Submitted to: Small Ruminant Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/4/2005
Publication Date: 11/15/2006
Citation: Hatfield, P.G., Goosey, H.B., Spezzano, T.M., Blodgett, S.L., Lenssen, A.W., Kott, R.W. 2007. Incorporating sheep into dryland grain production systems: III. Impact on changes in soil bulk density and soil nutrient profiles. Small Ruminant Research. 67:222-231. Interpretive Summary: Increased soil bulk density impedes water movement into soils, decreasing crop yield in semiarid environments. Purchase of essential nutrients for wheat represents a key cost for wheat producers in Montana. We conducted field trials investigating changes in soil bulk density and nutrient profiles from fallow plots grazed by sheep compared to conventional tillage with sweeps, burning, and untreated controls. In general, sheep grazing did not influence soil bulk density or nutrient profiles compared to untreated areas, burned or tillage.
Technical Abstract: Changes in soil bulk density and soil nutrient profiles are a major concern of dryland grain producers considering grazing sheep on cereal stubble fields. Although sheep grazing may reduce wheat stem sawfly over-wintering larval numbers and help control weeds on fallow ground, potential sheep impacts on the soil quality have not been investigated. Our objective was to compare burning, grazing, tilling, trampling and clipping wheat stubble fields on changes in soil bulk density and soil nutrient profiles. For Experiment 1, treatments were fall tilled (GT), fall grazed (GF), spring grazed (GS), fall and spring combined grazed (GFS), and an untreated control (GC). Five mature ewes were confined with electric fence for 24 h for GF and GS resulting in a stocking rate of 400 sheep d/ha. For GFS, the stocking rate was 800 sheep d/ha. For Experiment 2, treatments were fall grazed (BF), fall burned (BB), fall tilled (BT), and an untreated control (BC). In Experiment 3, treatments were fall trampling by sheep (TF), spring trampling by sheep (TS), fall and spring combined trampling by sheep (TFS), hand clipping to a stubble height of 4.5 cm (TCP), and an untreated control (TC). Trampling treatments were done at the same stocking rates as grazing treatments but sheep were muzzled to prevent intake. Treatment effects on soil bulk density and soil nutrients (nitrate-nitrogen, Olsen phosphorus, available potassium, organic matter, electrical conductivity, and soil pH) were determined using three samples per plot at each sampling time and site. In Experiment 1, soil bulk density of the grazing treatments was reduced (P<0.07) compared to GC at two sites, but did not differ (P>0.20) at the remaining six sites. Percent change in soil bulk density did not differ (P>0.15) at six of the eight sites when the mean of grazing treatments was compared to GT. Post treatment organic matter was greater (P=0.09) in the mean of the grazed treatments than GC. In Experiment 2, change in soil bulk density did not differ (P>0.25) between BB and BC or BB and BT at five of the six sites. Post treatment soil nitrate-nitrogen from 0 to 10 cm depth was greater (P<0.08) for BB than either BC or BF, but at the 10 to 20 cm depth, nitrate-nitrogen did not differ (P>0.73) between BB and BC, BT or BF. In Experiment 3, no differences were detected (P>0.13) for post treatment or change in soil bulk density. These results indicate a strong potential for grazing sheep on grain stubble without adversely impacting soil bulk density or nutrient profiles.