunflower losses. RodeoR may be cost-beneficial, especially if costs are amortized over a number of years. For example, if a 10 ha (25 A) cattail marsh harbors 20,000 blackbirds and each bird eats 14 g (1/2 ounce) sunflower per day (a conservative estimate, Besser 1979), this flock will eat 280 kg/day (617 lb/day) at a cost of $61.70/day (@ $0.10 lb). Over 30 days, the birds may damage 8,400 kg (18,518 lb) of sunflower at a cost of $1852.00. Cost of aerially applying RodeoR, using 5.8 L/ha (2.5 qt/A), is about $151.00/ha ($61.00/A). Most of the cost (88%) is for the herbicide. The cost of treating 70 to 100% of a 10 ha marsh with 5.8 L/ha RodeoR is $1057 - $1510. If the treatment is effective (i.e., a blackbird roost does not form), individual growers may recoup their costs for treating the marsh in 1 year. Additionally, the sunflower grower may enhance the value of the marsh by improving the habitat for marsh birds, especially waterfowl (Kantrud 1986, Solberg 1989).

Preliminary Recommendations For Applying Rodeo®
The U. S. Department of Agriculture, Animal and Plant Health Inspection Service, North Dakota Animal Damage Control and U.S. Department of Interior, Fish and Wildlife Service have received funding for demonstrating the use of RodeoR for controlling cattails used by roosting blackbirds (Louis Huffman, North Dakota Animal Damage Control, Bismarck, ND, pers. commun.) and to improve waterfowl habitat (Michael McEnroe, Wetland Habitat Office, Bismarck, ND), respectively. Therefore, we advance the following (albeit preliminary) recommendations: (1). For maximum cost-effectiveness, limit treatment to cattail marshes containing water and traditionally harboring large numbers of birds. The water will slow regrowth of cattails by inhibiting reproduction by seeds. (2). Apply RodeoR at 5.8 L/ha (2.5 qt/A). Under normal growing conditions, this rate should be adequate to kill the majority of the cattails (Cal Messersmith, Department of Crop and Weed Science, North Dakota State University, pers. commun.). (A 100 gal tank solution contains 12.5 gal RodeoR, 2 qt surfactant, 5 qt drift retardant, and sufficient water to bring to final volume). (3). At least 70% of the cattail should be killed by alternately spraying 15 m (50 ft) wide strips and skipping about 6.4 m (21 ft) between strips. Data gathered to date indicate that birds will not roost in marshes with narrow strips of live cattail. (4). Although we have evidence that RodeoR applications sprayed from mid-July to early September effectively kills cattails, ideally treatments should be made from August until first frost. This timing will (1) maximize herbicide efficacy, (2) decrease the possibility of spray drift damaging small grain crops, and (3) avoid most young waterfowl broods.

CONCURRENT AND FUTURE RESEARCH

In 1992, scientists from North Dakota State University and Denver Wildlife Research Center plan to (1) gather data on the efficacy of managing blackbird roosting sites for dispersing and reducing sunflower damage in 23 km2 blocks in southeastern North Dakota, (2) assess the effects of using RodeoR on marsh water quality, aquatic invertebrate populations, breeding bird populations, and winter cover for gallinaceous birds, and (3) continue to evaluate the response of cattails to various application rates of RodeoR.

MANAGEMENT IMPLICATIONS

LITERATURE CITED

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