Sheep Grazing Effect On Dryland Soil Properties and Wheat Yield in the Wheat-Fallow System.

Sheep (Ovis aries L.) grazing during fallow is an effective method of controlling weeds and pests in the wheat (Triticum aestivum L.)-fallow system. Little is known about the effect of sheep grazing on dryland soil properties and wheat yield. We evaluated the effects of fallow management for weed control and soil water conservation [sheep grazing (grazing), herbicide application (chemical), and tillage (mechanical)] and cropping sequence [continuous spring wheat (CSW), spring wheat-fallow (SW-F), and winter wheat-fallow (WW-F)] on extractable soil nutrients and chemical properties at the 0- to 60-cm depth and wheat yield. The experiment was conducted in a Blackmore silt loam (fine-silty, mixed, superactive, frigid Typic Argiustolls) from 2004 to 2008 in western Montana. From 2004 to 2007, P and K concentrations at 0- to 15-cm were lower in the grazing than in the chemical treatment in SW-F. In contrast, concentrations of Na at 0- to 30-cm and Ca at 30- to 60-cm were greater in the grazing or mechanical than in the chemical treatment. Magnesium concentration at 30- to 60-cm and SO₄-S content at 15- to 30-cm were greater in CSW than in WW-F. In 2008, P, K, and SO₄-S contents at 0- to 30-cm were lower in the grazing than in the chemical or mechanical treatment and SO₄-S content was lower in CSW than in SW-F or WW-F. In contrast, Mg and Na contents at 5- to 10-cm were greater in the grazing than in the chemical treatment. At 0- to 15-cm, P and K concentrations reduced from 2004 to 2006. Soil pH at 0- to 15-cm was greater in the mechanical than in the chemical and grazing treatments in CSW and WW-F. Cation exchange capacity (CEC) and electrical conductivity (EC) were also greater in the mechanical than in the chemical or grazing treatment and greater in WW-F than in CSW or SW-F. Annualized wheat yield was greater in CSW than in SW-F and WW-F and greater in 2004 than in other years. Although feces and urine returned nutrients to the soil, sheep grazing reduced soil P, K, and SO₄-S levels compared with other treatments probably by reducing crop residue inputs as a result of consumption. In contrast, tillage and grazing increased Ca, Mg, Na, pH, CEC, and EC probably by returning sheep feces and urines or incorporating crop residue into the soil. Such changes in soil properties, however, did not influence wheat yields within five years.