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Does Pasture Longevity Under Direct Grazing Affect Field-scale Sorghum Yield Spatial Variability In Crop-pasture Rotation Systems?
V. Pravia, J. A. Terra, Roel
National Institute of Agricultural Research-INIA

Crop yield spatial variability is usually related to terrain attributes and soil properties. In pasture systems, soil properties are affected by animal grazing. However, soil and terrain attributes relation with crop yield variability has not been assessed in crop-pasture rotations. Landscape attribute and soil properties relation with sorghum (Sorghum bicolor L.) grain yield were evaluated at the field-scale under three crop-pasture rotation systems during three years in Uruguay: i) continuous cropping (CC): ryegrass (Lolium multiflorum Lam.)-sorghum-ryegrass-soybean (Glycine max); ii) short rotation (SR): 2yr grass/legume pasture- 2yr like CC; iii) long rotation (LR): 4yr grass/legume pasture- 2yr like CC. Sorghum and soybean grain were harvested, while ryegrass and perennial pastures were grazed with steers. Sorghum yield variation was registered with yield monitors, and field variation was assessed with intensive grid sampling for soil chemical analysis, terrain topographic attributes and soil electrical conductivity (EC); and analyzed for spatial autocorrelation. Relations between variables were analyzed with factor and regression analysis. No spatial autocorrelation was detected for any soil variable but EC. Terrain attributes and EC explained 77% of the site variation and determined 14-69% of yield variability. In 2005-06, yield spatial autocorrelation was not detected, and terrain attributes and EC relation with yield was weak (R2=0.14 and 0.16 for CC and SR, and not significant for LR). However, in 2006-7 and 2007-08 dryer seasons, yield spatial dependence was strong (nugget/sill‹25%), with larger ranges in CC and SR than LR (165 vs 72m and 510 vs 125m, respectively). Terrain attributes relation with yield was stronger in SR than LR (R2=0.46 vs 0.21; 0.69 vs 0.27 in 2006-07 and 2007-08, respectively). Sorghum yield spatial variability and its relation with soil and terrain attributes seem to be affected by previous pasture longevity. Grazing animal effects on soil-plant spatial variability should be further studied in crop-pasture rotations.

Keyword: spatial autocorrelation, factor analysis, terrain attributes