Development of Winter Wheat Cover Crop Systems for Weed Control in Potatoes

Development of Winter Wheat Cover Crop Systems for Weed Control in Potatoes

Summary

Objectives
The overall goal of this project was to develop a winter wheat cover crop system that could be used for economical weed control in potatoes. A series of experiments were conducted to: 1) test the efficacy of using winter wheat as a cover crop for weed control in potatoes; 2) test the use of hybrid necrosis factors for replacing chemical removal of the cover crop once the potato crop was established; 3) generate and evaluate necrotic winter wheat hybrids for use as cover crops for potatoes; and 4) develop cytoplasmic male-sterile winter wheat adapted to the Pacific Northwest for commercial hybrid production.

Abstract of Results
Weeds are an economically important problem in potato production in the Pacific Northwest, but options for mechanical and chemical weed control are limited. Winter wheat is an effective competitor with summer annual weeds that infest potato fields in Washington, Oregon, and Idaho. Therefore, studies using a winter wheat cover crop system for weed control in potatoes were conducted at Aberdeen, Idaho and Patterson, Washington.

Fall-planted 'Weston' winter wheat was killed chemically at various times after planting potatoes the following spring. In 1992, winter wheat competed well with weeds and reduced weed biomass 43 to 97 percent, depending on treatment and location. However, U.S. No. 1 (marketable) tuber yields were reduced 51 to 60 percent by the combination of weed and cover crop competition. In 1993, the spring was much wetter than normal and the wheat cover crop system did not control weeds as well as in 1992. Weed biomass was reduced 15 to 94 percent at Aberdeen and 3 to 32 percent at Patterson. U.S. No. 1 yields were reduced 49 to 96 percent by the combination of weed and cover crop competition.

In addition to the cover crop trials, 55 necrotic hybrids were evaluated in two years. Significant variation in timing and severity of necrosis indicated that the hybrids may be useful as cover crops in a broad range of crops.

Economic Analysis
The economics of using a wheat cover crop weed control system were compared to a standard-practice weed management system used by commercial potato growers in Southeastern Idaho. Complete cost and return estimates, commonly referred to as enterprise budgets, were developed for both systems and were used as the basis for the economic comparison. The yield data from the Aberdeen plots were indexed and extended to a commercial-size Southeastern Idaho potato farm to facilitate the comparison.

All cover crop alternatives evaluated in both years show a decline in costs when compared to the standard-practice budget. However, the cost savings is not significant when compared to the total per acre costs of producing potatoes in Southeastern Idaho, which exceeds $1,400. Also, a substantial amount of the cost savings resulted from a decrease in the assessments paid by the grower. Assessments are based on yield, and the lower the yield, the lower the assessment cost.

The standard budget had herbicide costs of $34.37 per acre not found in the potato budgets using a wheat cover crop for weed control. In contrast, the wheat cover crop budgets had $11 of input costs not used in the standard budgets, including wheat seed and Roundup. Thus, the net difference in the value of herbicides and seed was approximately $23 per acre. Machine labor and fuel and lube were all higher with the standard-practice budgets. Machine repairs were slightly lower.

The relatively minor input cost savings with the wheat cover crop systems did not compensate for the substantial reduction in gross revenue, primarily from yield impacts and secondarily from quality adjustments to price. None of the cover crop weed control alternatives compete favorably with the standard practice budget on an economic basis

Potential Contributions
The technology being investigated is at a preliminary stage and is not presently amenable to on-farm testing. First, relying on a cover crop alone for weed control is too risky. Weed control with the cover crop system varied tremendously depending on environmental conditions in the spring. When weeds were not controlled, U.S. No. 1 yields were reduced 51 to 96 percent. Using a reduced rate herbicide application to supplement weed control with the cover crop may provide acceptable weed control. If the present difficulties with the cover crop system can be overcome, weed control with a modified winter wheat cover crop system could be accomplished with less herbicide input than is currently used, and soil erosion would be greatly reduced.
Potatoes generally require three to four weeks from planting until emergence, and fields are subject to severe wind erosion until sufficient canopy has developed to protect the soil. In addition, irrigated potatoes are also subject to water erosion from irrigation, especially when grown on sloping land. The winter wheat cover crop protects the soil from winter and spring wind erosion, and from spring water erosion. The wheat also could scavenge nitrogen not used by the previous crop and reduce nitrate leaching to groundwater.
Reported in 1995