LEGUME GREEN FALLOW EFFECT ON SOIL WATER CONTENT AT WHEAT PLANTING AND WHEAT YIELD

Authors: Nielsen, David; Vigil, Merle

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2005
Publication Date: 5/1/2005
Citation: Nielsen, D.C., Vigil, M.F. 2005. Legume green fallow effect on soil water content at wheat planting and wheat yield. Agronomy Journal. 97:684-689.

Interpretive Summary: The traditional wheat-fallow system of the central Great Plains can lose soil quality through erosion that occurs during the summer fallow period when weeds are controlled by tillage. A wheat production system that has shown some success in the northern Great Plains is to grow a legume cover crop in place of fallow. The legume is planted in early spring and terminated at a certain growth stage. The legume provides some nitrogen and protection from erosion to the soil. However, the legume also uses some stored soil water and precipitation that falls during the fallow period, thereby potentially lowering yields of the subsequent wheat crop. This study was conducted to determine which of four legume species would be best to use as a green fallow crop, and to determine when the legume should be terminated for this system to be successful. The legumes tested in this study were not different in their water use and the amount of water left in the soil at wheat planting time. The soil water available at wheat planting following an early legume termination (beginning flowering) was 57 mm lower than available soil water in the conventionally tilled treatment, resulting in a wheat yield that was reduced by 944 kg/ha compared with the conventional fallow treatment. When the legumes were allowed to go to maturity the soil water was reduced by 107 mm and the yield was reduced by 1759 kg/ha. The cost in water use by the legume and the subsequent decrease in wheat yield may be too great to justify the use of legumes as fallow cover crops in wheat-fallow systems in this environment.

Technical Abstract: Growing a legume cover crop in place of fallow in a winter wheat (Triticum aestivum L.)-fallow system can provide protection against erosion while adding nitrogen to the soil. However, the water used by the legume may reduce the following wheat yield. This study was conducted to quantify the effect of varying legume termination dates on available soil water content at wheat planting and subsequent wheat yield in the central Great Plains. Four legumes [Austrian winter pea, Pisum sativum L. subsp. sativum var. arvense (L.) Poir.; spring field pea, P. sativum L.; black lentil, Lens culinaris Medikus; hairy vetch, Vicia villosa Roth.) were grown at Akron, CO as spring crops from 1995 to 1999. The legumes were planted in early April and terminated at 2-week intervals (four termination dates), generally starting in early June. Wheat was planted in September in the terminated legume plots, and yields were compared to wheat yields from a conventional tillage wheat-fallow system. Soil water at wheat planting was significantly reduced with delayed legume termination date and legume growth, resulting in significant decreases in subsequent wheat yields. Wheat yield was linearly correlated with available soil water at wheat planting [kg/ha=15.71*(mm-70)]. There were generally no significant differences in available soil water at wheat planting due to legume type. The cost in water use by the legume and subsequent decrease in wheat yield may be too great to justify the use of legumes as fallow cover crops in wheat-fallow systems in this environment.