Weed Suppression and Enhancement of Wildlife and Beneficial Insect Habitat in Center Pivot Irrigated Field Borders and Corners

Weed Suppression and Enhancement of Wildlife and Beneficial Insect Habitat in Center Pivot Irrigated Field Borders and Corners

Summary

Objectives

1. Develop managed plant communities to increase animal and plant diversity and reintroduce native plant species into the corners and borders of center-pivot irrigated fields of the Columbia Basin.
2. Compare plant, insect, and wildlife populations in managed plant communities to existing range and herbicide-cultivated corners and field borders to determine if plant and animal species differ in these areas.
3. Determine if managed plant communities reduce weeds, insect pest populations, diseases and their vectors in adjacent crops.
4. Determine if managed plant communities increase biodiversity in field corners and borders when compared to existing plant communities.
5. Identify and promote the use of cover crops that have the potential to reduce soil erosion in fields adjacent to study sites when they are planted to low residue crops such as potatoes, beans, and peas.
6. Quantify fall and spring forage production of specific cover crops and assess their ability to suppress weeds and reduce soil erosion.
7. Conduct field research, demonstrations, and local meetings and prepare publications to inform growers of the environmental advantages, management feasibility, risks, and profitability of adapting plant community managed border habitats and cover crops.

Abstract

The idea behind this project was to improve the management of center-pivot irrigated field corners by establishing native species in those corners. These native grasses, forbs, and shrubs would compete against weedy species and, in the long-term, reduce weed seed in adjacent fields and reduce herbicide use. In the long-term, these re-vegetated corners might harbor wildlife and beneficial insects that would prey on pests in the adjacent crops. To test this idea, three sites were chosen for re-vegetation with native grass species. Due to low precipitation and resulting low germination of the seed, only one site was successful in establishing a stand of native grasses in a pivot corner. Native shrubs and wildflowers were later planted at this same site. Plant and insect populations were monitored in these re-vegetated sites, in conventionally managed corners (weeds managed with herbicides), and in adjacent native areas. The short-term results (three years of data at one site) did not reveal large differences between conventionally managed corners and re-vegetated corners in terms of insect populations. All three of the monitored sites had both pest and beneficial species of insects. The weed Downy brome was more prevalent in the conventionally managed corners, but it was present in the other two sites as well. In order for this to be a feasible strategy for farmers, the chance of success in re-vegetation must be increased. To do this may take a year of clean fallow before re-vegetation or supplemental irrigation after seeding. Long-term studies will be required to determine the changes in insect and plant populations that result when native plants are established in these field corners.

Specific Results

Native grasses were planted in corners of center-pivot irrigated fields at three sites. At site one, four native grasses, Sherman Big Bluegrass, Basin Wild Rye, Thickspike Wheatgrass, and Blue Bunch Wheatgrass, were planted on March 21, 1997, December 3, 1997, and on November 17, 1998. At site two, the same grasses were planted on December 3, 1997, and on November 19, 1998. Multiple plantings were required because of low precipitation over summer and winter resulting in a very low germination. Severe weed pressure from Downy brome and Russian thistle also caused problems with grass establishment. Site three was seeded with the same four grass species in September of 1996. This last site received supplemental irrigation and was the only site that had successful establishment of the native grasses.

In November of 1998, the following shrubs and wildflowers were planted at site three: Big Sage (Artemesia tridentate), Antelope Bitterbrush (Pushia tridentate), Low green Rabbitbrush (Chrysothamnus viscidiflorus), Rubber rabbitbrush (Chrysothamnus nauseaous), Arrowleaf Balsamroot (Balsunorhiza macrophylla), Globemallow (Sphaeralcea coccinea), Whiter yarrow (Achillea millefolium), Prairie Silky Lupine (Lupinus sericeus), Wooley lupine (Lupinus leucophyllus), Fern-leaf Desert Parsley (Lomatium dissectum), and Blanketflower (Glaillardia aristata). Many of the shrubs were alive in the spring of 1999, but it is too soon to tell if they or the wildflowers will establish successfully.

The plant and insect communities for re-vegetated corners, control corners (weeds controlled with herbicides), and native shrub-steppe were characterized at all three sites. Each spring for years 1997 through 1999, the sites were sampled twice, usually in May and late June or early July.

Corners at sites one and two did not have successfully established stands of native grasses, so the sites cannot be compared statistically. All observations are based on the comparisons between corners with established native grasses, corners with herbicide-managed weeds (control corners), and nearby native shrub steppe at site three.

Downy brome populations were generally higher in the re-vegetated corners and control corners than in the native shrub-steppe. Downy brome increased in the re-vegetated corners over the three years of the project, mainly between the established rows of native grasses. In addition, when Downy brome was controlled in the control corners, other weedy species took over, such as Russian thistle and kochia. These broadleaf weeds were mostly absent in the re-vegetated corners and in the native sites. The proportion of native grasses stayed relatively constant at about 25 percent of the ground cover, after establishment in the re-vegetated corners. The weed population in the native shrub-steppe was also stable, generally less than 20 percent of the ground cover.

No distinct trends were found in the insect sampling. Both the re-vegetated corners and the shrub-steppe had both pest and beneficial species. In the first sample date in the spring, pests tended to outnumber beneficial species at both sites. They were about equal in the control corners. The precipitation and date of sampling made a large difference in the number of insects found in these dry areas. The dry spring in 1998 resulted in the low number of insects and the high proportion of Downy brome covering the ground.

It will take much longer than two or three years to see the full results of re-vegetation. For this reason, the differences in animal species in each environment were not measured.

Because of difficulties in establishing native plant communities in the field corners the effects of the re-vegetation on the weeds, insects, and diseases in the adjacent fields was not measured. It is believed that any effect here would be in the long term.

Except for the seeded native grass species, the results of this study do not show that the re-vegetated corners support a greater diversity of plants, insects, or animals.

The farmers cooperating at the three sites did not plant low residue crops as planned for in the original proposal. Some cover crops were planted adjacent to the sites in 1997, but no formal measurements of wind erosion control or weed suppression were made.

Due to the failure to establish managed plant communities at two of the three sites, no on-farm field days, seminars, or workshops were held. One workshop on cover crops was held at a nearby research station. Several popular press articles covered the goals of the project at the beginning, but since the results are not conclusive, no extension publications nor journal articles were prepared.

Potential Benefits

This type of ecological research is complex, and the results may come only in the long term. There is still the potential for benefits to agriculture in pursuing the re-vegetation of center-pivot irrigated field corners, and another researcher at WSU Pullman has started a similar project in the same geographic area. However, the results are will not come for several years.

Farmer Adoption

Adoption of this concept by farmers is low, due to unproven benefits and high risk of failure at this time.

Future Recommendations

To get good results in this dry climate, it seems necessary to control the Downy brome and weeds for one full year (clean fallow) before re-vegetation. If supplemental irrigation is available, the year of clean fallow may not be necessary. Re-vegetation is only the first step towards creating a managed plant community. The re-introduction of a complete ecosystem of plants and insects is a long-term activity that we are just beginning to understand.

This summary was prepared by the project coordinator for the 2000 reporting cycle.

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